My Book by Pulkit Gupta

Semester

BUILDING TECHNOLOGY (UNIT – 1)

Preliminary Investigation

Building Technology â&#x20AC;&#x201C; Preliminary Investigation

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

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Table of Contents Principles of Planning General Principles of Site Selection Site Preparation and Setting Out of Works Excavation for Foundation Trenches Subsoil Drainage Orientation of Buildings Electricity Supply in Buildings Winter Building

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1 Topic

Principles of Planning The basic objective of planning of buildings is to arrange all the units of building on all floors and at level according to their functional requirements making best use of the space available for a building. The shape of such a plan is governed by several factors such as climatic conditions, site location, accommodation requirements, local bye-laws, surrounding environment etc. In spite of the certain principles or factors, which govern the theory of planning are common to all buildings of all classes intended to be used for residential purposes. These principles, enunciated below, are not rigid but just factors to be considered in planning: 1. Aspect 2. Prospect 3. Privacy 4

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4. Grouping 5. Roominess 6. Furniture requirement 7. Sanitation 8. Flexibility 9. Circulation 10. Elegance 11. Economy 12. Practical considerations 1. Aspect Aspect means peculiarly of the arrangement of doors and windows in the external walls of a building which allows the occupants to enjoy the natural gifts such as sunshine, breeze,

scenery

etc.

Aspect

very

important

consideration in planning as it provides not only comfort and good environment to live in but from hygienic point of view also. A room which receives light and air from a particular side is said to have aspect of that direction and all such rooms making a dwelling need particular aspect.

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From this angle, the following aspects for different rooms are preferred: a) For kitchen – E-aspect b) For dining room – S-aspect c) For drawing and living rooms- S-aspect or S-e aspect. d) For bed rooms – S-W –aspect or w-aspect e) E) For verandahs – S-w aspect or W-aspect f) For reading rooms, stores, class rooms, stairs etc,- Naspect. 2. Prospect Prospect in its proper sense, is the impressions that house is likely to make on person who looks at it from the out side. Therefore, it include the attainment of pleasing appearance by the use of natural beauties, disposition of doors and windows, and concealment of some undesirable views in a given outlook. Prospect and Aspect both demand disposition of doors and windows. For sake of either seeing or hiding certain views, window sites play a vital role. 6

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3. Privacy Privacy is one of the important principles in the planning of buildings of all types in general and residential buildings in particular. Privacy requires consideration in two ways: i) Privacy of one room from another. ii) Privacy of all parts of a building from the neighbouring buildings, public streets and by-ways. Privacy is of supreme importance in bed rooms, water closets, urinals, bathrooms etc. the kitchen apartment also should be kept out of the view of the passerby. 4. Grouping Grouping means the disposition of various rooms in the layout in a typical fashion so that all the rooms are placed in proper correlation of their functions and in proximity with each other. Services must be nearer to and independently accessible from every bed-room. The water closets , urinals etc must be far away from the kitchen and dining room and so on.

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5. Roominess Roominess refers to the effect produced by deriving the maximum benefit from the minimum dimensions of a room. In other words it is accomplishment of economy of space at the same time avoiding cramping of the plan. For giving better impression of roominess, the following points should be kept in view: i)

A great skill should be exercised in making suitable arrangements of the rooms, doors and passages for accommodation in such a way that the utility, liability, privacy and extension appearance are not adversely affected.

ii)

A square room appears relatively smaller in size and utility than a rectangular room of the same area. For a rectangular room the better proportion is to adopt length as 1.2 to 1.5 times of breadth.

iii)

A small room within ordinately high walls appears relatively smaller than its actual size.

iv)

The disposition of doors, windows and cupboards, such that they do not cross-cut this room area and obstruct the placing of furniture adds to roominess. 8

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The design of elements such as floors, walls, ceilings, lifts etc. should be such as to create a sense of space beyond its actual dimensions.

6. Furniture Requirements The functional requirement of a room or an apartment governs the furniture requirements. This is an important consideration in planning of buildings other than residential in particular and residential in general. 7. Sanitation Sanitation consists of providing ample light, ventilation, facilities for cleaning and sanitary conveniences. 8. Flexibility Flexibility means planning a room or rooms in such a way which, though originally designed for a specific purpose, may be used to serve other overlapping purposes also, as and when desired.

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9. Circulation Circulation means internal thoroughfares or the movement space provided on the same floor either between the rooms or within the room called horizontal circulation and between the different floors through stairs or lifts called vertical circulation. Passages, corridors, halls and lobbies serve the purpose of horizontal circulation. The following points should be considered in planning of a building: i) The links between entrances, passages and stair cases should be planned in a proper relation. ii) All passages in a building should be straight, short, sufficiently lighted and well ventilated. 10. Elegance Elegance is the effect produced by the elevation and general layout of the plan. Elevation should be impressive and

should

developed

simultaneously.

together

with

the

plan

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11. Economy: A structure designed for a good strength and solid character may be costly in its initial cost but may prove cheaper in the long run as it saves maintenance costs. 12. Practical Considerations i)

Strength and stability of structure, coupled with conveneience and comfort, should occupy the first place of importance in planning.

ii)

Simplicity and effect of strength lend a lasting beauty and mobility to a building.

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2 Topic

General Principles of Site Selection Site selection has an important bearing on planning and designing of buildings. Generally, therefore, an architect has either to make a choice of suitable site or to plan his building structure to suit the available site. The following general principles or factors should be borne well in mind in the selection of a site: a) The site should be selected keeping in view the general scope or the purpose of building and on the basis of extent of privacy desired. b) The site should be situated in locality which is already fully developed or which is fast developing. To secure happy living conditions, generally such a neighbourhood is preferred whether neighbours belong to an equal status in society and who should be social and friendly. 12

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c) The plot should be in a locality where

The various

facilities like a) Community services such as police and fire protection, clearing; b) Utility services such as water supply, gas, electricity and drainage; c) amenities such

schools,

hospitals,

libraries,

recreation

telephone etc; d) shopping facilities and e) means of transport are available. d) A site which comes within the limits of an area where the by-laws of the local authority enforce restrictions regarding proportions of plots to be built up, vacant spaces to be left in front and sides, heights of buildings etc., should be preferred. e) Area of the plot of land should be such that the house constructed, keeping in view the restrictions of the local authority, would meet the requirements of the owner, preferably with possibilities of future extensions. The site should not be irregular in shape or having any sharp corners.

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f) The site should be situated on an elevated place and also leveled with uniform slopes from one end to the other so as to provide good and quick drainage of rain water. g) The soil surface of the site should be good enough to provide

economical

foundations

for

the

intended

building without causing any problems. Generally, for most satisfactory constructions, the site should have rock, sand or firm soil below 60 to 120 cm layer of light soil or even black cotton soil. h) The situation of the site should be such as to ensure unobstructed natural light and air. i) The site should be available in a locality where natural beauty and man-made environments create healthy living and working conditions. j) The site should have a good landscape but away from quarries, kilns factories, etc. k) Besides these factors, the legal and financial aspects, which dictate upon ownership rights and the costs, should be given due consideration before the purchase of a plot. 14

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Site Plan The site plan or plot plan is included to locate the area belonging to any building, prepared before constructing the house and should represent the following information: i)

The boundary of the plot, shape of site and exterior house dimensions.

ii)

Setback line at the front, back and sides.

iii)

Any permanent boundaries or marks existing should be indicated on the plan.

iv)

Names and widths of existing streets and roads whether

concrete,

asphalt

etc.

should

indicated. Grade elevation at centre line should also be indicated. v)

Grade elevations at corners of plot and at corners of house should be indicated by means of contour lines.

vi)

Size and location of garage, if detached.

vii)

Number of plot and block , if any and names of adjoining properties.

viii)

Directions of prevailing winds and north line.

ix)

Footpaths, if any, widths and kinds of foot-paths.

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Zoning and building restrictions which may affect the site.

xi)

Size and location details of gas line, underground drainage, water-mains, manholes, ventilating pipes etc. should be indicated.

xii)

Location of fire hydrant should be marked.

Planning Regulations and By-Laws Municipal Requirements in Planning of Buildings When planning cities, towns and municipalities in India, we have to obey certain rules and regulations regarding the minimum size of plot with respect to the width of road in front, spaces to the left around the building for ventilation etc. For the rules applicable to each situation, the regulations published by the concerned authority should be consulted.

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Classification of Buildings According to the National Building Code of India (1970), buildings are classified into 9 groups according to its occupancy as follows: i)

Group A : Residential buildings

ii)

Group B : Educational buildings

iii)

Group C : Institutional buildings

iv)

Group d : Assembly buildings

Group E : Business buildings

vi)

Group F : Mercantile buildings

vii)

Group G : Industrial buildings

viii)

Group H : Storage buildings

ix)

Group I : Hazardous buildings

Some Definitions Building Line Building line is the line corresponding to the plinth of a building which adjoins the street or extension of the street. According to regulations, there is a minimum distance we have to keep between this line and the adjoining street line.

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Building Height Generally speaking, the height of a building is its height above the ground level. However, in some municipal regulations, it is measured with reference to the middle of the adjacent road level. In cases of buildings abutting a street, heights are measured differently for a flat roof construction from that of a sloped roof construction. Carpet Area This is the usable area of a building. In a residential house, it will exclude verandah, bathrooms, staircases etc. The carpet are of an office building can be 60% to 75% of the plinth area and in a residence, it can be as low as 50 to 65% of the plinth area. Plinth Area This is the built up covered area of a building measured at the floor level by taking the external dimensions of the building excluding the plinth offset. It also indicates areas of porches,.balconies etc. 18

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Floor Area It is the plinth area minus the area occupied by walls, door, openings etc Floor Area Ratio (FAR) or Floor Space Index (FSI) It is commonly known as the floor space index (FSI) and is given by FSI of flat = Total covered area (plinth area of all floors)/Total plot area of the building An FSI of 1.5 is nowadays allowed for flats in most cities. Front Setback (FSB), Rear Setback (RSB) and Side Setback (SSB) These are the setbacks specified by the competent authority from the boundaries of the building plot. The minimum front setback (FSB) to be kept when planning a building will be specified with reference to the area where the building is situated and also the width of a road in front. In many old congested commercial areas of cities, it is usually less than that specified for new developing residential areas. 19

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The minimum RSB and SSB are usually specified with reference to the height of the building and the width of the road in front. There are certain rules and regulations laid down by the municipalities

Town

Planning

authorities

urban

improvement boards, in their jurisdiction. These have to be considered by an architect while planning and designing the layout of the buildings. In rural areas, these by-laws are dictated by Revenue authorities. These by-laws and regulations govern the following building aspects 1. Lines of building frontages. 2. Built-up area of buildings 3. Open spaces around buildings and their heights 4. Provisions of size, height and ventilation of rooms and apartments. 5. Water supply and sanitary provisions 6. Structural deisgn or sizes and sections.

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1. Building The building line refers to the line of building frontage i.e., the line upto which the plinth of a building adjoining a street or an extension of street or on a future street may law-fully extend. This line is often known as setback or front building line and is laid down in each case parallel to

the plot

boundaries by the authority, beyond which nothing can be constructed towards the plot boundaries. Certain buildings such as cinemas, business centres, factories etc. which attract large number of vehicles should be further setback a further distance apart from the building line. This line which accounts for this extra margin is known as control line. Sometimes a line is fixed known as the â&#x20AC;&#x153;General Building Lineâ&#x20AC;? and no building or its portion should project beyond this front line. The fixation on building line depends upon the site of the proposed building, keeping in view the present width and widening requirements.

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Type

load

1.National

I N V E S T I G A T I O N

In open and

Ribbon

agricultural

development along

urban area

country

approaches

Actual limits in

Building

Control

Building

Control

Building

Control

line

30m

56m

18m

30m

45m

24m

45m

15m

24m

15m

24m

25m

12m

18m

15m

and State Highways 2.Major District Roads 3.Other District Roads 4.Village Roads

Generally, in urban areas, distance of control line is taken as

one half times that of building line. These distances are measured from the centre of the roadway.

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A minimum distance either from the boundary of the road or centre line is prescribed for the line of building frontage. Table gives the idea of such distances for building and control line specified for different types of areas. N.B. National Building Code specifies a minimum frontage of 6m on any street. Advantages of fixing such building lines are : i) It facilitates future widening of street. i) It keeps away the noise and dust of the street. ii) It prevents the formation of blind corners at the intersection of streets and provides open spaces.

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2. Built-Up (or Covered Area Limitation). The built-up or covered area equals to the plot area minus the area due for open spaces. Floor Area Ratio = (Covered area of all floors/ plot area) x 100 Generally

F.A.R for different occupancies and types of

construction as laid down by the authority based on various factors such as occupancy class; type of construction, width of street fronting the building and traffic load; locality and density, parking facilities and local fire-fighting facilities. The F.A.R values are specified in National Building Code for different occupancies and types of construction. The covered area is governed by F.A.R or F.S.I (Floor space Index). The following

limitations

for

built-up

area

have

been

recommended: i)

In a business area, the covered area shall not exceed 75 percent of the area of the site, provided sufficient space for parking, etc is available on the same site.

ii)

In an industrial area, the built-up or covered area shall not exceed 60 percent of the site area.

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iii)

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In residential areas, the covered areas are indicated in table below. Area of the plot

Maximum permissible built-up area

1.Less than 200 sq.m

60 percent with twostoreyed structure

2. 200 sq.m to 500 sq.m

50 percent of the site.

3. 500sq.m to 1000 sq.m

40 percent of the site

4. More than 1000 sq.m

33.33 percent of 1/3 rd of the site.

4. Open Space Requirements Around Buildings The open spaces should be left inside and around a building, particularly in residential type, to meet the lighting and ventilation requirements of the rooms abutting such open spaces. In case of buildings abutting streets in the front, rear or sides, the open spaces provided shall serve the purpose of future widening of such streets. All such open spaces whether interior or exterior shall be kept free from any erection thereon and shall be open to sky and no cornice, roof or weather shade more than 0.75m wide shall overhang or 25

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project over such open spaces. The National Building Code recommends the open space requirements for varying heights of the buildings. Provisions to Size, Height and Ventilation of Rooms and Apartments A) Size B) From the view of health and ventilation absolute minimum areas for individual rooms and Apartments have been laid down by NBC as follows: a) Habitable Rooms i)

If there is only one room, then minimum area = 9.5 sq.m with minimum width of room = 2.4 m

ii)

If there are two rooms, the minimum area for one room = 9.5 sq.m and area for other room shall be equal to 7.5 sq.m with a minimum width of 2.4m.

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b) Kitchen i)

For kitchen-cum-store, the minimum area = 5.5 sq.m with a minimum width of kitchen = 1.8 m

ii)

For kitchen having separate store, the minimum area for kitchen = 4.5 sq.m

iii)

For kitchen cum dining room, the minimum area = 9.5 sq.m with a minimum width of 2.4m.

c) Bath Rooms and Water-Closet i)

For bathroom, minimum size = 1.5 m x 1.2 m or area = 1.8 sq.m

ii)

For combined bath room and water closet Minimum floor area = 2.8 sq.m with a minimum width of 1.2m

iii)

For water closet, minimum floor area = 1.1 sq.m

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d) Height of Buildings and Rooms The height of the building is decided by two factors, either by the width of street on which it fronts, or the minimum width of rear space. In National Building Code the height and number of storeys are related to Floor Area Ratios and provisions of open spaces. The maximum height generally limited on the basis of the width of the street Table Maximum Height Limits of Buildings Width of street 1. Say â&#x20AC;&#x17E;Wâ&#x20AC;&#x; m

Height of the Building Height = 1.5W + front open space

2. Upto 8m

1.5 times the width of the street

3. 8m to 12m

Not more than 12 m

4. Above 12m

Not more than width of street and in no case more than 24m

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The minimum heights for individual rooms as specified by national Building Code of India are as follows: a) For habitable rooms = 2.75 m b) For habitable rooms if air conditioned = 2.4m c) For habitable rooms under row housing schemes = 2.6 m d) For bath-rooms or water closets = 2.2m C) Lighting and Ventilation of Rooms Rooms should have, for the admission of light and air, one or more apertures such as windows and fan lights, opening directly to the external air or into an open verandah. The area of such window openings exclusive of doors and inclusive of frames is specified as below: a) 1/10th of the floor area for dry hot climate and b) 1/6th of the floor area for wet hot climate. c) The aggregate area of door and windows shall not be less than 1/7th of the room. In addition to the above means of ventilation, every such room shall have ventilation of at least 0.3 sq.m in area near the top of each of two of the walls of such room and

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these ventilators preferably placed opposite to each other for thorough ventilation. 4. Water Supply and Sanitary Conveniences There are certain minimum requirements for water supply and sanitary conveniences which have been prescribed for different types of buildings. Dwellings with individual convenience should have at least the following figments: a) One bath-room with a tap. b) One water-closet c) One Nahani or sink 5. Structural Design or Sizes and Sections Regulations and By-laws also dictate the design stresses, safe loads and bearing capacities etc. which should be considered in the structural design of the building. Design requirements of each component of building should be taken into account.

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Depth of

foundation : The depth of foundation is

determined by the engineering formula but the minimum depth of foundation should be taken as below: a) For single storeyed building = 0.75 to 1.0 m below finished G.L b) For double storeyed buildings = 1.0 to 1.30 below finished G.L ii)

Width of foundation: The thickness of wall in spread foundations is extended by off-sets on each side equal to half-brick width i.e., 5 cm.

iii)

Plinth: This is the portion of building between the surface of the surrounding ground and ground floor level. The plinth level of a building is kept higher than the surrounding ground level such that adequate drainage of the site is assured. In case the plinth height is 20 cm or less, then there is no need to provide any step.

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3 Topic

Site Preparation and Setting Out of Works The first work to be taken before the actual construction of building is checking the dimensions of its boundaries as soon as the site is made available for construction. The vital boundary stones should be in their position and they should be checked with reference to the survey plan. Any difference that may be found regarding front, rear or side dimensions should be reconciled before the work is started. Site Layout The site layout for construction consists of the layouts of access roads, sheds etc. They should be made as follows:

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Access Roads An examination of the site drawing will determine the best layout for access roads. Wherever possible, access to the site for lorries and carts should be the shortest and capable of carrying materials either to a central place or various places of work, as may be desired. Sheds A study of the site drawing will indicate where weatherproof sheds must be erected for storage of materials such as cement, lime and other perishable materials. If the cement stores have to be large, they should be provided with two separate doors, one at each end-one for accepting delivery and the other for issue of materials.

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Site Clearing Site clearing means one or all of the following works: i)

Surface cleaning of grass, trees, anthills, hillocks, etc.

ii)

Cleaning of obstructions which may be above or below the ground level such as old foundations, old drainage works, old septic tanks, pit type latrines and soak pits.

iii)

Cleaning

obstructions

belonging

other

organizations such as drainage or water supply lines, under ground electric or telephone cables. Water Supply for Construction Water is an important building material. If groundwater is available, it should be tested suitably for various uses. Cost water comes to about one or two percent of the cost of civil works. If no groundwater is available and water connection can be obtained from the

municipal authorities, the pipes

should be so laid that they will become part of the permanent water

supply system after completion of the building. If

suitable water has to be brought by lorries, temporary or 34

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permanent water storage tanks should be built for storing sufficient quantity of water for each day and also for discharge

from the supply tanker. If good ground water is

available, sinking of a proper tube well at a suitable place will ensure a good supply of water during construction. Setting Out of Buildings Setting out of building consists of the following two operations: 1. The first operation is the setting out of centre lines. This means establishing the centres of the walls in case of a building with load-bearing walls, or the centre of columns in case of a framed building. 2. The second operation is the setting out of trenches or establishing the excavation lines for proceeding with the excavation.

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Setting Out Centre Lines for Bearing Walls The step-by step procedure for setting out the centre lines of walls can be stated as follows: Step 1: Establish a benchmark from which all levels for the various parts of the building can be established and which will not be disturbed during the building operations. This can be done by driving down a 50mm x 50mm angle 2m long or a steel rod of suitable diameter and 2m length in a previously dug hole so as to project about 10cm from the ground and then concreting

the base to a suitable depth below the ground

level to form a pedestal around it. Step 2: The second step is to mark a baseline from which all dimensions can be measured. The centre line of the longest outer wall of the building is usually taken as the baseline. This is marked with respect to the boundary.

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Step3: The third step is to start from the baseline and mark the corner points of the centre line of walls of a building by means of 50mm x 50mm wooden posts driven firmly to the ground projecting 25 to 50 mm above the ground. A nail or saw cut is placed on the peg to indicate the exact centre point. Setting dimensions are measured with steel tapes and ranging rods between corner posts. It is important that the 90 degree angles at corners are measured by using a builderâ&#x20AC;&#x;s square or by using the 3:4:5 principle or a theodolite. Check whether all the dimensions of the diagonals tally. Step 4: Using the corner points, transfer the centre line to the ground with dry lime by stretching lines between the pegs. 37

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Setting Out of Trenches for Excavation of Bearing Walls Having set up the centre line of corners and checked the dimensions of the building on the ground, we proceed to set out the lines for trenches using the centre line already established. The aim of setting out trenches is to mark the direction and width of excavations to be carried out and also to mark the width of the wall to be built. This is carried out by using pegs or by profile boards. These are masonry pillars or timber boards fixed to the ground some distance away from the excavation on which the excavation and wall boundaries can be marked as shown in Fig. These are set up at least 2m clear of the excavation as shown in Fig. the profile boards may be masonry or timber. The level of the top of the profile boards should be related to the side datum level and fixed at a convenient height if boning rod ( a traveller) is used to control the depth. The centre line, wall width snd trench width are marked on the profile board. The trench width is marked on the ground by lime powder after stretching strings between the profile boards.

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Setting Out a Framed Building For setting out the foundation of a framed building also, we first establish the benchmark and set out the centre line of columns. This is usally carried out by a theodoliet as the column centre lines are usually marked on a grid as shown in Fig. One axis is marked as 1,2,3,4 etc. and the other as A, B, C, D etc.

In this case, we first fix all the peripheral points as shown in Fig. We first fix point 4 with respect to the boundaries of the plot. Then we station a theodolite at point 4 and fix F4, E4 to B4. Turn 90o and fix A3 to A1. Secondly, fix theodolite at A1 40

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and fix B1 to F1. Thirdly, station theodolite at F and fix F2 to F4 (check F4). The internal points can then be fixed easily by theodolite or with stretched lines. Once the grid has been set out, profile boards can be fixed clear of excavation work to carry the excavation of the footings. Methods to Determine Depth of Excavation For construction of foundations and sewer drains, the depth of the base of the excavation is usually set out by means of sight rails and bonding rod ( also called traveller) as shown in Fig. In foundation, construction of the base is to be leveled and for drain construction, it is to be laid to the required gradient. This operation is fully dealt with in surveying. Another method that can be used for leveling of foundation is the use of a water level. A plastic tube is filled with water and is used as a water level. First the required

depth of

excavation is excavated in one place. In all the other sites, the level is determined by means of the water level with reference to this point. One more method is the use of the traditional leveling staff. 41

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The procedure of site preparation and setting out of works is an important item of work as the final dimensions of the building and the sizes of various rooms depend on this operation. It should be carried out with precision by an experienced person.

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4 Topic

Excavation for Foundation Trenches Generally, building work in the form of excavation starts from NE corner of the proposed construction. (NE is given great importance in Indian Vastu sastra). For ordinary buildings with continuous

footing

foundation,

excavation

made

trenches. For a building having a basement, the whole area in the plan of the building has to be excavated to the desired depth. For column footings excavation is made only around the column. The perimeter walls of framed buildings start on grade beams which may be placed much above the foundation level of the column footing. This aspect is very important in saving cost of foundation, especially in buildings where the foundations of the columns are very deep. The minimum depth of a foundation to be adopted is given as the depth not affected by climatic changes ( as in black cotton soil ) and also 43

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in walls, a depth in which rodents will not borrow through and enter the building. The minimum depth of 40 to 60 cm may be adopted for temporary buildings but the minimum value usually adopted is 90 cm to 1m for permanent buildings. It should be noted that in a newly developing areas, the general ground level of buildings should be kept well above ( not less than 45 cm) the proposed road level of the locality. Otherwise, the raising of the road level, which is bound to happen, the surface drainage of the plot will become a problem in subsequent years. Hence in newly developing sites, the decision regarding plinth level should be made on the basis of both safety and economy with reference to the future final ground level. If the excavation has to be made deeper than the minimum required level to meet good soil condition or if the filling of the site around the building has to be very large, we can save brickwork in foundation if fill up part of the depth of the foundation with soil of good bearing capacity like compacted coarse sand or hard material ( such as brick jelly concrete or

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lean concrete) instead of the expensive brickwork being started from a great depth. Procedure for Final Excavation for Foundation Foundation of buildings starts with sandfilling and base centre on top of the sandfill. Generally foundations are first dug to the approximate depth from the ground level and it is then leveled with sandfilling, to take care of variations of level in the excavation. We proceed as follows: Pegs are first driven in less than 3 metre intervals in the excavation already made to the approximate depth required so that the levels of the top of these pegs are at the level of the top of the foundation concrete. This can be accomplished by a level and a leveling staff, or by a water level or a boning rod. When using the boning rod, the end of the rod is set on the peg and the peg is driven until the cross head is on a level with the previously set profiles which is easily seen by sighting across them. Digging is then continued till the bottom of the trench reaches the required depth. The pegs are left to serve as a guide for sandfilling and leveling the base concrete. The levels of the top of the pegs can also be checked with the help of a 3m 45

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straight edge and a spirit level. In order to reduce any inaccuracy to a minimum, the straight edge and level should be reversed each time a fresh level is taken. If the excavation is shallow and the local soil will allow the sides to stand vertical, it can be carried out by trenches with vertical sides. Excavations which are deep or in loose or in weak soils should be made with sloping sides or provided with supports which are called timbering. Excavation work can be carried out by manual labour or machinery. Timbering of Excavation It is very important that when vertical trenches are excavated deeper than what the soil can sustain itself in a vertical cutand in any case, more than 1.5m ( the average height of a person)-the sides must be given some form of temporary support, the extent of which will depend on the depth of the trench, nature of the soil, the season and period for which the work lasts. A number of accidents often occur due to lack of supports of excavations. Traditionally, this has been done in timber and hence the work is called timbering of excavation. Excessive timbering with too many struts between the sides 46

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must also be avoided because of the difficult working condition that can occur in such situations. Deep and large excavations, as those required for basements, require specially 窶電esigned structures for withstanding the earth pressures. The conventional types of timbering for ordinary excavations are the following: 1. Vertical poling boards (open or closed) and horizontal waling pices and strutting for fairly firm soils. (Wood in contact with earth is called poling board). 2. Close horizontal sheeting (poling boards) with vertical waling pieces and strutting for deeper excavations in good soils. The vertical waling pieces can be anchored back to stakes driven into the ground at the back of the excavation. (wood that support poling boards is called waling). 3. Close vertical sheeting introduced in stage is called stage sheeting. Wooden sheet piles with iron shoe and strutting with longitudinal walings for soft

and loose

soils is called the runner system. The soil is to be excavated after driving the piles.

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For ordinary excavations, we use item 1 and 2. For deeper excavations, we use the expensive methods such as stage sheeting, runner system with special wooden planks with steel shoes, steel I soldier beams with horizontal sheetings and steel sheet piling, etc. In case of vertical sheeting (usually applied to deep excavations), it will be useful to extend the sheets above the ground to act also as a protection for people falling into the excavation. Simple types of timbering for moderate depths are indicated in Figures 1 and 2.

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After completing the excavation, the base concrete is laid and the foundation brickwork is constructed. Method of timbering of excavation for basement is shown in fig.

Firm subsoil open vertical timbering

Close vertical

with poling boards and struts

timbering

Close horizontal timbering

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Timbering of Excavation for Basements 1. Vertical face timbering projecting above G.L for safety of workers 2.Horizontal waling 3. Raking struts 4. Binders to struts provide on both sides 5.Vertical punchions at 2m centres 6. Sole plate

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5 Topic

Subsoil Drainage In places having high water table, excavation can be carried out only after dewatering of the foundation. The following methods are adopted: 1. By ditches and sumps for shallow excavations. 2. By simple and multistage well point systems with suction pumps. 3. By shallow wells of 30 cm or more diameter dug below the foundation level and water pumped up by suction pumps. 4. By deep well pumps for deep foundations. 5. By combination of deep wells and well points.

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Dewatering by Ditches and Sumps This method is used mostly in sandy soils which can drain easily. Shallow pits called sumps are dug at locations beside and along the periphery of the excavation as shown in Fig beyond and to a level below the excavations. Water collected in these sumps is constantly pumped out by a sump pump. If there is a tendency for the bottom of the sump to rise up, it is usually weighed down by a reverse filter which consists of successive layers of fine to coarse material with coarse materials on the top. Dewatering by Well Point Systems Single stage and multistage well point systems can be used in difficult situations of ground water. They essentially consist of perforated pipes of 5cm to 8cm diameter with filter points at their ends installed in the ground at 1m to 2m spacing to depths well below the depth of the ground water. These pipes are connected to a common leader connected to a suction pump. The system is kept under suction. The pump will be running all the time round the clock till the work in the

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excavation

I N V E S T I G A T I O N

completed.

This

system

pumps

the

groundwater and keeps the excavation dry as shown in fig.

Dewatering of excavation Drainage of Foul Water below Ground Level All the foul water from various sources to be collected together and disposed of through pipes, laid below the ground level. All rooms such as lavatories, urinals, bath room, kitchen which produce foul water should be planned, if possible to be grouped together and also located in such a way that the walls 54

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of one of these rooms form the periphery of the building. This facilitates an easy collection of the foul water and its conveyance to the house drain which usually runs around the perimeter of the building. The waste water can be collected separately and recycled but the sewage is usually disposed of in street sewers or septic tanks. General Layout of Drainage Systems The house drains (sewers) are usually laid around the perimeter of the building as shown in fig. Inspection chambers (manholes) are provided at the various points of discharge of foul water and also at changes of direction of flow. They are also placed at intermediate points of the drain if they are too long so that the spacing of the inspection chambers is not more than 6m in ordinary buildings ( and not more than 30m in street sewers ). These chambers allow access to the underground drainage system for its inspection as well as clearing of any blockage that may happen. The layout of a house drain is illustrated in Fig.

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6 Topic

Orientation of Buildings Orientation is defined as a method of setting or fixing the direction of the plan of the building in such a way that it derives maximum benefit from the elements of nature such as sun, wind and rain. Therefore once the site is chosen or accepted as available, for the construction of building the Architectâ&#x20AC;&#x;s first aim should be proper orientation prior to planning and design of building. Proper orientation means to utilize the natural gifts in achieving functional comfort inside the buildings though the planned aspect of the building units. The knowledge of orientation is the first prerequisite for good planning,

matter,

whether

depends

circumferences or it is to be decided by choice.

upon

the

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Alternatively, good orientation means proper placement of plan units of the building in relation to the sun, wind, rain, topography and outlook and at the same time providing convenient access both to the street and backyard. Orientation in case of non-square buildings is indicated by the direction of the normal to the long axis. For example, if the length of the building is East-west, its orientation is NorthSouth. It should be remembered that poor orientation of the buildings results in discomfortable conditions inside the building. Though the comfort conditions in such buildings can be created through the mechanical services or means but the operational cost of such devices is very high.

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Factors Affecting Orientation The various factors affecting orientation are as follows: i)

Solar heat gain, for which it is essential to know sunâ&#x20AC;&#x;s path throughout the year and its relative position with respect to the locality.

ii)

Prevalent breeze or wind, i.e., the direction of prevailing wind in summer when it is required and in winter when it is avoided.

iii)

Rainfall, i.e., the direction and intensity of rain.

iv)

Site conditions, i.e., location of site either rural, urban or sub-urban, neighbourhood and surroundings.

In view of the above data, particularly solar and climatic data, the orientation is made based on the need of summer or winter comfort. For places where summer causes greater thermal discomfort, the building as a whole should be oriented to intercept minimum solar radiation in summer and viceversa.

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Orientation Criteria under Indian Conditions Solar heat and humidity are the two controlling factors in the design of a building, particularly of residential type. Indian climate for design purpose is generally classified as either hotarid or hot-humid. Accordingly, India can be divided into zones from climate point of view, viz, i) Hot arid zones and ii) HotHumid zones ( wet zones). Hot Arid Zones Such zones having hot dry climate are found mostly in the interior of the country, away from the coastal belt. Hot dry climate is characterized by the high summer, day, time temperature, low relative humidity and wide range of temperatures between day and night and between summer and winter such climate for example prevails generally in the Northern India and Central India. Therefore, comfort requirements call for the removal of the hot air through walls, roof, doors and windows etc. for orientation in such Hot-Arid zones.

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Hot-Humid Zones Such zones having wet climate are found generally along the coastal belts of India. Here the comfort requirement call for the free movement of air through doors, windows and other openings and at the same time protection from the violent monsoon during four months for orientation in such Hot-Humid zones. Based on available data in India, on prevailing monsoon winds the following orientation have been suggested

by C.B.R.I.,

Roorkee keeping in view the indoor comfort conditions for a dwelling. For Hor-Arid Zones i)

Northern India (like Punjab): Orientation should be done along the direction East and West facing North.

ii)

Central India: Orientation should be done along the direction E-SE and W-NW, facing N-NE.

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iii)

I N V E S T I G A T I O N

Delhi Proper: the best position of a building from orientation point of view is considered when the longer side make an angle of 22 Â˝

on the east-West

line towards East-South. For Hot-Humid Zones or Wet Zones i)

West Coast Regions ( like Mumbai) Orientation should be along the direction S-E and n-W, facing S-W.

ii)

East Coast regions (like Madras). Orientation should be along the direction S-E and N-W facing N-W.

iii)

Bengal : The best

orientation is considered to be

along E and W, facing S. N.B for Hill Stations Like Kashmir, Shimla etc. At hill stations, the winter season causes more discomfort and therefore deserves greater consideration in orientation of buildings. The sole orientation for optimum orientation , therefore is to obtain maximum solar energy on the building in winter. The orientation of the buildings in hill stations should be such that living rooms are open on the South and West.

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Suggestions for Optimum Orientation of Buildings 1. The best orientation from solar point of view requires that the building as a whole should receive the maximum solar radiation in winter and the minimum in summer. For practical evaluation, it is necessary to know the duration of sun shine and hourly solar intensity on the various external surfaces on representative days of the seasons. 2. In hot climate, living rooms on the South and West sides should be protected by verandahs, baths, stores etc. Verandahs should not be provided on the north as far as possible. In long buildings such as hospitals, schools, one of the long sides should face North and south and West protected by verandahs. Drawing offices and dark rooms should be located on the North side. 3. The exposure of the house to the sun is also reduced by shady trees or bushes on the sunny side and by keeping the shorter walls . East and West so that minimum wall area is exposed by the sunrays. 4. In hot and humid areas orientation is governed by the direction of breeze. 63

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5. Generally all the rooms which are occupied in the day time should be placed on North and East side while the bed-rooms are placed in the direction of prevailing wind and at the same time protected by verandahs from the heat of the afternoon sun. Eastern or North eastern corner with cross ventilation is regarded best for kitchen. 6. The judicious location of rooms, specific shape of the building

with

its

external

surfaces

and

proper

ventilation, are imperative with the choice of proper orientation. From ventilation point of view, the height of a house should not be more than twice the width of the street. This is called 63 Â˝ o rule. According to this rule, the height is fixed by two imaginary lines, namely the horizontal line and the diagonal line. The horizontal line is drawn at right angles to the rod, through the centre of the front line. The location of this horizontal line is taken at the higher point along the line.

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The diagonal line is drawn in the direction of the building at 63 Â˝

from where the horizontal line meets the rear

boundary. No part of the building is allowed to project beyond the diagonal line as a rule except that for minor parts such as chimney turrets , etc.

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7 Topic

Electricity Supply in Buildings In India electricity is generated in generating stations as alternating current at 50 cycles per second. It is transmitted to the high voltage national grids by 132 kV high voltage lines to reduce the transmission losses. This electricity is tepped down at electric substations to 11kV, 3 phase, 50 cycles, which is considered economical for local distribution for industries and to the transformers located in various parts of a town. This supply voltage is further reduced by these local transformers and supplied to nearby buildings as single phase or three phase supply (50 cycles per second at 220 volts between the phase and the neutral). The supply is 230 volts between a phase and a neutral and it is 400 volts between the phases. In a single phase supply (used for low loads), we get on live phase wire and a neutral. In a three phase supply, we 66

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get three live phases and a neutral. The supply agency is responsible for the cables upto and including the electric metre. A system of fuse of 30 or 100 amp capacity is installed between the supply inlet and the electric meterboard by the supply agency to isolate the main supply from the building. From the fuses, the wires are led to the metre and from there, to the consumer unit, from where the electricity is distributed to the various parts of the building. It is advisable to put an indicator lamp to the line for each phase, as currents are cut off frequently in India and we can know from the indicator light, when the supplies or phases are cut off. Single and Three - Phase Supply Electricity is brought to a building by underground cables. When the electricity load is small and the total current drawn is less than 30 amp, as when the electricity is used only for lighting, we need only a single phase and a neutral. However, when the load is heavy, as when we use many equipments such as air conditioners, cooking ranges etc., it is necessary to draw current from three live line phases and a neutral. In this way, we distribute the load drawn amongst the three phases. 67

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The electric current metre for the three phase supply is different from that used for the single-phase supply. The metre and the main supply are located at a convenient point where the metre can be conveniently read by the agency. The ratings of all equipments used in residences must be 220 to 230 volts. When we distribute current from a three-phase supply, we should always avoid proximity of cables of two phases as there is a danger of high voltage leaking into the system. We should be very careful in cases where electricity is brought inside a building from the supply line by an overhead insulated cable instead of an underground cable. It is to be first run down the walls of the building by properly insulated cable and then turned up at the entry point of the building to prevent rainwater running along the cable into electric board. This is an important detail to be followed for all lines.

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Consumer Control Unit From the electric metre, the supply goes to the consumer control unit consisting of ELCB ( earth leakage circuit breaker), main isolation switch and distribution points. This unit provides a compact and effective means of controlling and distributing electricity to different parts of the building. Apart from the isolation switch, it contains live phases, neutral and the earth bars as well as individual circuit breakers or old fashion fuses which are put on the live phases. It is important to note that the fuses should always be placed on the live phase and not on the neutral. The modern practice is to use miniature circuit breaker (MCB) instead of the old fuses. The miniature circuit breakers give overload protection only. In addition, earth leakage circuit breaker (ELCB) can also be provided to protect equipments as well as for childrenâ&#x20AC;&#x;s safety. With an ELCB, as soon as there is even a small leakage of current through the earth, the power supply is immediately cut off. The ELCB is introduced before the main switch.

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Wiring of Buildings Types of Wiring The following three types of wiring are used: 1) Sheathed wiring by surface fixing 2) Conduit installation on walls and ceilings 3) Concealed conduit wiring Nowadays concealed PVC conduit wiring is the fashion in most buildings. Conduit wiring can also be made on the surface. It must be remembered that PVC has a high coefficient of expansion. For concealing in concrete, the conduits are laid before concreting and in masonry, a chase is made before the final plastering. In each case, care should be taken to see that there will be no chance for water to enter into these pipes from any place, after they are finally laid . When laying in concrete, it is a good practice to surround the conduit in chicken mesh reinforcement.

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Use of Flexible Cords Flexible cords are used in many electrical appliances such as electric iron, pendent lights, etc. Rating of Strands: Flexible cords are made of small strands and not a single wire. The usual sizes of available flexible cords are as given below: Rating of Strands Cross

Number and

sectional area diameter (mm) of

Current rating (ampere)

mm2

strands

0.5

16/0.20

0.75

24/0.20

1.0

32/0.20

1.25

40/0.20

1.5

30/0.20

2.5

50/0.25

4.0

56/0.30

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From table, it can be observed that for lighting and light 窶電uty appliances the sizes to be used are 0.5 and 0.75 mm2. Typical sizes for various electric ratings can be in terms of cross sectional area. When used for suspended lights always preferable to employ additional steel straining wires, suitably positioned, which will take the weight away from the cords. Further, when a nonmetallic outlet box of thermoplastic material (like PVC) is used for the suspension of a light fitting, care is necessary to ensure that the box temperature does not exceed 60oC. The mass suspended from the box must not exceed 3.2 kg. Colour-Code Indentification for Flexible Cords Appliances with 3-core flexibles are usually identified by Live core

Brown, Red

Neutral core

Blue

Earth

Green/Yellow

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Precautions Necessary in Light Fittings and Pendents Heat-resisting cords are necessary for most connections between the ceiling rose and lamp holder where tungsten filament lamps are to be used due to the abnormally high temperatures generated by these. Light fittings and shades especially if flush-mounted and totally enclosed, require heatresisting insulation suitable for the temperatures likely to be encountered. Here heat-resisting sleeves are also used. They should be fitted over the individual cores of the flexible cables in such a way that the normal insulation of the cores is not relied upon to prevent a short circuit between the conductors or an earth fault. Similar methods should be employed for accessories and appliances which are subject to such heating conditions. Methods of Earthing of Electricity Supply in Buildings Earthing of electrical installations is considered in two wayssystem and equipment earthing. System earthing is the earthing associated with current carrying conductors while equipment earthing is the system used for safety of equipments and prevention of shocks. In buildings, generally, 73

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earthing is used for the second purpose. There are two systems of earthing, namely pipe electrode type and plate electrode type. Pipe Electrodes This is installed by digging an auger hole 30 cm in diameter and 3.75 metres depth. Into this pit is inserted a 38 mm diameter

GI pipe with holes at 7.5 cm from the centres

(staggered on two perpendicular diameters) and 2.5m from its lower end. The pipe is inserted into the hole and the 2.5m depth is packed with alternate layers of charcoal and common salt, each of 50 cm height, with the bottom layer commencing with the charcoal. The rest of 1.25 m depth is filled with earth and the earth wire connection is made on a hole at top of the pipe with GI nuts and washers.

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Plate Earthing In plate earthing, a convenient pit is dug to a depth of 2.5m.earth connections are made to a plate of 60 cm x 60 cm and 6.3 mm thick of galvanized iron and thick charcoal all round, under and above it. Earth connections are made to this plate. The top soil over these earthings may be planted with flower bed which can be watered regularly to keep the soil surrounding the earthing moist.

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8 Topic

Winter Building Each year an increasing number of builders are finding it advantageous to continue building houses throughout the winter. By avoiding shutdowns in cold weather, the builder can retain the services of his key men, schedule completions more closely to market demand and generally improve his ability to make long range plans. Despite this, records show that houses under construction at mid-winter are still less than in the peak building months of the summer. Increased house building activity in winter can be of major benefit to the entire community. It reduces seasonal unemployment. It levels out the demand for mortgage money and provides a steadier flow of new houses to meet market requirements. 76

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By enabling the builder to spread his overhead throughout a twelve month building season instead of the present eight to ten months, such house building can be carried out at only a small increase in cost. Why then are more houses not built in winter? There are probably two main reasons. One is the somewhat natural assumption that all winter construction is difficult and unduly expensive. The other is the belief that successful winter building techniques are only possible on large construction projects. There is perhaps a third reason too, and this is the tendency on the part of some potential home owners to assume that houses built in winter are lower in quality than those built in summer. There is no evidence to support such views. The quality of a house is much more dependent on the contractor is skill and reputation than on the season in which it is built. In fact, if winter building has any effect at all on quality, it will probably be a beneficial one due to the improved job conditions arising from the protection required to combat cold weather. There is mounting evidence from many areas that the techniques of winter building can "be 77

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applied successfully to all types of housing projects and applied with relative ease and economy, providing the work is carefully planned. Indeed, if there is one single item that holds the key to successful winter building, it is this vital question of planning. The object of such planning is to schedule each phase of the job so that the least possible inconvenience is suffered due to the weather. A knowledge of local weather conditions is therefore an essential requirement. A glance at the climate records indicates a wide range of winter conditions. Average minimum temperatures, Wherever possible, the winter-sensitive operations of a project are completed before the onset of intense frost or heavy snowfall. The provision of access roads, drainage, water and sewer services and completion of lot layout are all jobs that can be done much more cheaply in mild weather.

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The ideal situation results when the excavation and foundation are also completed before winter. Where this is not possible, techniques have been developed that permit such work to be continued under winter conditions even when the ground is frozen. 1ÂŁ the frost has only penetrated a few inches, normal excavation methods can be used, often with greater ease than in spring or fall when rain and soft ground may seriously hinder the movement of construction machines. With greater depths of frost, the ground must be broken by construction machines equipped with special ripper or frost breaker attachments. Heating the ground prior to digging, by burning a layer of straw and

coal was once a common method of preparing

frozen soil for excavation. It is still used where some pre-thawing is necessary because of particularly deep frost penetration. Regardless of the method, it is good practice to keep all machinery and personnel off the site until excavation begins, because the snow and

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vegetative cover are excellent insulators and will retard frost penetration. Winter concreting no longer poses serious problems for the builder. It is now common practice to use heated concrete delivered by ready-mix trucks. The fresh concrete requires protection during freezing weather but this need not be elaborate or costly. Footings can often be protected sufficiently with a layer of straw over the newly placed concrete. Basement walls, because of their greater exposed area, generally require some additional heat to keep the concrete warm during the curing period. One technique is to build the wood subfloor first, supporting the joists on the basement wall forms, thus creating an enclosed space which can be heated with portable oil or gas heater s. Another method consists of partial shelters formed with tarpaulins which are simply hung over the wall forms and portable heaters placed beneath. Some builders provide additional protection by using insulated foundation forms which slow down the loss of heat from the warm 80

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concrete. Most people are aware that fresh concrete needs protection from the cold but it is not generally realized that high temperatures in summer can also reduce the quality of concrete work. In fact, well controlled winter concreting provides greater assurance of top quality concrete than when the material is placed in summer and exposed to wide fluctuations in the weather. When concrete blocks are used for the foundation walls, it is usual to require that both block and mortar be warm when laid and that the finished wall be protected from freezing for at least forty-eight

hours. This often necessitates an

enclosure over the entire foundation area. With the foundations in, the main problems of winter building are solved. Work on the superstructure which, in most cases, is of wood frame offers no difficulties in winter. Once the building frame is up and enclosed, most operations can proceed in much the same way as in summer and often under more desirable working conditions, free from the heat and humidity of the summer months. Prefabrication 81

techniques

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can assist the builder to close in the structure quickly but even using conventional methods of on-site construction, an average size house can be closed in within five days. With the protective shell of the house complete, some interior work can begin even before heat is supplied. For those jobs that do require heat, such as plastering and interior painting, the house furnace can be installed on its pad of concrete in the basement or temporarily hung from the floor joists. This also permits the basement concrete slab to be placed under controlled conditions. Some exterior finishes such as wood and prefinished aluminum siding can be installed without protection from the weather. Asphalt shingle roofing can also be placed without protection, providing the shingles, which tend to become brittle in cold weather, are handled carefully. Even steps and sidewalks can be installed if precast concrete units are used. The application of stucco, or painting of exterior woodwork, cannot be done in winter without some form of protection. But since these

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operations do not normally delay occupancy of the house, they are often left until the warmer spring weather. Builders who use brick or stone as a finish generally rely on heated partial shelters to protect their work. These consist of canvas or polyethylene sheets draped from the eaves of the house to the ground and supported on a light framework. Occasionally, builders will plan to take advantage of breaks in the weather and complete their masonry work without protection during periods of relatively mild temperatures. Another interesting development is the use of large shelters which completely enclose the house during construction. These have received more extensive use in areas such as Toronto where solid masonry walls are used instead of wood frame but they have also been tried with varying success in other locations. In Ottawa a few years ago, a local builder constructed a single story house with basement under a plastic enclosure as part of a winter' construction cost study sponsored by the Division of Building Research. Enclosures may be built of various materials and some air inflatable types 83

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have been proposed. Recently, plastic tarpaulins which remain flexible at low temperatures have been introduced. These transparent plastics have the great advantage of trapping solar heat so that the temperature inside such enclosures may be as much as 45째 above outside air temperature during sunny weather. This often provides all the heat that is needed during daylight hours. Any discussion of winter construction must inevitably consider this important matter of cost since in most cases, the decision to build or not to build in winter is based on this factor. It is a difficult question to resolve. Builders have reported cost increases in carrying out construction in winter but these direct costs will vary with each builder's operation, reflecting, often to a major extent, the degree of planning that has gone into the project. There are clear indications that, in a well planned project, these direct costs can be largely offset by the indirect savings resulting from higher productivity and uninterrupted schedules achieved through improved control of the "weather" on the 84

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job. In assessing winter work, it is easy to overlook the time lost on a project in spring and summer caused by rain and the necessity of pumping out excavations, by slowdowns due to high temperatures, and by delays in delivery of materials. The builder would also do well to consider the cost of not building in winter. By completely shutting down the operation when cold weather arrives, he risks the probable loss of key men and the almost certain loss of potential sales in the early spring market. Undoubtedly the strongest recommendation for winter building comes from the increasing number of Canadian house builders who have tried it and found that a year 'round building program makes the most effective use of their men, materials and capital.

P R E L I M I N A R Y

I N V E S T I G A T I O N

END OF THE BOOK

Building Technology â&#x20AC;&#x201C; Preliminary Investigation

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

Semester

BUILDING TECHNOLOGY (UNIT – 2)

Site Selection and Sub Structures

Building Technology â&#x20AC;&#x201C; Site Selection and Sub Structures

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

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Table of Contents Site Selection Foundation Types of Foundations Deep Foundations Machine Foundations

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1 Topic

Site Selection General Principles of Site Selection Site selection has an important bearing on planning and designing of buildings.Generally, therefore, an architect has either to make a choice of suitable site or to plan his building structure to suit the available site. The following general principles or factors should be borne well in mind in the selection of a site: The site should be selected keeping in view the general scope or the purpose of building and on the basis of extent of privacy desired.

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The site should be situated in locality which is already fully developed or which is fast developing. To secure happy living conditions, generally such a neighbourhood is preferred whether neighbours belong to an equal status in society and who should be social and friendly. The plot should be in a locality where the various facilities like a) Community services such as police and fire protection, clearing; b) Utility services such as water supply, gas, electricity and drainage; c) amenities such

schools,

hospitals,

libraries,

recreation

telephone etc; d) shopping facilities and e) means of transport are available. A site which comes within the limits of an area where the by-laws of the local authority enforce restrictions regarding proportions of plots to be built up, vacant spaces to be left in front and sides, heights of buildings etc., should be preferred.

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Area of the plot of land should be such that the house constructed, keeping in view the restrictions of the local authority, would meet the requirements of the owner, preferably with possibilities of future extensions. The site should not be irregular in shape or having any sharp corners. The site should be situated on an elevated place and also leveled with uniform slopes from one end to the other so as to provide good and quick drainage of rain water. The soil surface of the site should be good enough to provide

economical

foundations

for

the

intended

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The site should have a good landscape but away from quarries, kilns factories, etc. Besides these factors, the legal and financial aspects, which dictate upon ownership rights and the costs, should be given due consideration before the purchase of a plot. Types of Building as Per NBC National Code of India (SP:7-1970) defines the building as any structure for whatsoever purpose and of whatsoever materials constructed and every part thereof whether used as human habitation or not and includes foundations , plinth, walls, floors, roofs chimneys, plumbing and building services, fixed platforms, verandah etc., and outdoor display structures. According to National Building Code of India (1970), buildings are classified, based on occupancy as follows: Group A : Residential buildings Group B : Educational buildings Group C : Institutional buildings Group D : Assembly buildings Group E : Business buildings 93

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Group F : Mercantile buildings Group G : Industrial buildings Group H : Storage buildings Group J : Hazardous buildings 1. Group A: Residential Buildings These are those buildings in which sleeping accommodation is provided for normal residential purposes, with or without cooking or dining or both facilities, except any building classified under category C. Buildings of group A are further sub-divided as follows: i) Sub-Division A-1 : Lodging or Rooming Houses These include any building or group of buildings under the same management, in which separate sleeping accommodation for a total of not more than 15 persons, on either transient or permanent basis with or without dining facilities, but without cooking facilities for individuals is provides. A lodging or rooming house is classified as a dwelling in sub-division A-2 if no room in any of its private dwelling units is rented to more than three persons. 94

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ii) Sub-Division A-2 : One or Two Family Private Dwellings These include any private dwelling which is occupied by members of a single family and has a total sleeping accommodation for not more than 20 persons. If rooms in a private dwelling are rented to outsiders, these should be for accommodating more than 3 persons. If sleeping accommodation for more than 20 persons is provided in any one residential building, it should be classified as a building sub-division A-3 or A-4 as the case may be. iii) Sub-Division A-3 : Dormitories These include any building in which group sleeping accommodation is provided, with or without dining facilities, for persons who are not members of the same family, in any one room or series of closely associated rooms under joint occupancy and single management, for example, school and college dormitories , students and other hostels and military barracks.

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iv) Sub-Division A-4 : Apartment Houses (Flats) These include any building or structure in which living quarters are provided for three or more families living independently of each other and with independent cooking facilities, for example, apartment houses, mansions and chawls v) Sub-Division A-5 : Hotels These include any building or group of buildings under single management in which sleeping accommodation , with or without dining facilities is provided for hire to more than 15 persons who are primarily transient, for example hotels, inns, clubs and motels. 2. Group B: Educational Buildings These include any building used for school, college or daycare purposes for more than 8 hours per week involving assembly for instruction, education or recreation and which is not covered by Group D.

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3. Group C: Institutional Buildings These include any building or part thereof, which is used for purposes such as medical or other treatment or care of persons suffering from physical or mental illness, disease or infirmity; care of infants, conval escents or aged persons and for penal or correctional detension in which the liberty of inmates is restricted. Institutional buildings ordinarily provide sleeping accommodation for the occupants. Buildings under group C are further sub-divided as follows i) Sub-Division C-1 : Hospitals and Sanitaria The sub-division includes any building or group of buildings under single management, which is used for housing persons suffering from physical limitations because of health or age, for example, hospitals, infirmaries, sanitaria and clinics.

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ii) Sub-Division C-2: Custodial Institutions This sub-division includes any building or group of buildings under single management, which is used for the custody and care of persons such as children, convalenscents and the aged, for example, homes for the aged

and infirm,

convalescent homes and orphanages. iii) Sub-Division C-3 : Penal Institutions This sub-division includes any building or a group of buildings under single management, which is used for housing persons under restraint, or who are detained for penal or corrective purposes, in which the liberty of the inmates is restricted, for examples, jails, persons, mental hospitals, mental sanitaria and reformatories.

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4. Group D : Assembly Buildings These include any building or part of a building, where group of people congregate or gather for amusement, recreation, social, religious, patriotic, civil travel and similar purpose, for example, theatres, motion picture houses,

assembly

halls,

auditoria,

exhibition

halls,

museums, gymnasiums , dance halls , passenger stations and

terminals

air,

surface

and

marine

public

transportation service, recreation piers and stadia. Buildings under group D are further subdivided as follows: i) Sub-Division D1 This subdivision includes any building primarily meant for theatrical or operatic performances and exhibitions and which has a raised stage, proscenium curtain, fixed or portable scenery loft, lights, motion picture booth, mechanical appliances or other theatrical accessories and equipment and which is provided with fixed seats over 1000 persons.

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ii) Sub-Division D-2 This subdivision includes any building primarily meant for use as described for sub-division D-1 but with fixed seats for less than 1000 persons. iii) Sub-Division D-3 This sub-division includes any building, its lobbies, rooms and other spaces connected thereto, primarily intended for assembly of people, but which has no theatrical stage or theatrical and/or cinematographic accessories and has accommodation for more than 300 persons, for example, dance halls, night clubs, halls for incidental picture shows dramatic, theatrical, educational presentation lectures or other similar purposes, having no theatrical stage except a raised platform and used without permanent seating arrangement,

art

galleries,

museums,

lecture

halls,libraries, passenger terminals and buildings used for educational purposes for less than 8 hours per week.

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iv) Sub-Division D-4 This sub-division includes any building primarily intended for use as described in sub division D-3 but with accommodation for less than 300 persons. v) Sub-Division D-5 This sub-division includes any building meant for outdoor assembly of people not covered by sub-division D-1 to D-4, for example, grand stands, stadia, amusement park structures, reviewing stands and circus tents. 5. Group E : Business Buildings These include any building or part of a building, which is used for the transaction of business (other than that covered by building in Group F); for the keeping of accounts and records and similar purposes; doctors and dentists, service facilities, such as new stands, lunch counters serving less than 100 persons, barber shops and beauty parlours.

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City halls, town halls, court houses and libraries should be classified in this group in so far as the principal function of these is transaction of public business and the keeping of books and records. Minor office occupancy incidental to operation is another type of occupancy should be classified under the relevant group for main occupancy. 6. Group F : Mercantile Buildings These include any building or part of a building, which is used as shops, stores, markets, for display and sale of merchandise, either wholesale or retail. Office, storage and service facilities incidental to the sale of merchandile and located in the same building should be included under this group. Minor merchandising operations in buildings primarily meant for other uses should be covered by group under which the predominant occupancy is classified.

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7. Group G: Industrial Buildings These include any building or part of a building or structure in which products or materials of all kinds and properties are fabricated, assembled or processed, for example, assembly plants, laboratories, dry cleaning plants, power plants, pumping stations , smoke houses, gas plants, refineries, diaries and saw mills. 8. Group H: Storage Buildings These include

any building or part of a building, used

primarily for the storage or sheltering ( including servicing , processing or repairs incidental storage) of goods, wares or merchandise ( except those that involve highly combustible or explosive products or materials), vehicles or animals, for example, warehouses, cold storages, freight depots , transit sheds, store houses, truck and marine terminals garages, hangers ( other than aircraft repair hangers) grain elevators, barns and stables.

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9. Group J : Hazardous Buildings These include any building or part of a building which is used for the storage , handling, manufacture or processing of highly combustible or explosive materials or products which are liable to burn with extreme rapidity and/or which produce poisonous fumes or explosions, for storage , handling , manufacturing or processing which involve highly corrosive , toxic or noxious alkalies, acids or other liquids or chemicals producing flame, fumes and explosive, poisonous irritant or corrosive gases.

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2 Topic

Foundation A foundation is that part of the structure which is in direct contact with the ground. It transfers the load of the structure to the soil below so as to avoid over loading of the soil beneath.

It prevents the differential settlement by evenly

loading the sub-strata. It provides a level surface for building operations. It also increases stability of structure by taking the structure deep into the ground. Every structure consists of two major parts namely foundation and superstructure. Foundation is the lower part of a structure which transmits the live loads, wind pressure and the weight of the superstructure, to the subsoil. Foundations are designed such that the load transmitted through them should not

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produce a stress beyond the safe bearing capacity of the underlying subsoil. Foundation are generally built of bricks, stones, concrete and steel etc.,.The selection of material and type of foundation depends upon the type of structure and the nature of the underlying soil. Objectives of Providing Foundation Following are the main objectives of the foundations: 1. To distribute and transmit the total load coming on the structure to a larger area of underlying soil. 2. To

prevent

excessive

settlement

and

differential

settlement of subsoil. 3. To provide stability to the structure against wind, rain and earthquake forces. 4. To provide a level surface for the construction of superstructure.

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Selection of Site for a Building The following points should be kept in mind while buying an area for a residential building: 1. Residential building should be planned near the places where the basic amenities like water and electricity are already available. 2. The site should be nearer to rail or road (Transport facilities) 3. A flat site is preferred. 4. Proper drainage facilities should be available. 5. The site should have good foundation soil. 6. The site should be far away from industrial area to avoid nuisance due to sound and air pollution. 7. The ground should slope away from the site so that the rain water gets easily drained out.

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Requirements of a Good Foundation Following are the requirements of a good foundation 1. The area of foundation provided should be such that the overloading of the soil beneath is avoided. 2. It should give stability to the supported structure. 3. The load should be evenly distributed to substratum, so that unequal settlement is prevented. 4. Foundation should provide a level surface for building operations. 5. The type of foundation provided should be such that abnormal settlement of foundation is avoided. 6. The foundation should be designed to take up the future extension of the building also.

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Loads on Foundations There are four types of loads transmitted to the foundation: 1. Dead loads 2. Live loads 3. wind loads and 4. Seismic loads 1. Dead Loads It is the sum of self weight of the structure ( i.e., weight of walls, partitions, floors , roofs etc.,), weight of footings,

foundations

and

all

other

permanent

constructions in the building. 2. Live Loads Live load or superimposed load consists of moving or variable loads due to people using the structure, temporary stores and snow loads etc., The value of live loads considered for the design of floors and roofs in the buildings are referred from the IS codes.

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3. Wind Loads Wind force acts horizontally on the exposed vertical surfaces of walls, columns and inclined roof of the structure. This wind force acts as uniform pressure on the component of the structure on which it acts and tends to disturb the stability of the structure. The basic wind pressure is decided by paying due regard to the

meteorological data, local conditions, location of

structure and characteristics and duration of wind. Causes of Failure of Foundations 1. When the pressure intensity at the base of foundation exceeds the safe bearing capacity of the soil, shear failure occurs in the underlying soil. 2. Excavation of soil in the adjacent ground results in movement of the soil which leads to the failure of foundation. 3. Failure takes place in block cotton soils if excessive expansion and shrinkage occurs due to seasonal changes. 110

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4. When the bearing capacity of the is not uniform unequal settlement of supporting soil takes place. 5. Due to earthquakes and heavy rains. 6. Failure

foundation

may

takes

place

due

overturning effect produced by wind forces acting on the structure. 7. Shrinkage of subsoil due to variations in the water table.

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3 Topic

Types of Foundations Foundations are broadly classified into the following two types: 1. Shallow foundation 2. Deep foundation 3. Machine Foundations Shallow or Spread Foundations A Foundation in which the depth is equal to or less than its width is called as shallow foundation. This is the most common type of foundation and can be laid using open excavation by allowing natural slopes on all sides. This type of foundation is practicable for a depth upto 5m and is normally convenient above the water table.

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The base of the structure is enlarged or spread to provide individual support. In a spread foundation the total load transferred to the base of the structure is spread over a larger area. The intensity of load transmitted to the supporting soil is less than its allowable bearing capacity. The various types of spread footings are: i)

Wall footing

ii)

Isolated column footing

iii)

Combined footing

iv)

Grillage foundation

Stepped foundation

vi)

Raft or mat foundation

i) Wall Footing This type of footing is proved along the length of the walls. Wall footings or either simple or stepped in shape. These footings can be either simple or stepped. The base course of these footings can be of concrete or entirely of one material.

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Simple Footings have only one projection outside the width of the wall on its either sides. This type of foundation is provided to the walls carrying light loads. The width of concrete base should be atleast equal to twice the width of wall. The depth of concrete bed is at least equal to the projection. Generally the projection provided in the footing is kept as 150 mm on either side and the concrete mix comprises of cement, sand and aggregates in proportion of 1:3:6 or 1:4:8. Concrete bed is economical where width of foundation is considerably high. Brick footing is generally stepped over a level concrete bed as shown in Figure 2.1. Stepped Footings have more than one step of brick footings. When the width of the foundation is considerably more than the wall width, it is economical to make the brick footing, stepped over a level concrete bed.

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The plain concrete used in footings consists of ordinary Portland cement, sand and stone chips in the mix proportion of 1: 3:6 or 1:4:8. Wall

Wall

G.L

Step

Simple footing

Concrete block

Stepped footing

Figure 2.1 Wall footing

The minimum depth of foundation is obtained using the Rankineâ&#x20AC;&#x2122;s formula as:

p 1 sin Depth of foundation w 1 sin

where p safe bearing capacityof thesoil w unit weight of soil angle of reposeof the soil

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For load bearing walls the minimum depth of footing is limited to 900mm. Width of the footing is obtained by the following expression: Width of foundation

W p

where, W = Total load on the wall per metre length p = Safe bearing capacity of the soil. ii) Isolated Column Footing In framed structures individual isolated column footing is provided for each R.C.C column to safely transfer the load to the soil bed. Isolated column footing may be in the form of masonry or in concrete. Columns carrying light loads have a single spread at its base called footing. But in heavily loaded columns two or three steps called stepped footing are provided at their bases. An offset of atleast 150mm is provided in all sides of the concrete bed. In sloped column footing the width of the footing is increased gradually towards the bottom.

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In brick masonry columns the footing is stepped down in all four sides in regular layers with 50mm offsets.

Elevation

Concrete base

Elevation

column

Plan

Plan Simple footing

Stepped footing

Sloped footing

Figure 2.2 Isolated column footing

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iii) Combined Footing A combined footing supports two or more columns in a row. Generally they are constructed of reinforced concrete. When the space between the two columns is so small that the foundation for individual column overlaps, a common footing

called

combined

footing

provided.

The

construction of combined footing becomes essential when the external column is situated near the boundary line and it is not possible to project the footing symmetrically on both sides of the column. Combined

footings

are

generally

rectangular

trapezoidal shape. Combined footing are proportioned in such a way that the centre of gravity of loads from the two columns coincides with the centre of gravity of the combined footing. Figure 2.3 shows the rectangular and trapezoidal combined footings.

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Columns

Footing

Elevation

Plan

Rectangular combined footing

Trapezoidal combined footing

Figure 2.3 Combined footing

In the design of footings, the footing is assumed to be rigid and resting on a homogeneous soil.

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iv) Grillage Foundation Grillage foundation is used to transfer the heavy structural loads from a single steel column or more than one column to a soil having low bearing capacity. This type of foundation is constructed with rolled steel joists placed in single or double layer. In double tier grillage, the top tier is placed at right angles to the bottom tier. The steel joists are kept in position by pipe separators and nuts. The distance between the flanges of steel joists is generally kept as 2 to 3 times the width of the flange. To protect the steel joists from corrosion, they are completely embedded in concrete. As shown in Figure.2.4. The concrete filling is not supposed to take any load but it keeps the steel joists in position and prevents corrosion.

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Figure 2.4 Grillage foundation

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v) Raft or Mat Foundation When the load coming on the soil is practically uniform and the soil is soft clay or made-up ground with low bearing capacity, raft foundations are found to be suitable. A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the columns. When the allowable soil pressure is low or the structure loads are heavy, the use of spread footings would cover more than half of the building area, and it may prove more economical to use raft-foundation. They are also used where the soil mass contains compressible lenses so that differential settlement would be difficult to control. The raft tends to bridge over the erratic deposits and eliminates the possibility of differential settlement. Raft foundation is also used to reduce settlement above highly compressible soils by making the weight of the structure and raft approximately equal to the weight of soil excavated.

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This type of foundation is constructed of reinforced concrete slab covering the whole area of the bottom of the structure. Steel reinforcing bars are provided in the slab in both the directions and on its both the faces (top and bottom).

In the case of heavily loaded columns, main

beams and secondary beams are constructed monolithically with the raft slab as shown in Figure 2.5

Figure 2.5 Plan of raft foundation

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4 Topic

Deep Foundations These foundations carry loads from a structure through weak compressible soils or fills on to the stronger and less compressible soils or rocks at depth. Foundations with relatively larger depths compared to its width are called as deep foundations. Generally when the depth of the foundation is more than 3m ii is called as deep foundations.

There

are

three

foundations namely, i)

Pile foundation

ii)

Pier foundation and

iii)

Well or caisson foundation

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common types

deep

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i) Pile Foundation Pile is an element of construction used as foundation and it is driven into the ground vertically or with some inclination

safely

transfer

the

load

from

the

superstructure to the soil. Pile foundations are provided under the following specific conditions: a) When a deep bed of sandy soil exists at the site. b) When the bearing capacity of the soil under the soil is very poor. c) When the live load and dead load transferred from the structure is considerably large. d) When the construction of grillage or raft foundation is not economical. e) When the structure is situated near the sea â&#x20AC;&#x201C;shore where the foundation is likely to be affected by the scouring action of water.

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Classification of Piles Piles are classified on the following basis: 1. Based on the mode of load transmission a) End bearing piles b) Friction piles c) Partly bearing piles d) Compaction piles e) Sheet piles f) Under reamed piles 2. Based on the materials used for the piles a) Concrete piles b) Steel piles c) Cast-iron piles d) Timber piles

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1. Classification Based on Mode of Load Transmission a)

End Bearing Piles End bearing piles are driven to a greater depths and made to rest on a hard stratum.This type of piles act as columns or piers and transmit the load on the hard stratum at its bottom end. The loose soil available above the hard stratum is not considered for supporting the load.

Friction Piles Friction piles transfer the super-imposed load to the soft soil

the

skin

friction

developed

between

the

surrounding soil and the periphery of the pile along its length. The depth of a friction pile is such that the frictional resistance developed at the sides of the piles equals the load transferred through the piles.

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Pier cap Pier cap

Side friction

Loose soil

Hard strata (a) End Bearing Pile

(b) Friction Pile Pier cap

Side friction

S I T E

Figure 2. 6 Pile foundations 128

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c) Partly Bearing Piles This type of pile acts partly as an end bearing pile and partly as a friction pile. When the hard stratum is available at reasonable depth below sand, a part of the load is transferred by the friction between the piles and the surrounding soil, while a part of the load is transferred to the end hard stratum. d) Compaction Piles To improve the bearing capacity of loose granular soils, compaction piles can be driven into the ground. e) Sheet Piles Sheet pile consists of thin member of steel or timber. This type of piles are used in pervious cut off to reduce seepage and uplift under hydraulic structures.

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f) Under Reamed Piles Under-reamed piles have one or more enlargements called bulbs in its vertical shaft. These bulbs are known as under-ream. In block cotton soil and other types of soils, buildings often crack due to relative ground movements. This can be prevented by using under-reamed piles.This type of piles are also suitable for tower footings, retaining walls and abutments. In sandy soil with higher water table, it is very difficult to build conventional footings but under reamed piles are found to be suitable.

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2. Classification Based on Materials a) Concrete Piles Concrete piles are further classified as cast in situ concrete piles and precast concrete piles. The factors which govern the choice between different types of piles are as under: 1. Nature of soil at the site 2. Type, size and weight of the structure to be supported. 3. Depth, extent and nature of the strata for supporting the piles. 4. Availability of material for piles 5. Number of piles required 6. Facilities for driving piles 7. Durability required 8. Comparative costs of different types of piles.

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a) Single under-reamed pile

S T R U C T U R E S

b) Double under-reamed pile

Figure 2.7 Details of Under-reamed pile foundations

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Cast - In Situ Piles This type of piles are cast at the site itself. A hole is made in the ground by excavating the soil with auger or by driving a casing. The hole is filled up with cement concrete. The casing may be withdrawn after the hole is made or it may be left in its position. The common types of cast-in situ piles are i) Raymond piles ii) simplex pile and iii) Mono-tube piles. (i) Raymond Pile It consists of a thin corrugated steel shell closed at bottom which is driven into the ground with collapsible steel mandrel or core in it. On reaching the required depth, the mandrel is collapsed and withdrawn. The shell is reinforced with spirally wound hard drawn wire at a suitable pitch. Finally concrete is poured in the shell and finished up. In Raymond step-taper concrete piles the diameter of the piles increases in steps.

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Raymond standard

Raymond step-taper

concrete pile

concrete piles Figure 2.8

(ii) Simplex Pile In this type of pile no casing will be left in the ground. A steel tube fitted with cast-iron shoe is driven into the ground upto the required depth. Concrete is then poured into the tube and then the tube is slowly withdrawn, leaving behind the cast iron shoe.

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Precast Piles Precast concrete piles are manufactured in a factory or at a place away from the construction site and then driven into the ground at the required place. Precast piles are generally available in square, octagonal and round shapes. To take up the handling stresses during transport and driving operations, steel reinforcements are provided in the pre-cast piles. It consists of main bars of 16mm to 30mm diameter and lateral ties of 8mm or 10mm diameter. The size of piles ranges from 300mm to 600mm and length may be of 15m or more. A steel shoe is generally provided at the toe of the pile. Timber Piles In this type, trunks of trees are used as pile with an iron shoe provided at the bottom of the pile and a steel plate fixed at the top. These piles are either circular or square in shape. To avoid buckling, the length of timber pile is limited to 20 times its top width.

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Steel Piles In this type rolled steel sections or fabricated steel sections are used as piles. The common types of steel piles are a) H-beam pile b) Box piles c) Tube piles

Steel piles

Figure 2.9

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Pier Foundation Pier foundation is in the form of a cylindrical column of larger diameter. This type of foundation transmits the load through bearing only. It is generally shallower than the pile foundation. Well or Caisson Foundation Well foundation is a type of under water foundation provided for bridges and ducks. This type of foundation resembles a well in which the load is transmitted through the wall around called steining. The well is constructed and brought to the site. The well is driven gradually by digging the soil from inside. The bottom is plugged with concrete and the hollow portion is filled with sand. The whole well is covered with a well cap and the super structure rests on this well cap.

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Caissons are preferred in sandy soils. The caissons can be divided in the following three groups: i) Box caissons ii) Open caissons or wells and iii) Pneumatic caissons i) Box Caissons A box caisson is a strong water-tight vessel open at the top and close at the bottom. They are generally built of timber, reinforced concrete or steel. This type of caisson is suitable where bearing stratum is available at shallow depth and where loads are not heavy. To place the caisson in position, it is launched and floated to pier site where it is sunk in position. ii) Open Caissons or Wells The open caissons are open both at the top and at the bottom. They are used on sandy or soft bearing stratum liable to scour and where no firm bed is available for large depth below the surface.

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They are generally built of timber, metal, reinforced concrete or masonry. They form the most common types of deep foundations for bridges in India. Well foundations are provided in the following different shapes Circular Twin circular Double D Dumb well Twin hexagonal Twin octagonal Rectangular The choice of a particular shape depends upon the following factors: a) The dimensions of the base of the pier or abutement b) The ease of sinking c) The cost of sinking and shuttering d) The vertical and horizontal forces acting on the well e) The considerations of tilt and shift during sinking

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Figure 2.10 Types of wells

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Well Components and Their Functions Figure illustrates the various components of a well foundation. In brief the purpose of each element is as follows: 1. Cutting edge: It provides a comparatively sharp edge to cut the soil below during sinking operation. It usually consists of a mild steel equal angle of side 150 mm. 2. Curb: it has a two-fold purpose. During sinking it acts as an extension of cutting edge and also provides support to the well steining and bottom plug while after sinking, it transforms the load to the soil below. It is made up of reinforced concrete using controlled concrete of grade M20.

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Figure 2.11 Details of Well Foundation

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3. Steining: It is the main body of the well. It also serves dual purpose. It acts as a cofferdam during sinking and a structural member to transfer load to the soil below afterwards. The steining may consists of brick masonry or reinforced concrete. The thickness of steining should not be less than 450mm nor less than that given by the following equation t

H D 100 10

where, t = minimum concrete steining thickness H = well depth below bed D = external diameter of well K = a constant which is 1.0 for sandy strata. The value of K is 1.1 for soft clay, 1.25 fr hard clay and 1.3 for hard soil with boulder.

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4. Bottom plug: Its main function is to transfer load from the steining to the soil below. 5. Sand filling: It is supposed to afford some relief to the steining by transferring directly a portion of load from well cap to the bottom plug. 6. Top plug: It serves as a shuttering for laying well cap. 7. Reinforcement: It provides requisite strength to the structure during sinking and service. 8. Well cap: It is needed to transfer the loads and moments from pier to trhe well or wells below. The shape of well is similar to that of well with a cantilevering of about 150mm.

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5 Topic

Machine Foundations The design of machine foundations requires a special knowledge on the vibrations due to dynamic loads produced during the operation of machines like turbines, motors, compressors, forge hammers and other machines. Since the dynamic loads cause differential settlement of the soil and foundation, conventional foundations cannot be provided for machines. Usually mass concrete is used for machine foundations. The excessive vibrations produced due to the operation of machines can be eliminated by using heavy foundations. Based on the experience, the suitable weight of foundation is generally recommended by the manufacturers of the machine.

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While designing the machine foundation, the safe bearing pressure of soil is assumed as half of the safe bearing pressure under static loads. General Requirements of Machine Foundations For a machine foundation the following requirements should be satisfied based on design considerations i) The

foundation should

be able

transmit

the

superimposed loads without causing shear or crushing failure. ii) The settlement of the foundation must be within the allowable limits. iii) Resonance should not be produced. To avoid the resonance, the natural frequency of foundation soil must be either too small or too large compared to the operating frequency of the machine. iv) The combined centre of gravity of machine and the foundation should lie in the same vertical line with the centre of gravity of the base plane.

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v) Machine foundations should be isolated from the adjacent building parts by providing expansion joints with suitable damping materials.

Figure 2.12 Foundation resting on elastic support

Figure 2.13 Foundation Resting on Piles 147

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Foundations for Reciprocating Type Machines According to the Indian Standard code of practice ( IS:2974 Part I:1964) the following criteria should be followed in the design of foundations for reciprocating type machines : 1. The depth of the foundation should be such as to rest the foundation on a good bearing strata and to ensure stability against rotation in a vertical plane. 2. The size of the foundation block should be larger than the bed plate of the machine with a minimum clearance of 150mm alround. 3. In all vertical machines, the width of the foundation should be atleast equal to the distance of the centre of gravity of the crank shaft to the bottom of foundations. 4. The combined centre of gravity of machine and the foundation block should be as much below the top of the foundation as possible. 5. Wherever possible, the operating frequency should be lower than the natural frequency of the foundation soil system and the frequency ratio should be less than 0.5.

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6. The permissible amplitude for vertical vibrations should not exceed the limiting amplitude for the machine prescribed by the manufacturer. Design of Foundations for Impact Type Machines In this type of foundation, anvil rests on a foundation block, which is a mass of reinforced concrete. This foundation block rests on the soil or on an elastic mounting such as springs, cork layers etc., The R.C.C trough rests on a sole plate or sometimes supported by group of piles.

An elastic joint is

provided between anvil and the foundation block in order to prevent bouncing of the anvil and large impact stresses.

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Indian standard code (IS:2974:Part II:1996) gives the following criteria for the design of foundations for heavy impact type machines such as drop and forge hammers: 1. The centre of gravity of the anvil and of foundation block, as well as joints at which the resultants of the forces in the elastic joints act coincide with the time of fall of hammer tup. While determining the centre of gravity of the foundation block, the weight of the frame of the tup could also be considered. 2. The foundation block should be made of reinforced concrete and reinforcement should be arranged along the three axes and also diagonally to prevent shear as shown in Figure.2.14

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Figure 2.14 Typical reinforcement details 3. The stresses developed at the time of impact in the foundation base should be within 0.8 times the allowable static stresses. 4. The maximum vertical vibrational amplitude of the foundation block should not be more than 1.2mm. In case of foundations on sand below the ground water the permissible amplitude should not be more than 0.8 mm.

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5. For the anvil, the permissible amplitude, which depends upon the weight of the tup should be taken from the table given in the IS code. 6. The weight of anvil may be generally kept as 25 times the weight of the tup. 7. The area of the foundation block should be such that the safe loading intensity of the soil never exceeded during the operation of the hammer.

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END OF THE BOOK

Building Technology â&#x20AC;&#x201C; Site Selection and Sub Structures

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Semester

BUILDING TECHNOLOGY (UNIT – 3)

Super Structure

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Building Technology â&#x20AC;&#x201C; Super Structure

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

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Table of Contents Introduction Brick Masonry Composite Masonry Load Bearing Walls Cavity Walls Partition Walls Reinforced Brick Masonry

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1 Topic

Introduction Masonry is defined as the construction of building units bonded together with mortar. Masonry is normally used for the construction of foundations, walls, columns and other similar structural components of buildings. The basic advantage of masonry construction lies in the fact that it can be used in load-bearing structures. It performs a variety of functions such as i)

Supporting the loads

ii)

Subdividing the space

iii)

Providing thermal and acoustic insulation

iv)

Affording fire and weather protection etc.,

Depending upon the types of building units used, masonry is divided into five types namely stone masonry, brick masonry, 158

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Hollow concrete blocks masonry, reinforced brick masonry and composite masonry. Stone Masonry Depending upon the arrangement of stones in the construction and dressing of stones the stone masonry is classified as (i) Rubble Masonry (ii)Ashlar Masonry i) Rubble Masonry In this masonry, the stones are used either undressed or comparatively roughly dressed. The masonry has wide joints, since stones of irregular sizes are used. The rubble masonry is classified as 1. Random Rubble Masonry a) Un coursed Random Rubble Masonry b) Coursed Random Rubble Masonry 2. Square Rubble Masonry a) Un coursed Rubble Masonry b) Coursed Rubble Masonry 159

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3. Dry Rubble Masonry 4. Polygonal Rubble Masonry 5. Flint Rubble Masonry 1. Random Rubble Masonry Uncoursed Random Rubble Masonry This is the roughest and cheapest type of masonry in which stones are used as it is from quarry without dressing or with minimum dressing and the stones are arranged randomly without forming a course or layer.

Figure 3.1 160

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Here the stones are of not uniform size and shape and hence care must be taken to arrange them in such a way to avoid continuous vertical joints. Larger stones are selected for quoins and jambs to give good appearance and strength. Coursed Random Rubble Masonry In this masonry, the stones are roughly leveled up to form courses varying from 30 to 40 cm thick. But all the courses are not of the same height. For this masonry, first the stones are placed at the edges first and line is stretched in between and the course is maintained using different size of stones as show in Figure.3.2 This masonry is better than uncoursed random rubble masonry.

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Figure 3.2 2. Square Rubble Masonry Square rubble uses stones having straight bed and sides. The stones are usually squared and brought to hammer dressed or straight cut finish.

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Uncoursed Square Rubble Masonry In the uncoursed square rubble, the stones with straight edges and sides are available in different heights. They are arranged on face in several irregular pattern. Good appearance can be achieved by using risers, leveler and sneck in a pattern, having their depths in the ratio of 3:2:1 respectively as shown in figure. Snecks are used to prevent the occurrence of long continuous joints. Hence this type of masonry is also called as Square-snecked rubble masonry.

Figure 3.3

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Coursed Rubble Masonry This type of masonry also uses the same stones as used for uncoursed masonry. But the work is leveled up to courses of varying depth. The courses are of different heights. Each course may consist of quoins, jamb stones, bonders and throughs of the same height, with smaller stones built in between them up to the height of the larger stones to complete the course.

Figure 3.4

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Figure 3.5 3. Polygonal Rubble Masonry In this masonry, the stones are hammer finished on face to an irregular polygonal shape. These stones are bedded in position to show face joints running irregularly in all directions. There are two types of polygonal this type of masonry: first type in which the stones are only roughly shaped, resulting in only rough fitting. Such a work is known as rough picked. In the second type, the faces of stones are more carefully formed so

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that they fit more closely. Such a work is known as closepicked work.

Figure 3.6 4. Flint Rubble Masonry The stones used in this masonry are flints or cobbles, which vary in width and thickness from 7.5 to 15 cm and in length from 15 to 30cm. These are irregularly shaped nodules of silica. The stones are extremely very hard. But they are and therefore may break easily. The face arrangement may be either coursed or uncoursed. 166

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Dry Rubble Masonry In this type of masonry, mortar is not used in the joints. The type of construction is cheaper but requires skilled labours. This may be used for non load bearing walls like compound walls. (i) Ashlar Masonry In this type of construction, no irregular stones are used. The stones used in this masonry are rectangular blocks and dressed finely with chisel. The courses are not necessarily being the same height. It may vary from 25 to 30 cm. Following are the different types of masonry:

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1. Ashlar Fine Masonry In this type of masonry, each stone is cut into uniform size and all the edges are sharp and rectangular in shape. Hence the stone gives perfectly horizontal and vertical joints with adjoining one. This masonry is very expensive.

Figure 3.7

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2. Ashlar Rough Tooled Masonry In this type of ashlar masonry, the beds and sides are finely chisel dressed but the face is made rough by means of tools. A strip about 25mm wide, made by means of chisel is provided around the perimeter and the face is made rough using tools. 3. Ashlar Rock or Quarry Faced In this type of ashlar masonry, a strip about 25mm wide, made by means of chisel is provided around the perimeter as in the case of rough tooled ashlar masonry. But the face is left as it is from quarry.

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4. Ashlar Chamferred Masonry In this type of ashlar masonry, the strip is provided as above. But it is chamfered or beveled at an angle of 45Ë&#x161; for a depth of 25mm using chisel.

Figure 3.8 5. Ashlar Block in Course Masonry This is the combination of rubble masonry and ashlar masonry. In this type, the face work is of ashlar masonry for good appearance and the back side is made rubble for economical consideration.

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Points to be observed in the Construction of Stone Masonry 1. Stones should be strong, tough and hard. 2. Each stone should be well watered before use. 3. Each stone should be laid on their natural bed. 4. Proper bond should be maintained and continuous vertical joints should be avoided. 5. Small stone pieces should be used for facing. 6. The wall should be raised uniformly throughout its length. 7. Stones should be in properly dressed. 8. Mortar for joints should be in proper proportion. 9. After the construction, the whole work should be kept wet at least for 2 to 4 weeks.

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2 Topic

Brick Masonry Introduction Brick masonry is made of brick units bonded together with mortar. Two essential components of brick masonry are therefore 1. Bricks, 2. Mortar Brick masonry is sometimes preferred over other types of masonry due to the following reasons: 1) All the bricks are of uniform size and shape, and hence they can be laid in any definite pattern. 2) Brick units are light in weight and small in size. Hence these can be easily handled by brick layers by hand. 3) Bricks do not need any dressing.

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4) The art of brick laying can be understood very easily, and even unskilled masons can do the brick masonry. Stone masonry construction requires highly skilled masons. 5) Bricks are easily available at all sites, unlike stones which are available only at quarry sites. Due to this, they do

not

require

transportation

from

long

distances. 6) Ornamental work can be easily done with bricks. 7) Light partition walls and filler walls can be easily constructed in brick masonry. Some Definitions Course A courser is a horizontal layer of masonry. Stretcher A stretcher is the longer face of the brick (i.e.19cm X 9cm) as seen in the elevation of the wall. A course of bricks in which all the bricks are laid as stretchers on facing is known as a stretcher course or stretching course. 173

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Header A header is the shorter face of the brick (i.e.9cm X 9cm) as seen in the elevation of the wall. A course of bricks in which all the bricks are laid as headers on the facing is known as header course or heading course. Stretcher Course A course of brick work showing only the stretchers on the exposed face of the wall Header Course A course of brick work showing inly header on the exposed face of the wall Lap Lap is the horizontal distance between the vertical joints of successive brick courses.

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Perpend A perpend is an imaginary vertical line which includes the vertical joint separating two adjoining bricks. Bed Bed is the lower surface (19cm X 9cm) of the brick when laid flat.

Figure 3.9 Elevation of a brick wall 175

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Closer It is a portion of a brick with the cut made longitudinally, and is used to close up bond at the end of the course. A closer helps in preventing the joints of successive sources (higher or lower) to come in a vertical line. Closers may be of various types, defined below. Queen-Closer It is a portion of a brick obtained by cutting a brick lengthwise into two portions. Thus, a queen-closer is a brick which is half as wide as the full brick. This is also known as queen-closerhalf. When a queen-closer is broken into two pieces, it is known as queen-closer-quarter. Such a closer is thus a brick piece which is one-quarter of the brick size. King Closer It is the portion of a brick which is so cut that the width of one its end is half that of a full brick, while the width at the other end is equal to the full width. It is thus obtained by cutting the triangular piece between the centre of one end and the

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centre of the other(lay) side. It has half-header and halfstretcher face. Bat It is the portion of the brick cut across the width. Thus, a bat is smaller in length than the full brick. If the length of the bat is equal to half the length of the original brick, it is known as half bat. A three-quarter bat is the one having its length equal to three-quarters of a full brick. If a bat has its width beveled, it is known is beveled bat. Quoins It is a corner or the external angle on the face side of a wall, generally at right angles

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Figure 3.10 Various forms of Brick portions

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Bonds in Brick Work Rules for Bonding For getting good bond, the following rules should be observed: 1) The bricks should be of uniform size. The length of the brick should be twice its width plus one joint, so that uniform lap is obtained. Good bond is not possible if lap is non-uniform. 2) The amount of lap should be minimum Âź brick along the length of the wall and Â˝ brick across the thickness of the wall. 3) Use of brick bats should be discouraged, except in special locations. 4) In alternate courses, the centre line of header should coincide with the centre line of the stretcher, in the course below or above it. 5) The vertical joints in the alternate courses should be along the same perpend. 6) The stretchers should be used only in the facing; they should not be used in the hearting. Hearting should be done in headers only.

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7) It is preferable to provide every sixth course as a header course on both the sides of the wall. Types of Bonds Following are the types of bonds provided in brick work 1) Stretcher bond, 2) Header bond, 3) English bond, 4) Flemish bond, 5) Facing bond, 6) English cross bond, 7) Brick on edge bond, 8) Dutch bond, 9) Raking bond, 10)

Zigzag bond,

11)

Garden wall bond.

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1. Stretcher Bond Stretcher bond or stretching bond is the one in which all the bricks are laid as stretchers on the faces of walls. The length of the bricks are thus along the direction of the wall. This pattern is used only for those walls which have thickness of half brick (i.e.9cm) such as those used as partition walls, sleeper walls, division walls or chimney stacks. The bond is not possible if the thickness of the wall is more.

Figure 3.11 Stretcher bond

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2. Header Bond Header bond or heading bond is the one in which all the bricks are laid as headers on the faces of walls. The width of the brick are thus along the direction of the wall. The pattern is used only when the thickness of the wall is equal to one brick (i.e. 18cm). The overlap is usually kept equal to half the width of brick (i.e.4 .5 cm). This is achieved by suing three-quarter brick bats in each alternate course as quoin. This bond does not have strength to transmit pressure in the direction of the length of the wall. As such, it is unsuitable for load bearing walls. However, the bond is especially useful for curved brick work where the stretchers, if used, would project beyond the face of the wall and would necessitate inconvenient cutting. This is also used in construction of footings.

Figure 3.12 Header bond 182

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3. English Bond This is the most commonly used bond, for all wall thicknesses. This bond is considered to be the strongest. The bond consists of alternate courses of headers and stretchers. In this bond, the vertical joints of the header courses come over each other; similarly, the vertical joints of the stretcher courses also come over each other. In order to break the vertical joints in successive courses, it is essential to place queen closer after the first header (quoin header) in each heading course. Also, only headers are used for the hearting of thicker walls. Figure shows the general elevations of the English bond. Following are the essential features of English bond. 1) Alternative

courses

will

show

either

headers

stretchers in elevation. 2) Every alternate header comes centrally over the joint between twp stretchers in course below. 3) In the stretcher course, the stretchers have a min. lap of 1/4th their length over headers. 4) There is no continuous vertical joints.

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5) Walls of even multiple of half bricks (i.e. 1 ½ brick thick wall, 2 ½ brick thick wall, 3-bricks thick wall) present the same appearance on both faces. Thus a course showing stretchers on the front face will also show stretchers on the back face.Wall of odd multiple of half bricks (i.e.1 ½ brick thick wall, 2 ½ brick thick wall etc.) will show stretchers on one face and headers on the other face.

Figure 3.13

English bond

6) The hearting (middle portion) of each of the thicker walls consists entirely of headers. 7) At least every alternate transverse joint is continuous from face to face. 184

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8) A header course should never start with queen’s closer, as it will get displaced. The queen’s closer should be placed just next to the quoin header. Queen’s closers are not required in stretcher courses. 9) Since the number of vertical joints in the header course are twice the number of vertical joints in the stretcher course, the joint in the header course are made thinner than the joints in the stretcher course. 4. Flemish Bond In this type of bond, each course is comprised of alternate headers and stretchers. Every alternate course starts with a header at the corner (i.e. quoin header). Quoin closers are placed next to the quoin header in alternate courses to develop the face lap. Every header is centrally supported over the stretcher below it. Flemish bonds are of two types: a) Double Flemish bond, b) Single Flemish bond.

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a) Double Flemish Bond In the double Flemish bond, each course presents the same appearance both in the front face as well as in the back face. Alternate headers and stretcher are laid in each course. Because of this, double Flemish bond presents better appearance than English bond.

Figure 3.14 Flemish bond

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Figure 4.14 shows the general elevation of Flemish bond, for all the wall thicknesses. Figure shows the double Flemish bond in plan, for walls of various thicknesses. Special features of double Flemish bond: 1) Every course consists of headers and stretchers placed alternately. 2) The facing and backing of the wall, in each course, have the same appearance. 3) Quoin closers are used next to quoin headers in every alternate course. 4) In walls having thickness equal to odd multiple of half bricks, half bats and three-quarter bats are amply used. 5) For walls having thickness equal to even multiple of half bricks, no bats are required. A header or stretcher will come out as header or stretcher on the same course in front as well as back faces.

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b) Single Flemish Bond Single Flemish bond is comprised of double Flemish bond facing and English bond backing and hearting in each course. This bond thus uses the strength of the English bond and appearance of Flemish bond. However, this bond can be used for those walls having thickness at least equal to 1 Â˝ brick. Double Flemish bond facing is done with good quality expensive bricks. However, cheaper bricks can be used for backing and hearting. Comparison of English Bond and Flemish Bond English Bond Alternate course of Header and Stretcher Course

Flemish Bond Each Course will consists of alternative Header and Stretcher

For thicker walls, this is For thicker walls, it is more stronger and comparatively weak compact Appearance is not as good Gives a good appearance ad Flemish bond Skilled labours are It requires no skilled labour required Mortar requirement is less Mortar requirement is more 188

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Raking Bond This bond is used in thick walls. In this type of bond, the bonding bricks are kept at an inclination to the direction of the wall. Due to this, the longitudinal stability of thick wall built in English bond is very much increased. This bond is introduced at certain intervals along the height of the wall. Following are special features of raking bond: i.

The bricks are arranged in inclined direction, in the space between the external stretchers of the wall.

ii.

The raking or inclination should be in opposite direction in alternate courses of raking bond.

iii.

Raking bond is not provided in successive courses. It is provided at a regular interval of four to eight courses in the height of a wall.

iv.

The raking courses is generally provided between the two stretcher courses of the wall having thickness equal to even multiple of half-bricks, to make the bond more effective.

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Figure 3.15 Raking bonds Raking bonds are of two types 1) Diagonal Bond In this type of bond, bricks are arranged at 45 0 in such a way that extreme corners of the series remain in contact with the external line of stretchers. Bricks cut to triangular shapes and of suitable sizes are packed in the small triangular spaces at the ends. This bond is best suited for walls which are 2 to 4 bricks thick. The bond is introduced at regular vertical interval, generally at every fifth or seventh course. In every alternate course of the bond, the direction of bricks is reversed.

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2. Herring-Bone Bond This bond is more suitable for walls which are thicker than four bricks thick. Bricks are arranged at 450 in two opposite direction from the centre of the wall thickness, as shown. The bond is introduced in the wall at regular vertical interval. In every alternate course, the directions of bricks are changed. The bond is also used for ornamental finish to the face work, and also for brick flooring. Zig Zag Bond This bond is similar to herring-bone bond, except that the bricks are laid in zig-zag fashion, as shown. This bond is commonly used for making ornamental panels in the bricks floorings. Garden Wall Bonds As the name suggests, this type of bond is used for the construction of garden walls, boundary walls, compound walls, where the thickness of the wall is one brick thick and the height does not exceed two metres.

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This type of bond is not so strong as English bond, but is more attractive. Due to this reason, it is sometimes used in the construction of outer leaves of cavity walls. Garden wall bonds are of three types: 1) Garden wall English bond. 2) Garden wall Flemish bond. 3) Garden wall monk bond. 1. Garden Wall English Bond In this bond, the header course is provided only after three to five stretchers courses. In each header course, a queen closer is placed next to quoin header, to provide necessary lap. In stretcher courses, quoin headers are placed in alternate courses.

Figure 3.16 192

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2. Garden Wall Flemish Bond In this bond, each course contains one header after three to five stretchers continuously placed, throughout the length of the course. Each alternate course contains a three â&#x20AC;&#x201C;fourth brick bat placed next to the quoin header, develop necessary lap, and a header laid over the middle of each central stretcher. This bond is also known as scotch bond or Sussex bond. 3. Garden Wall Monk Bond This is special type of garden-wall Flemish bond in which each course contains one header after two successive stretchers. Every alternate course contains a quoin header followed by a Âž brick bat. Due to this, the header rests over the joints between two successive stretchers.

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Figure 3.17 Points to be observed in the Construction of Brick Masonry 1. Bricks should be soaked in water before use. 2. The beds of courses shall be perpendicular to the line 3. Proper bond should be maintained and continuous vertical joints should be avoided. 4. Use of brick bats should be minimized. 5. The wall should be raised uniformly throughout its length. 6. Mortar for joints should be in proper proportion. 7. The height of masonry constructed in one day should be restricted to 1m. 8. After the construction, the whole work should be kept wet at least for 2 to 4 weeks. 194

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Comparison between Stone Masonry and Brick Masonry Stone Masonry

Brick Masonry

Cost of construction is more

Cost is less

Complicated lifting device is Complicated lifting device is required

not required

Less fire resistance

Brick resist fire better than stone

Only cement mortar can be Any mortar can be used used Dead weight is more. Hence Dead weight is less preferred

under

water

construction Stone Masonry is quite strong

Brick work is comparatively weak

Stones are more water tight

Water absorption is more for bricks

Thick mortar joints

Thin mortar joints

Gives massive appearance

Does

not

give

solid

appearance Skilled labours are required

Skilled labours not required

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3 Topic

Composite Masonry Composite masonry is the one which is constructed out of two or more types of building units of or different types of building materials. The composite masonry may be adopted due to two reasons: (i) Improvement in the appearance of walls, etc., (ii) Use of available materials, to obtain optimum economy. Composite masonry may be of the following types 1. Stone-composite masonry. 2. Brick stone composite masonry. 3. Cement concrete masonry. 4. Hollow clay tile masonry. 5. Reinforced brick masonry. 6. Glass block masonry.

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Stone Composite Masonry Composite stone masonry generally, consists of a combination of ashlar masonry and rubble masonry. Rubble masonry is generally very cheap, while ashlar masonry gives pleasing appearance. Hence rubble masonry is used in backing of the wall while the ashlar masonry is used in the facing, as shown in Figure 3.18.

Figure 3.18

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In order that both the facing and backing of the wall act monolithically, it is essential to observe utmost care during construction. The following points should be specifically attended to: (1) Through stones should be used at regular interval, and in sufficient number. (2) The

backing

and

facing

portions

should

constructed in rich cement mortar. (3) Construction of both the backing and facing should be carried out simultaneously so that proper bond is obtained. (4) If necessary, metal cramps, dowels, lead plugs etc. should be provided between facing and backing. Brick-Stone Composite Masonry Bricks and stones can be simultaneously used in three forms of composite masonry: i.

Brick â&#x20AC;&#x201C; backed ashlar masonry

ii.

Brick â&#x20AC;&#x201C; backed stone slab facing

iii.

Rubble-backed brick masonry.

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Figure 3.19 a) shows brick-backed ashlar masonry. The ashalr may be rough tooled. It is preferable to use the height of ashlar as a multiple of brick thickness plus masonry joints, so that coursed masonry is obtained. Cement mortat should be used for construction. Bricks should laid in proper bond. Alternate courses of ashlar may be headers. Under each projecting course of ashlar, header bricks should be used.

Figure 3.19 Brick stone composite masonry Figure 3.19 b) shows the facing of stone slabs or stone tiles. The backing consists bricks laid in courses with proper bond. This type of construction is quite common, since stone tiles may be of marble stone. If stone slabs are used, they are fine dressed, and are used in big panels. It is preferable to use 199

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metal cramps to connect the facing and backing masonry of the wall.

Figure 3.19c) shows a rubble-backed brick masonry. It is commonly used at locations where rubble stone is available in large quantities, but ashlar is not available. In that case, the facing of the wall may be done in bricks laid in courses. Each alternate brick course consists of quoin header.

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4 Topic

Load Bearing Walls Types of Walls Walls is one of the most essential components of a building. The primary function of a wall is to enclose or divide space of the building to make it more functional and useful. Walls provide support to floors and roofs. Walls should therefore be so designed as to have provision of adequate. i.

Strength and stability

ii.

weather resistance

iii.

Durability

iv.

fire resistance

thermal insulation and

vi.

sound insulation.

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A wall may be defined as a vertical load-bearing member, the width (i.e., length) of which exceeds four times the thickness. In contrast to this a column is an isolated load-bearing member, the width of which does not exceed four times the thickness. Walls may be basically divided into two types: (a) Load-bearing, and

(b) Non-load bearing

Each type may further be divided into external (or enclosing) walls and internal or divide walls. Load-bearing walls are those which are designed to carry super-imposed loads (transferred through roofs etc.), in addition to their own weight (self weight). Non-load-bearing walls carry their own-load only, they generally serve as divide walls or partition walls. The external non-load-bearing wall, commonly related to framed structures is termed as panel wall (Figure 3.20 a).

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Figure 3.20 A partition wall is a thin internal wall which is constructed to divide the space within the building into rooms or areas. It may either be non-load-bearing or load bearing. A loadbearing partition wall is called an internal wall. A party wall is a wall separating adjoining buildings belonging to different owners or occupied by different persons. It may, or may not, be load-bearing. A separating wall is a wall separating different occupancies within the same building.

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A curtain wall is a self-supporting wall carrying no other vertical loads but subject to lateral loads. It may be laterally supported by vertical or horizontal structural members where necessary [Figure 3.20 b)]. Cross-wall construction is a particular form of load-bearing wall construction in which all the loads are carried by internal walls, running at right angles to the length of the building. Load bearing walls nay further be divided into the following types: (a) Solid masonry wall (b) Cavity wall (c) Faced wall (d) Veneered wall. Solid masonry walls are the most commonly used. These walls are built of individual blocks of material, such as bricks, clay or concrete blocks, or stone, usually in horizontal courses, cemented together units throughout its thickness. However, it may have openings for doors, windows etc.

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A cavity wall is a wall comprising two leaves, each leaf being built of structural units and separated by a cavity or is filled with non-load-bearing insulating and water proofing material A faced wall is a wall in which the facing and backing are of two different materials which are bonded together to ensure common action under load (See Figure). A veneered wall is a wall in which the facing is attached to the backing but not so bonded as to result in a common action under load. Design Considerations Load-bearing wall may be subjected to a variety of loads, viz., love loads (super-imposed loads), dead loads, wind pressure, earthquake forces etc. Live loads and dead loads act in vertical direction. When the floor slabs transferring the loads to the wall are not supported through the full width of the wall, the loads act eccentrically, causing moments in the wall.

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Load-bearing walls are structurally efficient when the load is uniformly distributed and when the structure is so planned that eccentricity of loading on the wall is as small as possible. The strength of a wall is measured in terms of its resistance to the stresses set up in it by its stability by its super-imposed loads and by lateral pressure such as wind, etc; its stability by its resistance to overloading by lateral forces and bucking caused by excessive slenderness.

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5 Topic

Cavity Walls A cavity wall or hollow wall is the one which consists of two separate walls, called leaves or skins, with a cavity or gap inbetween. The two leaves of a cavity wall may be of equal thickness if it is a non-load-bearing wall, or the internal leaf may be thicker than the external leaf, to meet the structural requirements. The two portions of the wall may be connected together by metal pins or bonding bricks at suitable interval. Cavity walls are often constructed for giving better thermal insulation to the building. It also prevents the dampness to enter and acts as sound insulation. Thus they are normally the outer walls of the building. The size of cavity varies from 4 to 10 cm. The inner and outer skins should not be less than 10 cm each (half brick).

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Advantages Cavity walls have following advantages over other walls. 1. There is no direct contact between the inner and outer leaves of' the wall (except at the wall ties). Hence the external moisture (dampness) cannot travel inside the building. 2. The cavity between the two leaves is full of air which is bad conductor of heat. Hence transmission of heat from external face to the inside the room is very much reduced. Cavity walls have about 25% greater insulating value than the solid walls. 3. Cavity walls also offer good insulation against sound. 4. The nuisance of efflorescence is also very much reduced. 5. They are cheaper and economical. 6. Loads on foundations are reduced because of lesser solid thickness.

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General Features of Cavity Walls

Figure 3. 21 shows the vertical sections of various types of cavity walls for flat and inclined roofs. In the case of brick cavity wall, each leaf is half' brick thick. Such a wall is capable of taking load of' two storeyed building of the domestic type. However, if heavier loads are to be supported, the thickness of inner leaf' can be increased in the multiple of' half brick thickness. The cavity should neither be less than 40 mm nor more than 100 mm in width. The inner and outer skins are adequately tied together by means of special wall ties placed in suitable arrangement, at the rate of atleast five ties to a square metre of wall area. According to Building Regulations of U.K., the ties must be placed at distances apart not exceeding 900 mm horizontally and 450 mm vertically. The ties are staggered. Ties must be placed at 300 mm vertical intervals at all angles and doors and window jambs to increase stability.

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Figure 3.21 Brick Cavity Walls 210

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Since the cavity separates the two leaves of the wall, to prevent moisture to enter, it is essential to provide a vertical damp proof course at window and door reveals. The damp proof coed should be flexible. Construction of Cavity Wall Generally, the cavity wall is set centrally over the concrete base,

without

any

footings.

According

I.S.

recommendations, the lower portion of the cavity may be filled with lean concrete upto a few centimeters above the existing ground level. The top of the filling should be sloped with weep holes at 1 m intervals along the outer leaf of the wall. The inner leaf' may be of common bricks and the outer leaf with any designed kind of facing bricks or it may also be common bricks finished with rendering. The two leaves should be tie together with wall ties. Bonds for cavity wall construction should consist of stretcher bond for half brick leaves and any ordinary bond, such as English bond or Flemish bond for leaves which are one brick or more in thickness. Where solid walls are joining cavity walls, 211

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bonding of former into the latter should conform to the principle shown in Figure. 3.22. Stretchers in the solid wall should extend half brick into the inner leaf of the cavity wall and closers as shall be used for good bonding

Figure 3.22 Junction between Solid wall and cavity wall Bricks should be laid very carefully to leave the cavity free from mortar droppings. Two leaves of the wall should be raised simultaneously and uniformly. The position of wall ties should be predetermined so as to have uniform spacing preferably in centres. The cavity should be made free from rubbish and mortar droppings by means of a timber batten 25 212

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mm thick and width about 12 mm less than the cavity, resting over the ties.

Figure 3. 23 Cavity Wall construction The battens may be lifted by means of wires or rails attached to the battens, as shown in Figure.3.23. The batten is supported on wall ties and the brick work is carried out on either side of the batten, to the height where next row of wall ties are to be provided. After this, the batten is lifted up, cleaned of mortar droppings and replaced over the next row of wall ties. 213

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6 Topic

Partition Walls A partition wall is a thin internal wall which is constructed to divide the space within the building into rooms or areas. A partition wall may be either non-load-bearing or load-bearing. Generally, partition walls are non-load bearing. A load-bearing partition wall is called an internal wall. For a load-bearing internal wall, strength is an important factor of design; a partition, on the other hand, need only be strong enough to support itself under normal conditions of service. Weather exclusion and thermal insulation do not arise as criteria in the design of internal walls. However, sound insulation is an important requirement. A partition wall, separating two adjoining rooms must often provide a barrier to the passage of sound from one to another. An additional requirement in all partition walls is their capacity to support a surface suitable

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for decoration and which is able to withstand the casual damage by impact to which the occupation of the building is likely to subject, them. On ground floors, partitions rest either on flooring concrete or on beams spanning between the main walls. In multi-storeyed buildings, partitions are supported on concrete beams spanning between columns. The total self weight of partitions may considerably affect the total load carried on the frame work and on the foundations. The lighter the partitions, the lighter and smaller will become the structural elements, and the building as a whole will become more economical. The thickness of partitions will affect the amount of usable floor space available in the building. However, light and thin partitions often raise problems of sound insulation and fire resistance.

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Requirements to Be Fulfilled To summarise, a partition wall should fulfill the following requirements: 1. The partition wall should be strong enough to carry its own load. 2. The partition wall should be strong enough to resist impact to which the occupation of the building is likely to subject them. 3. The partition wall should have the capacity to support suitable decorative surface. 4. A partition wall should be stable and strong enough to support some wall fixtures, wash-basins etc. 5. A partition wall should be as light as possible. 6. A partition wall should be as thin as possible. 7. A partition wall should act as a sound barrier, specially when it divides two rooms. 8. A partition wall should be fire resistant.

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Types of Partition Walls Partition walls are of the following types 1. Brick partitions. 2. Clay block partitions. 3. Concrete partitions. 4. Glass partitions. 5. Metal lath partitions. 6. Asbestos sheet or G.I. sheet 7. Plaster slab partitions. 8. Wood-wood slab partition 9. Timber partitions. Brick Partitions Brick partitions are quite common since they are the cheapest. Brick partitions are of three types: 1. Plain brick partitions 2. Reinforced brick partitions. 3. Brick nogging partitions.

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Plain Brick Partitions Plain brick partitions are usually half brick thick. The bricks are laid as stretchers, in cement mortar. Vertical joints are staggered alternate blocks. The wall is plastered on both the sides. The wall is considerably strong and fire resistant. Reinforced Brick Partitions These are stronger than the ordinary brick partitions, and is used when better longitudinal bond is required, and when the partition wall has to carry other super-imposed loads. The thickness of the wall is kept equal to half 'brick (10 cm). The reinforcement consists of steel meshed strips, called Exmet, made from thin rolled steel plates which are cut and stretched (or expanded) by a machine to a diamond network. Such a strip is known as expanded metal and is provided at every third course. Another form of meshed reinforcement, called Bricktor is made of a number of straight tension wires with binding wires

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Brick Nogging Partitions Brick nogging partition wall consists of brick work (half brick thickness) built up within the frame work of wooden members. The timber frame work consists of (i) sill, (ii) head, (iii) vertical members, called studs, and (iv) horizontal members called nogging pieces. The vertical members or studs are spaced at 4 to 6 times the brick length. The nogging pieces are housed into the studs at vertical interval of 60 to 90 cm. The framework provided stability to the partition against lateral loads and vibrations caused due to opening the adjoining door. The brick work is plastered on both the sides. The bricks are usually laid flat, but they may be laid on edge also. Cement mortar, 1: 3 is used. The surfaces of the timber frame work coming into contact with brick work is coated with coal tar. Clay Block Partition Walls The blocks used for such partition wall are prepared from clay or terra-cotta, and they be either solid or hollow. For light partitions, hollow clay blocks are commonly used. They are good insulators for heat and sound. They are also fire resistant. The hollow clay blocks are usually 30 cm long, 20 cm 219

S U P E R

S T R U C T U R E

high and 5 to 15 cm wide. The blocks are provided with grooves on top, bottom and sides. Grooves provide rigid joints, and serves as key to plaster. The blocks are laid in cement mortar. Concrete Partitions Concrete partitions consists of concrete slabs, plain or reinforced, supported laterally between vertical members. These slabs may be either precast or cast-in-situ. Cast-in-situ concrete partitions are usually 80 to 100 mm thick, cast monolithically with the inter-mediate columns. Such partitions are rigid and stable along both vertical and horizontal directions. However, such partitions require costlier form work. Pre-cast slab units are commonly used for partitions. These slabs may be quite thin (25 mm to 40 mm) and are se-cured to precast posts, as shown in Figure. 10.3 (b). Concrete mix usually adopted is M 15 (1 : 2 : 4). The joints are filled with cement mortar. 220

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Another form of concrete partition is made from precast Tshaped or L-shaped units, as shown in Figure. 10.4. A light weight, hollow partition is obtained, without any necessity of vertical post etc. Cement mortar (1 : 3) is used for jointing. Glass Partitions Glass partition walls are constructed using either glass sheets or hollow blocks. Glass Sheet Partition In this, a wooden frame work is used in which glass sheets are fixed. The wooden frame work consists of a number of' horizontal and vertical posts, suitably spaced, to divide the entire area into a number of panels. The glass sheets are kept in position in the panels either by using timber beadings or by putty which is made of linseed oil and whiting chalk. Such partitions are light weight, vermin-proof, sound-proof and damp-proof. However, ordinary glass is quite weak, and require frequent replacement. Nowadays, strong varieties of glass, such as wired glass, bullet-proof glass and three-ply glass are also available. 221

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Hollow Blocks Hollow glass blocks are translucent units of glass, which are light in weight and are available in different sizes and shapes and thicknesses. They are usually square (14 x 14 cm or 19x19 cm), with a normal thickness of 10 cm. The jointing edges are painted internally and sanded externally to form a key for mortar. The front an back faces may be either decorative or plain. The front and back faces are sometimes fluted. The glass blocks are usually laid in cement-lime mortar (1 : 1 : 4), using fine sand. All joints should be filled carefully. For blocks upto 15 cm in height, expanded metal strip reinforcement is placed in every third or fourth course. If the height of the block is more than 25 cm, the reinforcement is placed in every course. Provision for expansion should be suitably made along the jambs and head of each panel.Another type of Glass bricks walls glass blocks are in the form of glass bricks with joggles and end grooves. Glass blocks or glass bricks walls provide good architectural effected and also admit light. They are sound-proof, fire-proof and heat-proof to some extent.

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7 Topic

Reinforced Brick Masonry Reinforced brick work is the one in which the brick masonry in strengthened by the provision of mild steel flats, hoop iron, expanded mesh or bars. It is adopted or used in the following circumstances: 1. When the brick work has to bear tensile and shear stresses. 2. When it is required to increase the longitudinal bond. 3. When the brick work is supported on soil which is susceptible to large settlement. 4. When the brick work is to resist lateral loads, such as in retaining walls etc. 5. When the brick work is to resist lateral loads, such as in retaining walls etc., 6. When the brick wall is to carry heavy compressive loads.

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7. When the brick work is to be used in seismic areas, since it can also resist lateral loads. Reinforced brick work uses first class bricks with high compressive strength. Dense cement mortar is used to embed the reinforcement. The reinforcing material may be (i) hoop iron, (ii) mild steel bars, (iii) mild steel flats and (iv) expanded mesh. The reinforcement is laid either horizontally or vertically. (a) Horizontal Reinforcement Horizontal reinforcement for wall consists of either (i) wrought iron flat bars, known as hoop iron, or (ii) steel mesh. Figure 3.22 shows the hoop iron reinforcement for a brick wall. Generally, two strips of hoop iron are used per header brick and one hoop iron per stretcher brick i.e., one stand of hoop iron for each half brick thickness of wall. Mild steel flat bars may have width between 22 to 32 mm and thickness equal to 0.25 to 1.6 mm. Protection against rust is provided by dipping the bars in hot tar; these are then at once sanded to increase the adhesion of the mortar. At the ends (quoins), the bars are 224

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beaten flat and then double hooked to bars coming from transverse direction. At the junctions, the bars crossing each other are interlaced and single hooked. Hoop iron is now rarely used because of its higher cost and because of its thickness, unless thicker joints are used.

Figure 3.24 Horizontal reinforcement in walls 225

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Another form of horizontal reinforcement, which is more commonly used, is the provision of steel meshed strips called Exmet, made from their rolled steel plates which are cut and stretched (or expanded) by a machine to diamond network. Such a strip is known as expended metal (Exmet) and is provided at every third course. These strips are available in widths of 65 mm, 178 mm and 230 to 305 mm, with thicknesses of 0.6mm, 0.8 mm and 1 mm. They are supplied in coils of 83 m length. To prevent corrosion, the metal in the coil form is coated with oil and then dipped in asphaltum paint. Cement mortar is first trowelled on the bed and the Exmet is uncoiled and pressed down in the mortar. Another form of meshed reinforcement, called Bricktor, is made of a number of straight tension wires (1.4 mm) interlaced with binding wires (1.1 mm). One such strip is provided for every half-brick thickness of wall.

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Figure 3.25 Reinforced brickwork lintels 227

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Horizontal reinforcement is also used for brick lintels, as shown in Figure 3.24. Generally, mild steel bars (6 mm to 12 mm dia.) are provided through the vertical joint, all along the span of lintel. If the lintel carries heavy loads, resulting in heavy shear force, 6 mm dia. Steel wire stirrups are provided at every 3rd vertical joint, as shown in Figure 3.25. the longitudinal steel bars (main reinforcement) should extend 150 mm beyond the jambs. (b) Vertical Reinforcement Vertical reinforcement, in the form of mild steel bars, is provided in brick columns, brick walls and brick retaining walls. In such a circumstance, special bricks, with one or two holes extending upto the face, are used. Vertical mile-steel bars are then placed in the holes. These bars are anchored by steel plate or wire-tie at some suitable interval. Figure 3.26 shows the details of reinforced rick work piers.

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Figure 3.26 Reinforced brickwork piers

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Brick retaining walls are often reinforced since such a work is cheaper than the reinforced cement concrete, when the height of the wall is upto 3 m. Vertical reinforcing bars are placed vertically near each face, in addition to steel meshed strips at every fourth course. The bricks opposite each bar are purpose made, having a groove. The size of the groove is kept slightly more than the diameter of the bar so that it may be grouted in which mortar, to prevent corrosion. Steel wire ties may be provided at every fourth course. In all types of reinforced brick work, it is essential to embed the steel reinforcement in rich cement mortar (usually 1: 3), with proper cover so that reinforcement is not corroded. Corrosion will result in expansion of the joint and consequent cracking. The bricks should also be of high quality, possessing high compressive strength so that optimum use is made of all the materials (i.e., bricks, mortar and reinforcement).

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Figure 3.27 Reinforced brickwork retaining walls

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END OF THE BOOK

Building Technology â&#x20AC;&#x201C; Super Structure

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

232

Semester

BUILDING TECHNOLOGY (UNIT – 4)

Flooring and Roofing

233

Building Technology â&#x20AC;&#x201C; Flooring and Roofing

Excel Soft Technologies Pvt. Ltd. Mysore, Karnataka (INDIA) Web: www.excelindia.com

234

F L O O R I N G

A N D

R O O F I N G

Table of Contents Flooring Types of Floors (Ground Floors) Construction of Upper Floors Types of Pitched Roofs or Sloping Roofs Shell Structures Roof Covering For Pitched Roofs and Their Selection

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R O O F I N G

1 Topic

Flooring Floors are the horizontal elements of a building structure which divide the building into different levels for the purpose of creating more accommodation within a restricted space one above the other and provide support for the occupants, furniture and equipment of a building. A floor consists of the following two components: i) A Sub-Floor (or Base Course or Sub-Grade). The purpose of this component is to impart strength and stability to support floor covering and all other superimposed loads. Live loads to be considered for design of floors are given to Appendix 9.

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R O O F I N G

ii) Floor Covering (or Paving or Flooring). This is the covering over the sub-floor and is meant to provide a hard, clean, smooth, imp 'ious, durable and attractive surface to the floor. Selection of Floorings for Ground Floors Each type of floor has its own merits and there is not even a single type which can be suitably provided under all circumstances and more so when floors have to serve different purposes in different types of buildings, such as residential, institutional, industrial, assembly, etc. However, the selection of flooring, i.e., floor covering should be made considering the following factors: Initial Cost Appearance Cleanliness Durability Damp-resistance (or Damp-proofing) Sound Insulation (or Noiselessness)* Thermal Insulation

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R O O F I N G

Smoothness Hardness Comfort criteria Fire-resistance* Maintenance considerations. Components and Construction of Ground Floors Since the ground floors directly rest on the ground, hence they do not require the construction of a sub-floor. But, to ensure proper drainage, a floor may consist of a system of drains constructed below it, such that the whole water leads outside the building. However, in normal construction of ground floors, the space above the ground, up to a height of about 25 to 30 cm below the plinth level, is first filled with some inert material to prevent the rise of water into the floor. This porous layer of inert material may be made of materials, such as sand, gravel, crushed stone, cinder, etc. In some cases, the asphalt layers also help in the general drainage of the surface. Over this uniform and even surface or layers, a floor covering or wearing surface or finish is provided.

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Note: A basement floor is similar in construction and otherwise also, to a ground floor, except, its location. Generally, the following materials are used for ground floor construction: (i) Bricks, (ii) Stones, (iii) Wooden Blocks, and (vi) Concrete. The materials usually employed for floor finishes or coverings are as follows: (i) Mud and muram (ii) Stones, (iii) Bricks, (iv) Wood or timber, (v) Concrete, (vi) Mosaic, (vii) Terrazzo, (viii) Asphalt, (ix) Tiles, (x) Rubber, (xi) Linoleum, (xii) Cork, (xiii) Magnesite, (xiv) Glass, (xv) Marble and (xvi) Plastic or P.V.C.

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2 Topic

Types of Floors (Ground Floors) The various types of floorings used in the ground floor constructions, on the basis of materials used in their formation, are designated as below Mud-flooring and muram flooring, Flag-stone flooring or Stone flooring, Brick on edge flooring or Brick flooring, Wood-block flooring or Timber flooring, Cement concrete flooring or Concrete flooring, Mosaic flooring or China mosaic tile floors, Terrazzo flooring or Cast in-situ terrazzo flooring. Granolithic flooring, Tiled flooring, Rubber flooring, Linoleum flooring, 240

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Cork flooring or Cork tiles flooring, Magnesite flooring, Glass flooring, Marble flooring, Plastic or P.V.C. flooring, and Asphalt flooring or Mosaic asphalt flooring. A brief description of above floorings, with regard to their uses, methods of construction and salient features, is given below in serial order. 1. Mud-Flooring and Muram Flooring a) Mud-flooring.

Mud

floors

are

generally

used

for

unimportant buildings, particularly in villages of India. They are cheap, hard, fairly impervious, and easy in construction

and

maintenance.

They

maintain

equable (i.e, uniform) temperature (because of good thermal insulation property) both in the summer and winter seasons and hence are best suited under Indian conditions where extreme temperature variations exist.

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For mud floor construction, a 25 cm thick layer of se.' ted moist earth is spread over the prepared bed; it is then rammed well to get a consolidated thickness of about 15 cm. To prevent the formation of cracks after drying, a small quantity of chopped straw (acts as reinforcement) is mixed with the moist earth before ramming.

The

floors

are

maintained

giving

occasionally (once or twice a week), a wash of cementcow dung (1 cement: 2 or 3 cow dung), which is objectionable from sanitary point of view. b) Muram flooring. Muram or disintegrated rock floors are also used in villages of India and possess the same advantages as mud floors. For muram floor construction, first of all, a hard- bed or hard core or sub-grade is prepared by laying about 25 cm thick layer of hand-packed rubble boulders or broken hard brick-bats and then wetted and rammed hard. Upon this hard-bed, a 15 cm thick layer of muram, with coarser pieces at bottom and finer at the top, is laid. 242

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Over this layer of muram, another 25 mm thick layer of powdery muram is spread. Water should then be sprinkled on the entire surface and rammed well. After ramming, the surface is saturated with water, so that a thin layer or film of about 6 mm of water is formed on the top of the rammed surface. The surface is welltrampled under the feet of work-men and leveled until the cream of muram rises to the top and then left to itself for about a day. The surface is then rammed again with wooden rammers called thapies for about three days. After this, the surface is smeared or rubbed with a thick paste of cow dung, and the floor rammed for two days in the morning. Finally, over the dry hard surface, a thin coat of cement cow dung plaster (1cement: 4 cow dung) is applied evenly and wiped clean immediately by hand. To maintain muram floors in good condition, these are given occasionally (once a week) a wash of cement cow dung plaster (1cement: 2 or 3 cow dung).

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Merits and Demerits of Mud Flooring and Muram Flooring May Be Summarised as Below Merits i) Mud and muram floorings are cheap in initial and maintenance cost. ii) They provide a smooth, hard and fairly impervious surface. iii) They offer good insulation against heat, and hence equal temperature in all seasons can be maintained. iv) They are easy in construction, repair and maintenance. v) They possess sufficiently long life, if properly maintained. Demerits For proper maintenance, the floors are required to be given a wash of cement cow dung plaster once or twice a week which is objectionable from sanitary considerations.

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2. Flag-Stone Flooring This flooring consists of thin slabs of stones laid on concrete bedding. The usual sizes of stone slabs are 30 cm x 30 cm, 45 cm x 45 cm, 60 cm x 60 cm and 45 cm x 60 cm and their thickness varies from 2 cm to 4 cm. The slab stones may be square, rectangular or oblong in shape with squared edges. For the construction of stone flooring (See Figure. 4.1), first of all, after excavating to the required depth, the earthen base is levelled, rammed and watered. On this surface a layer of 10 to 15 cm thick lime concrete is laid and properly rammed. Over this concrete bed or subgrade, well dressed flag stones are laid and fixed with thin layer (2.5 cm) of mortar. When the stone slabs are properly set, mortar in joints is raked out to depth of about 2 cm and flush pointed with concrete mortar (1: 3). A slope of 1 in 40 is generally provided in flag stone flooring for proper drainage.

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During the construction of stone flooring, the following points require due consideration: i. For stone flooring above the damp and black cotton soil, a porous layer of sand or rubble should be provided as a cushion below the base. ii. For laying of stone slabs, the work is started by first laying only two stone slabs from diagonally opposite corners over a mortar layer of 2.5 cm thickness and then intermediate slabs are brought up from both the sides. Moreover, a string is stretched touching the tops of these two diagonal stones at required gradient (1 to 40) to facilitate drainage. All other intermediate stone slabs will be touching the string at their top. iii. All stone slabs used should be hard, durable, tough, and of even and good quality. iv. Normally, the joints width between the individual stone slabs should not exceed 0.5 cm(i.e. Â˝cm).

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v. All stones should be dressed on edges before use. Stones with rough surface should be used on rough works, like godowns, sheds, stores, etc., whereas the stones with polished surfaces are used in superior type of work, such as schools, workshops, hospitals, etc. and that too in a definite pattern.

Figure 4.1 Flag-Stone Flooring

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Merits and Demerits of Flag-Stone Flooring are as Follows Merits i) It provides a hard, durable and wear-resisting floor surface and as such can be used for godowns, stores, workshops, etc. ii) It is easy in construction, repair and maintenance. iii) In places, like Tamil Nadu and Andhra Pradesh, where slab stones are available in abundance, this type of flooring can be used with economically. Demerits (i)

does

not

give

pleasing

and attractive

appearance, so cannot be used in residential buildings or important public buildings. (ii)

Being bad in conductivity, poor in shock-absorbing characteristics and not offering perfectly even surface, the use of this flooring is not comfortable for living purposes.

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3. Brick Flooring This type of flooring is suitable for cheap construction, and for places where heavy articles are to be stored as in case of warehouses, stores and godowns. Brick flooring is commonly used in alluvial places like U.P., Punjab, etc., where stone is scarce and well burnt bricks of good quality are readily available. The brick flooring may be laid with bricks laid flat, or on edge arranged in herring-bone pattern, or set at right angles to the walls. The bricks, whether laid flat or on edge, are set in ordinary mortar and pointed with cement, or set in hydraulic mortar. Brick-on edge is preferred to bricks laid flat, because the former being less liable to crack under pressure than the latter and also having the higher depth gives a greater thickness in the former case to resist the moisture penetration.

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The method of brick flooring construction is as follows (See Figure. 4.2).

Figure 4.2 Brick Flooring First of all, an excavation is made about 40 cm or so below the intended surface or level of the floor depending upon the nature of soil and the type of structure. The earth is then levelled, watered and well rammed, until it becomes dry and hard. On the bed so prepared, the sub-grade should be made with a 25 cm layer of rubble or brick bats, and covered with 10 to 15 cm thick layer of lime concrete or lean cement concrete (1 cement : 3 sand : 6 C.A). Upon the prepared subgrade, the bricks are laid in desired shape (may be in 250

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R O O F I N G

parallel rows or herring-bone pattern) and set in cement or lime mortar. The joints should preferably be 1.5 mm in thickness. The joints, sometimes, may be required to be pointed to have better appearance. For pointing purposes, the mortar is first raked out from the joints to a depth of about 2 cm and then pointed with cement mortar. For brick floorings outside the building, the brick joints are grouted with dry sand and the process is termed as sand grouting. The merits and demerits of using brick flooring are as follows: Merits a. It offers a durable and sufficiently hard floor surface. b. It provides a non-slippery and rice-resistant surface. c. It is cheaper in initial cost as compared to cement concrete, mosaic, terrazzo flooring, etc. d. It is easy in maintenance. Demerits The only drawback of this flooring is that it is absorbent.

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4. Timber Flooring This flooring is not much used for ground floors in India. However, this type of flooring is, sometimes, used for carpentry halls, dancing halls, auditoriums, etc. The use of timber floors is best suited for buildings on hill stations or in localities where the climate is damp. In timber floors, the prevention of dampness is most important and hence all possible measures should be taken to check the dampness from rising above. The entire area of the building below the ground floor of timber is covered with an impervious material in order to prevent dampness. This material may be either cement concrete or asphalt. Generally, a 15 cm layer of concrete known as oversite concrete, is placed all over the bed, and D.P.C. courses are inserted throughout the width of the wall immediately below the wall plate. Timber floors essentially consist of boarding sup-ported on timber joists called bridging joists or floor joists, which are nailed to the wall plates at their ends. In case of large 252

F L O O R I N G

rooms,

A N D

R O O F I N G

where

the

distance

between

the

wall

considerable, inter-mediate walls, called sleeper or dwarf walls, are constructed to support the joists along their length.

Longitudinal

timber

members

called

'sleeper

plates', are fixed on the top of sleeper walls and the timber joists are secured to the sleeper walls by nailing them to the sleeper wall plates.

Figure 4.3 Supported Timber Flooring

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The hollow space between the flooring and the oversite concrete is kept dry and fully ventilated by keeping openings in the main walls above the ground level. Timber floors are elastic in nature and possess enough resistance to wear. Timber flooring can be provided in the following alternative ways: i) Strip-Flooring. This is made up of narrow and thin strips of timber which are jointed to each other by tongue and groove joints (Normally strips, 6 to 10 cm in width and 2 to 2.5 cm in thickness are used). ii) Planked-Flooring. In this type of flooring, wider planks are used and they are also jointed by tongue and groove joints (Normally, plank width is about 20 cm). iii) Wood-Block Flooring (Figure). This consists of short but thicker wood blocks which are laid in suitable designs over a concrete base. The blocks are properly jointed together with the ends of the grains exposed. (Normally, wooden blocks of size varying from 20 x 8 cm to 30 x 8 cm, with thicknesses ranging between 2 to 4 cm, are used) 254

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R O O F I N G

iv) Parquet Flooring. This is similar to block flooring except thin (Max. thickness = 10 mm) blocks are supported on sub-floors. The blocks are laid by means of hot glue in desired pattern and then nailed with panel pins. This flooring of plywood is becoming popular these days. 5. Concrete Flooring This type of flooring is most commonly used these days in residential, commercial, institutional and public buildings of all types. This flooring is also known as Indian Patent stone flooring. The concrete flooring consists of two components. a) a base course on subgrade, and b) a wearing course on floor finish. In floor construction, the floor finish over the base course (or subgrade) may be placed in two ways. Firstly, nonmonolithically, i.e., topping or floor finish is laid after the base has hardened and secondly, monolithically, i.e., topping is laid immediately after laying the base layer while the base is still in plastic state. Generally, the first method of nonmonolithic construction is preferred because monolithic 255

F L O O R I N G

A N D

R O O F I N G

method has got the following draw-backs: a. The floor finish or topping may get damaged by the subsequent building operations. b. There is possibility of developing hair cracks on account of settlements, which may occur in base course after laying. c. The finish once damaged is difficult to repair due to monolithic character of the flooring. d. The construction progress is slow as the finish can only be laid when base course has sufficiently set. However, the monolithic construction has the advantage of smaller thickness due to good bond. It should be noted that good results can be obtained with either method of construction; if the base is properly prepared to secure adequate bond between the wearing course and the base. It is essential to provide a strong base so as to cut a stable foundation for the floor finish.

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The concrete flooring consists of topping of cement concrete 2.5 to 4 cm thick laid on a 10 to 15 cm base of suitable mix of concrete (either lime concrete or cement or cement concrete, 1 : 3 : 6 or 1 : 2 : 4), de-pending upon the loading anticipated and type of earth filling on which the base is placed. The actual construction of concrete flooring consists of the following operations (See Figure. 4.4) i) Ground preparation, ii) Formation of base course on subgrade and laying lime concrete, iii) Laying of the topping concrete, iv) Laying of wearing coat or floor finish, v) Grinding and polishing, and vi) Curing. i) Ground Preparation First of all, the surface of the ground for receiving the floor is levelled, well-watered and rammed.

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ii) Formation of Base Course on Subgrade and Laying Lime Concrete. Upon the above prepared surface of the ground, a 15 cm thick layer of broken stones or hard bricks is evenly spread and consolidated, provided that the ground is made up of loose or soft soil. This sub-base on subgrade so prepared is also called as hard core. Thereafter, a layer of lime concrete (1: 2 : 4), about 15 cm thick is laid on the hard core or directly on the surface of ground if made up of good soil. Necessary slope is given to the surface of lime concrete. To facilitate the washing down of the finished floor; usually a slope 1 in 120 to 1 in 240, is sufficient for inside floors and an outward slope of 1 in 36 to 1 in 40 is recommended for bath-rooms and verandah floors. The lime concrete layer should be watered and well rammed for two days and on the third day the topping concrete should be laid.

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Figure 4.4 Concrete Flooring iii) Laying of the Topping Concrete. On the third day, the surface of lime concrete is first well moistened with great care so that no pools of water are formed. [In case of large floor area, the entire area is divided into small sections in the form of strips, or rectangular, or square panels not exceeding 2.5 m in length or 5 m 2 in area, in order to avoid the formation of cracks due to contraction during setting. The topping concrete, in such cases, is placed in alternate bays or panels formed by dividing the floor with battens of sufficient width (usually equal to the thickness of concrete topping)].

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A small quantity of dry cement is then carefully sprinkled over the surface, and this is swept with a broom and immediately a layer of well-mixed cement concrete having proportions 1:2:4 (1 cement : 2 sand : 4 C.A.), about 2.5 to 4 cm in thickness, is laid evenly above the finished grade. This uniform topping concrete is rammed well or compacted with the aid of wooden rammers or tampers. The surface is then smoothened with a wooden float or power float and finally finished by means of steel trowels. During laying of topping concrete, the following points should be taken care of: a) The thickness of topping concrete varies with the type of floor finish. However, when the topping is laid monolithic with the base, it should not be less than 1.5 cm and when laid non-monolithically, it should either have a minimum 4 cm thickness or combined thickness of topping and underbed should be 4 cm (1.5 cm topping + 2.5 cm underbed monolithic). Moreover, the thickness of concrete topping over a base course other than that of cement concrete such as lime concrete base,

should

atleast 260

whether

placed

F L O O R I N G

A N D

R O O F I N G

monolithically or non-monolithically. b) The concrete may be either machine or hand mixed, but the former being preferred. Care should be taken in mixing the sufficient quantity of water such that slump value does not exceed 4 cm. An excess of water should always be avoided as it reduces the strength and wearing properties of concrete and produces a dusty floor. The concrete floors under heavy loads or those exposed to heat may have the main reinforcement at 2 cm below finished surface. c) In current practice, instead of laying one monolithic layer of topping concrete, it is laid in two layers. First, the lower layer, 2.5 cm thick, of lean concrete (1: 3: 6) is laid and rammed, and then another upper layer of 1.5 cm in thickness of richer concrete (1: 1: 2) is laid after half an hour. The coarse aggregate in rich concrete should be crushed well-graded hard stone below 8 mm size. d) For laying concrete over R.C.C. slab, it is essential to first allow the slab to harden and then cleaned of dust, dirt, etc. to obtain a clean base. 261

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e) For the joints between the panels or sections, or bays of concrete floors, a 1.5 mm hoop steel lining is generally used which is protected by oiling against adhesion with concrete. Sometimes, strips of teak wood, ebonite iron or brass are also introduced between the panels and are oiled or white washed. f) For getting a coloured finish, either the coloured cement (though best but costly) or colouring pigments are used in the topping concrete layer. The following pigments per cubic metre of concrete in topping are specified: Red colour ... 1/10 cu.m red oxide of iron powder (or 10%) Terra-cotta colour ... burnt yellow ochre (nearly 10%) Black colour ... 1/20 cu.m of manganese dioxide (or 5%) Buff-colour ... 1/25 cu.m of yellow ochre (or 4%)

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g) Masons are generally found in the habit of sprinkling or spreading either dry cement or a mixture of cement and sand over the wet surface of concrete, to make the surface dry in a lesser time to facilitate early finish. This must not be permitted as the dry cement forms hair cracks and scales which is chipped off in due course, resulting in poor-floor strength and dusty surface. iv) Laying of Wearing Coat or Floor Finish. After an hour of laying the cement concrete, a finishing surface or wearing coat, about 2 cm thick, is laid on the surface of the former (i.e., concrete surface). The finishes of several types, such as mortar finish, mosaic finish, terrazzo finish and granolithic finish, are used in different thicknesses for different purposes of floors, over the concrete base. The special finishes are discussed in subsequent types of floorings, namely, Mosaic flooring, Terrazzo flooring and Granolithic flooring.

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It should be noted well that all the surfaces of the cement concrete floors are not finished and for common works, the wearing coat is generally omitted. Sometimes, in ordinary type of concrete flooring, a thin coat (1 to 2 cm thick) of cement mortar (1 : 1) is applied over the hard dry concrete base. This is known as mortar finish. If a very hard wearing surface is required, as in case of floors in factories, then 2 to 3 coats of sodium or calcium silicate may be applied, each being placed after the previous coat has thoroughly dried. Cement paints (which are water paints) are available in various colours and are applied over the concrete surfaces for achieving a hard, wear-resisting, damp-resistant and pleasing surface. Generally cement paints are supplied in powder form which is stirred into water just before use.

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v) Grinding and Polishing. A hard wearing surface of attractive appearance can otherwise be produced by grinding and polishing the concrete surface by hand or surface grinding machines. The grinding should be done after 3 to 4 days of laying concrete. This grinding removes laitance or loose material and produces a smooth finish. For grinding with hand, it is first done with a coarse grained carborundum stone and then, after five days, with a finer grained carborundum stone. After each grinding, the floor is washed thoroughly, and all pores and holes are filled with cement mortar of the same materials as the floor surface. Final grinding should, generally, be done after a gap of 10 days. For grinding with a machine, the first cut should not be made till the coloured surfacing layer has been down for 14 days.

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vi) Curing. After the flooring is completed, the whole surface is covered with wet socks or bags or with 5 cm of wet sand and kept wet for atleast 10 days by sprinkling water at suitable intervals. The curing of concrete surface is important as it helps in developing strength, weatherresisting and wear-resisting qualities. Merits and Demerits of Concrete Flooring Merits i) It is non-absorbent and hence offers sufficient resistance to dampness. This is used for water retaining floors as well as stores. ii) It possesses high durability and hence is employed for floors in kitchens, bath-rooms, schools, hospitals, drawing rooms, etc. iii) It provides a smooth, hard, even and pleasing surface. iv) It

easily

cleaned

and

has

proved

overall

economical due to less maintenance cost. v) The concrete being non-combustible material, this flooring offers a fire-resistant floor, required for fire-hazardous buildings. 266

F L O O R I N G

A N D

R O O F I N G

Demerits i) The defects, once developed, in concrete floors whether due to poor workmanship or materials, cannot be easily rectified. ii) The concrete flooring cannot be satisfactorily repaired by patch work. iii) It

does

not

possess

very

satisfactory

insulation

properties against sound and heat. 6. Mosaic Flooring (or China Mosaic Tile Floor). This flooring which consists of tiles available in variety of pattern and colours, is commonly used in operation theatres, temples, bath-rooms and superior type of building floors. For construction of mosaic flooring, first of all, a hard concrete base (as discussed under concrete flooring) is laid. Over this concrete base, while it is still wet, a 2 cm layer of cement mortar (1: 2) is evenly laid. [Sometimes, instead of cement mortar using monolithically, a lime surkhi mortar is first spread to a depth of about 6 cm and levelled; over which a layer of paste or cementing material (2 slaked lime 267

F L O O R I N G

A N D

R O O F I N G

: 1 powdered marble : 1 pozzolana) is laid in thickness not exceeding 3 mm]. Upon the bed of cement mortar (or cementing material), small pieces of broken tiles (of China glazed or of cement) are arranged in definite patterns. After this, cement or coloured cement is sprinkled at the top and surface is rolled by light stone roller (usually, having dia. = 30 cm and length = 45 to 60 cm) till the even surface is attained. This surface is left for 24 hours to dry and then it is rubbed with pumice stone to get a smooth and polished surface. The polished surface is finally allowed to dry for about two weeks before use. 7. Terrazzo Flooring. This is a special type of concrete flooring in which marble chips are used as aggregates, and this concrete on polishing with carborundum stone presents a smooth surface. Any desired colour can be obtained by using marble chips of different shades and sizes and also by using different colours of cement. The aggregate shades are exposed by grinding the surface. Normally, terrazzo mix having proportions 1 : 2 to 3 (1 cement : 2 to 3 marble chips) depending upon the size of marble chips, is used. 268

F L O O R I N G

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R O O F I N G

The terrazzo flooring is becoming very popular these days for providing floor finishes in banks, hotels, office buildings and other public or social buildings, on account of its excellent wear-resisting properties and decorative effects. Terrazzo finish is atleast 10 mm thick and comprises a mixture of desired cement (i.e., coloured cement), marble powder and coarse aggregate, such as chippings of marble, quartzite, pearl, glass, etc., of selected colours and of sizes graded from 2 mm to 8 mm. Sometimes, larger particles up to 10 mm are used. This terrazzo finish is installed over the concrete base course (as discussed under concrete flooring) in following two ways or methods: In one method, the cement concrete base is covered uniformly by a 6 mm sand cushion, over which a tar paper is placed. On this paper, a layer of rich mortar (1 : 3) about 30 mm thick is deposited.

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F L O O R I N G

A N D

another

R O O F I N G

method,

terrazzo

finish

applied

monolithically. First of all, a thin coat of cement is spread over the wet concrete base. This layer is then swept with a broom and a layer of cement mortar (1 cement: 2 sand) 12 mm thick is evenly spread immediately over it. The metal dividing strips, about 30 mm in width and 1.3 mm in thickness, are inserted on edge in the mortar bed (prepared by either of the above methods) in desired patterns before it hardens. When the mortar bed has sufficiently hardened, a terrazzo mixture (1 cement : 3 marble chips), 6 to 12 mm thick, depending upon the size of chips, with water just sufficient to make a workable mix for the mosaic finish, is applied. This terrazzo layer is then rolled lengthwise as well as crosswise. About 85% of the marble or aggregate should be exposed over the finished surface and to achieve this, it may be necessary to add additional chips during the rolling process.

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After curing for several days, the surface is care-fully polished by means of a grinding machine fitted with carborundum grinding stone disc. During the process of grinding, the surface is kept wet. Holes or pores, if any, are filled with a thin grout of cement paste, having same tint as the terrazzo finish. After this, the surface is again cured for few days and finally ground by grinding machine fitted with a finer carborundum stone disc. Finally, the whole surface is washed with a weak solution of soft soap in warm water. The surface, so produced, is pleasing in appearance and sanitary in nature. 8. Granolithic Flooring. This flooring is a finishing coat over the concrete surface, which is used to provide a hardwearing surface for the floors in offices and public buildings like schools, hospitals, banks, etc. Granolithic finish is economically employed for those floors which are required to resist heavy wear and for which attractive appearance is not required.

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Granolithic flooring consists of rich concrete made with very hard and tough quality coarse aggregate, such as granite so that the surface will have the maximum resistance to abrasive action. The thickness of granolithic finish should not be less than 25 mm when laid monolithically with the concrete base and not less than 35 mm when laid over a hardened base (i.e., when laid nonmonolithically). The composition of concrete granolithic finish varies with the type of service a floor is expected to perform. For light duty floors, granolithic concrete consists of mix proportions as 1 : 2 : 3 (1 cement : 2 sand : 3 C.A., C.A. is usually well graded granite but sometimes, C.A. of basalt, lime stone or quartzite may be used. The coarse aggregate is usually graded from 10 mm to 4.75 mm. For heavy duty floors, this finish consists of mix proportions as 1 : 1 : 2 (1 cement : 1 sand : 2 granite chips).

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To improve further the wearing qualities of such a finish, the sand should be replaced by fine aggregate of crushed granite. In case, exceptionally hard surface is required then an abrasive grit may be sprinkled uniformly over the surface during floating operation @ 1.6 to 2.2 kg/sq. m. The surface is finally smoothened by means of steel trowel. 9. Tiled Flooring. Tiles, either of day (pottery) or cement (concrete) or terrazzo are manufactured in square, hexagonal and various other shapes, sizes and thicknesses these days. These tiles are used commonly for flooring purposes in residential, high class hotels, offices and other public buildings, etc., where floors are required to be installed in shorter time with pleasing appearance and good durability. The method of laying a tiled flooring is similar to that used for stone flooring, but more care and skills are required in the execution of this type of flooring.

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For construction of tiled flooring, first of all, the ground for receiving the floor is leveled, well watered and rammed, and on this a sub-grade of lime concrete (usually 15 cm thick) or of R.C.C. (if heavy loads and heat effects are expected) is made. Over the sub-grade thus prepared, a thin layer (25 mm thick) of lime sand mortar with mix proportions 1 : 3 (1 lime : 3 sand) or 1 : 1 cement mortar, is laid to serve as a bedding mortar for receiving tiles. The bedding mortar is allowed to harden for few hours and then a neat cement slurry is spread over the surface. At this stage, readymade tiles are laid flat on this surface, with a thin paste of cement applied on their sides. Great care is exercised to see that the sides of the tiles have a thin coat of cement mortar over their entire surface for proper adhesion. The joints are made as thin as possible and extra mortar that comes out through the joints to the surface is immediately wiped clean with saw dust.

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A N D

R O O F I N G

After 2 or 3 days, the joints are well rubbed, with a carborundum stone so that slight projections rising above the surface are levelled. The entire surface of the floor is then polished with the aid of soft carborundum stone first and with a pumice stone thereafter. This polishing of surface can be done by rubbing machine or hand. Finally, the surface is washed with a weak solution of soft soap in warm water. N.B. 1. Flooring tiles in India are being manufactured in square sizes of 20 x 20 cm, 25 x 25 cm and 30 x 30 cm at present. Whereas the sizes recommended are 19.85 x 19.85 cm, 24.85 x 24.85 cm and 29.85 x 29.85 cm with the assumed thickness of joints as 1.5 mm. 2. For damp-resisting floors, hollow tiles are sandwiched between the base of cushion below and concrete topping above.

275

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Merits and Demerits of Tiled Flooring Merits i) It provides a non-absorbent, decorative and durable floor surface. ii) It permits quick installation or laying of floors. iii) It is easily repaired in patches. Demerits i) Tiled flooring is generally costly in initial cost as well as in maintenance cost. ii) On becoming wet, it provides a slippery surface. 10.

Rubber Flooring. Rubber floorings are being used to a

large extent in public and industrial buildings because of their good wearing qualities, resiliency (i.e., elasticity) and noise insulation. The flooring material is made up of pure rubber mixed with fillers, such as cotton fibre, granulated cork or asbestos fibre and the desired colouring pigments. This type of flooring is manufactured in the form of sheets or tiles, in a variety of patterns and colours. The thickness of tiles or sheets ranges between 3 to 10 mm. 276

F L O O R I N G

A N D

R O O F I N G

For the construction of rubber flooring, a base of concrete R.C.C. or wood is prepared, with a caution that concrete slab has been water-proofed properly. The rubber tiles are then cemented to the smooth and dry base of concrete or wood by means of a special adhesive. Though rubber flooring is expensive in its initial cost, yet it provides a durable wearing surface. However, oil, grease and gasoline make the floor slippery and it becomes difficult to restore it in good condition. N.B. 1. Rubber sheets are supplied usually, in sizes, 500 x 90 cm, 350 x 90 cm, and 250 x 90 cm. 2.

Rubber tiles are supplied usually, in sizes, 20 x 20 cm,

30 x 30 cm and 45 x 45 cm. 3.

For three sizes given in (1) and (2), the corresponding

thicknesses are specified as 3.2 mm, 4.8 mm and 6.4 mm respectively. Note: For summary of merits and demerits of rubber flooring, please refer Linoleum flooring. 277

F L O O R I N G

11.

A N D

R O O F I N G

Linoleum Flooring. Linoleum is a covering, generally

laid over the wooden or concrete floors of residential and public buildings, in order to improve their durability and appearances but hide the defects. Linoleum flooring, in domestic bath rooms, kitchens, laundries, etc., should be used with care as it is subjected to rotting when kept wet for sufficient time. Linoleum material is lubricated by mixing oxidized linseed oil with gum, resins,, pigments, wood floor, corkdust and other filler materials. It is available in varieties of colours, in both plain and printed rolls. Normally the rolls are supplied in widths of about 2 metres or 4 metres, in varying thicknesses from 2 to 7 mm. Higher thicknesses (between 5 to 7 mm) are used for floors subjected to heavy wear such as floors of cinemas, restaurants, hospitals, etc., whereas lower thicknesses (between 2 to 2.5 mm) are used for floors, such as offices, shops, houses, etc. Linoleum tiles are also available for specific uses.

278

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Linoleum covering over the wooden or concrete base is laid in the following 3 ways or methods: i) In the first method, linoleum coverings (sheets or tiles) are laid loose on dry and smooth sub-floor or base. ii) In the second method, the coverings, laid loose as in method (i), are fixed to the sub-floor by means of a suitable adhesive, in order to have proper bond and high durability. iii) In the third method, the coverings of linoleum over the prepared wooden base (i.e., resin bonded plywood) are nailed down at the ends, in order to prevent tearing of linoleum due to movements of timber base below. N.B. 1. Linoleum coverings, if varnished and waxed, provide surfaces which have longer life with ease in cleaning. 2. Linoleum coverings are used only over effective damp-proof course.

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Merits and Demerits of Linoleum Flooring Merits i) It provides an attractive, resilient, durable and cheap (rather economical) surface. ii) It offers surface which can be easily washed and cleaned. iii) Being moderately warm with cushioning effect, it gives comfortable living and working conditions. iv) It offers adequate insulation against noise and heat. Demerits i) It is subjected to rotting when kept wet for sufficient time and its use is not recommended for basements. ii) It

does

not

offer

resistance

against

fire,

being

combustible in nature. iii) This covering when applied over wooden base may get torn, under excessive sub-floor movements. Note: Merits and demerits, mentioned above for linoleum flooring, hold true for rubber flooring as well as cork flooring also. 280

F L O O R I N G

12.

A N D

R O O F I N G

Cork Flooring. Natural cork is nothing but the outer

bark of the cork oak tree which is used in the flooring after manufacturing it in the form of tiles and carpets like Linoleum flooring, also provides a warm, noiseless, nonslippery and resilient flooring, and possesses good heat insulation qualities. The cork flooring is generally used to obtain a noiseless floor as in case of libraries, churches, radio-broadcasting

studios,

theatres,

hospitals,

art

galleries, schools, etc. This type of flooring should not be used in basements or such other floors which are constantly subjected to dampness. For construction of cork flooring, first of all a sub-floor or base made of 3 : 1 or 4 : 1 sand cement screed finished with a wood float or any other dry level floor (concrete or wooden) is prepared for receiving cork flooring. Over this base, cork tiles or cork carpet are laid in a similar manner as a linoleum covering (Refer 3 ways of laying linoleum covering).

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Regarding cork flooring, the following salient features should be noted: i) Cork tiles are manufactured from high grade cork bar and are available in standard sizes (from 10 cm x 10 cm to 30 cm x 90 cm) with varying shades in different thicknesses (from 5 mm to 15 mm). ii) Cork tiles are usually made in two qualities, ordinary and heavy density. The heavy density tiles are used when heavy wear is expected, like floors of theatres, schools, art galleries, etc. Tile joints are either tongue and groove type or butt types. iii) Cork carpet is prepared by heating granules of cork with linseed oil, etc. and then compressing it by rolling on canvass. The cork carpet, being more absorbent, is difficult to maintain it clean. Note: For summary of merits and demerits, please refer linoleum flooring.

282

F L O O R I N G

13.

A N D

Magnesite

R O O F I N G

Flooring

(or

Magnesium

Oxychloride

Composition Flooring). This flooring made of patent flooring materials is generally named as 'composition flooring' or 'jointless flooring' and offers a less noisy floor than popular floors such as tiled floors, terrazzo floors, marble floors, etc. This flooring is occasionally used, depending upon the availability of its flooring materials, as flooring for schools, office buildings, light factories, etc. The composition flooring consists of a dry mixture of magnesium oxide, an inert material such as asbestos, wood floor or saw dust and a pigment. Liquid magnesium chloride is added to this powder or dry mixture at the state of work to make it a plastic material. For construction of magnesite flooring, the plastic material is directly spread over the base of wood, stone, concrete or steel plates and then the surface is finished smooth by a trowel. This type of flooring is generally cheap and is laid in thickness of about 12 mm over the rough surface.

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R O O F I N G

Regarding composition flooring, the following features should be carefully noted: i) The success of flooring depends upon the proper composition of the ingredients and the skill of the labour employed. The excessive amount of magnesium chloride in the mix results in sweating of the floor surface. ii) The finished floor should be protected against moisture by the application of wax polish or oil at regular intervals. iii) When flooring is provided over a wooden base, it is necessary to provide a foundation of metal lath to increase its holding power for receiving composition. Thus, metal lath is protected by a costing of bituminous paint before use. 14.

Glass Flooring. This type of flooring can be used for

special purposes where it is desired to transmit light from an upper floor to a lower floor such as from a ground floor to a basement. This flooring is not commonly used for floors, in general. 284

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R O O F I N G

In the construction of glass flooring, the structural glass in the form of tiles, blocks, etc. is fitted within frames of various types. The structural glass is available in different forms of varying thicknesses, usually from 10 to 30 mm. The frame-work containing structural glass blocks should be so closely spaced that the glass flooring can safely sustain the anticipated loads. 15.

Marble Flooring. This flooring is commonly used for

superior type of floor construction, particularly, where sanitation and cleanliness are required as in case of hospitals, temples, bath-rooms, theatres and other such buildings. The construction of this flooring is exactly same as that of mosaic flooring already discussed, except that the.0 of marble pieces instead of mosaic tiles is made. I

285

F L O O R I N G

16.

A N D

R O O F I N G

Plastic or P.V.C. Flooring. The plastic flooring is a

recent development in the floor construction and has been successfully used as a covering over the concrete floor base in all type of buildings such as residential buildings, hospitals, churches, hotels, schools, offices, shops, etc. This plastic material, called P.V.C. (Poly-Vinyl-chloride), is fabricated in the form of tiles and is available in different sizes in varying colours and shades. The plastic flooring is laid in the similar way as cement or pottery tiles, discussed already under Tiled Flooring. Regarding plastic tiles, the following points should be noted: i) Though the plastic tiles or thermo-plastic tiles provide a hard and durable surface but are found to be slippery, and hence they are more suitable for using on walls of sanitary rooms such as W.C.'s, bath rooms, kitchens, etc., rather than on floors. ii) The use of plastic tiles on a wooden floor base is not advised as the cost of preparing of wooden surface for receiving the tiles is considerably high. 286

F L O O R I N G

17.

A N D

R O O F I N G

Asphalt Flooring. Formerly, this type of flooring was

disliked due to bad smell and ugly appearance, particularly for indoor floors. But with the development of technology now it is possible to have not only a dustless, elastic, durable and waterproof but also an attractive acid-proof asphalt flooring. The asphalt flooring is carried out in a variety of colours and in different forms such as asphalt tile, asphalt terrazzo, mastic asphalt, special acid-proof asphalt blocks, etc. Asphalt tiles and asphalt terrazzo are used for flooring in schools, offices, shops, hospitals, restaurants, etc. Mastic asphalt flooring, being non-slippery and noiseless, is generally

recommended

for

use

commercial

and

industrial buildings such as factories, loading platforms, swimming pools, dairies, breweries, etc., and terrace floors, etc. Special asphalt blocks flooring is used for floors in chemical laboratories, acid plants, dye-houses, storage battery buildings and similar structures where strong acid solutions are either manufactured or handled.

287

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A N D

R O O F I N G

The composition of asphalt flooring material of above popular forms is as follows: i) Asphalt tiles. These are produced from asphalt, asbestos fibres and mineral pigments in different sizes (generally, in 20 to 45 cm square size), colours and thicknesses varying from 3 to 6 mm. ii) Asphalt terrazzo. This is obtained in-situ by the combination of black or coloured asphalt with marble chips in hot condition. iii) Mastic asphalt. Like asphalt terrazzo, it is obtained by mixing a mineral filler (may be lime stone dust, sharp sand or grit) and coarse aggregate with black bitumen when heated to a liquid form. iv) Acid-proof asphalt blocks. These asphalt blocks consist of inert crushed rock aggregate bound

288

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3 Topic

Construction of Upper Floors The upper floors are generally classified on the basis of arrangement of beams and girders, or the framework, for supporting the flooring and the materials used in the entire floor construction. Types of Upper Floors The various types of floors commonly used are as follows: 1. Timber floors. a) Single joist floor b) Double joist timber floor and c) Framed or triple joist timber floor

289

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A N D

R O O F I N G

2. Timber floor supported on steel joists 3. Steel joists and flag stones or precast concrete slabs 4. Steel joists and jack arches of brick or concrete 5. R.C.C slab floor 6. Steel girders and R.C.c slab floor 7. R.C.C beam and slab floor 8. Flat slab floor 9. R.C.C ribbed floor 10. Precast concrete floor. 1. Floors of Jack Arches of Brick or Concrete These floors are formed by constructing brick or concrete arches, called jack arches on the lower flanges of mild steel joists. The joists provided, in turn, are made to rest either on wall or on beam. The floors bear the anticipated load either from reinforcement or by arch action. The joists are spaced at 80 to 120 cm. the jack arches are usually given a small rise of 1/12 th of the arch span. The floor is finished on the top with any kind of paving, such as stone, cement, concrete, tile etc.,. For large spans rolled steel joists are provided to support the joists. 290

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A N D

R O O F I N G

This type of floor suffers from the following draw backs (i)

The ceiling of the floor is not plain from below.

(ii)

The rolled steel joists, if embedded in lime concrete, are liable to rust due to the action of lime.

(iii)

The arch action of the floor exerts thrust on the side walls, which require extra tie rods of mild steel in the end spans at intervals.

The methods of construction of brick jack arch floors and concrete jack arch floors are as follows: A. Construction of Brick Jack Arch Floors. First of all, a timber centering is made to the required size and shape (usually made segmental in shape, having about 4 cm thickness), and is laid on edge with the circular part (known as intrados) upwards, on the lower flanges of the rolled steel joists at a distance of 7.5 cm from the wall, well burnt and saturated bricks are then laid on the centering board from both the ends or joists of the arch and the work is closed at the centre. The end bricks are suitably cut to the required shape to enable them to fit into the joists properly and the joint next to the joist is 291

F L O O R I N G

A N D

R O O F I N G

filled with cement mortar to prevent contact with lime (as it results in rusting of joists). The first ring should consist of alternate bricks, 20 cm and 10 cm, long respectively, so as to maintain a continuous bond between the first and successive rings. The central brick known as the key is laid in stiff mortar and the joints on either side of the 'key' brick should be filled tight. The centering board is then removed and rest of the arches in series are constructed as described above. The entire arch work is well watered for at least 10 days and then the upper space is levelled by filling with. cement or lime concrete, muram or light weight concrete, or any other desired fill. This surface is then finished with required type of flooring (See Figure. 4.5). The underside of the arches is plastered and white (or colour) washed. N.B. Hollow concrete or clay blocks or clay tiles can also be used instead of bricks for constructing jack arches.

292

F L O O R I N G

A N D

R O O F I N G

Figure 4.5 Brick Jack Arch Floor B. Construction of Concrete Jack Arch Floors The

construction

concrete

jack

arch

floors

comparatively simple. The centering consists of 3 mm thick mild steel plate bent to the exact shape of intrados and having holes at the two ends, at 75 cm c/c longitudinally (i.e., along the length of the plate). To support this mild steel centering, two iron rods, about 12 mm in diameter and of suitable length, are hooked at the ends so as to form eyes large enough to pass a 12 mm rod ; and each iron rod is passed through the eyes of the other in such a way that by sliding the eyes, the total length of the two iron rods 293

F L O O R I N G

A N D

R O O F I N G

can be increased or decreased. These iron rods are made to pass through the hole made in the mild steel plate, and are supported on the lower flanges of the floor joists.

Figure 4.6 Concrete Jack Arch Floor This centering is formed over the R.S. joists already placed on the walls at intervals of 75 cm. Concrete of specified mix is now laid on the top of the centering to the required thickness and well consolidated by means of rammers (See Figure. 4.6). The flooring is then completes with desired type of flooring.

294

F L O O R I N G

A N D

R O O F I N G

The entire floor work is well watered for atleast 10 days, and the centering is removed by removing the wooden packing piece and then hammering the eyes in the rods towards each other. The underside of the arches is finished with plastering. A typical concrete Jack Arch floor with rolled steel beams, suitable for large spans is shown in Figure.4.6. Important Considerations (or Precautions) in Jack Arch Construction The following important considerations or precautions should be observed in the construction of brick or concrete jack arch floors: i)

Steel joists, if allowed to come in contact with lime mortar or lime concrete, are liable to rust dueto the action of lime. Hence, they should be protected from the action of lime by interposing a screed of either cement mortar or cement concrete (See Figure. 4.5).

295

F L O O R I N G

ii)

A N D

R O O F I N G

It should be ensured that the steel joists are well secured on either side to the supporting wall usually (by means of bolts and nuts) against the outward horizontal movement, before starting the arch construction. (See Figures. 4.5 and 4.6).

iii)

For span between the walls when more, than 4 metre, the R.S. beams, as shown in Figure. 4.6, should be placed from wall to wall at spacing of 2 to 2.5 metre, and their ends should be placed on stone or concrete bed blocks embedded in masonry.

iv)

The arch work should be cured or watered atleast for 10 days.

Filling and finishing of the floor, over the laid work, should start after the completion of curing period.

2. R.C.C. Slab Floor. R.C.C. slabs are becoming very popular in the construction of floors for modern buildings, because RC.C. is a combination of two materials, cement concrete and steel. Concrete is weak in tension and to overcome this, steel, which is strong in tension, is introduced to form a composite material. In R.C.C. the steel reinforces or 296

F L O O R I N G

A N D

R O O F I N G

strengthens the concrete, and hence R.C.C. is equally strong both in compression and tension For small spans of floors up to 4 metre which do not carry heavy loads, a simple R.C.C. slab may be used. When the ratio of length of the room to its breadth is greater than 1.5, slabs are designed as one-way slab to span along the shorter width. In this case, main reinforcement is nonparallel to the shorter walls, while distribution bars are laid parallel to the length of the room (See Figure. 4.7).

Figure 4.7 Details of One way R.C.C slab

297

F L O O R I N G

A N D

R O O F I N G

The thickness of the slab depends upon several factors, such as concrete mix used, anticipated floor loads, the span or floor plan, etc. Depending upon the building plan, the slab may be simply supported on the wall, or it may be continuous over intermediate walls. The design of a continuous floor slab is different from that of a simply supported one. In case of simply supported slabs, to allow the slab a freedom of movement, the top of walls is covered with a layer of plaster and then with a thin coat of bitumen over it. If the building is of R.C.C. framed construction, then the slab is cast monolithically with the supporting beams.

Figure 4.8 Details of Reinforcement in Two way R.C.C slab

298

F L O O R I N G

A N D

R O O F I N G

When the length to breadth ratio of the room is less than 1.5, (i.e., room is nearly square), the floor slab is designed spanning in both directions and called two-way slab. In this case, main reinforcement in the slab is provided in both the directions parallel to the sides of the room. At corner, suitable mesh reinforcement is provided at top and bottom (See Figure. 4.8). R.C.C. Slab Floor Construction is Carried out as Follows i) A well designed centering [Refer chapter 231 or false work either of steel or timber, is erected to support its own weight (dead load) and the super-imposed load. ii) After centering, the reinforcement is placed on the interior surface which has been finished first with a thin coat of oil and then with a thin layer (2 to 5 cm) of cement concrete. iii) The cement concrete is then poured around the reinforcement upto required thickness of the slab, and well consolidated by means of rammers. iv) The concrete is now cured for about 2 weeks to attain its full strength. 299

F L O O R I N G

A N D

R O O F I N G

v) After

the

concrete

has

sufficiently

hardened,

the

formwork is removed and the upper and lower surfaces of the slab are treated as desired. N.B. K.C.C. floors are highly efficient, fire-resisting and economical but have resonance qualities. 3. Steel Girders and R.C.C. Slab Floor. When the spans are larger (more than 4 metre), the use of R.C.C. slab floor becomes uneconomical. There-fore, under such conditions additional supports of steel girders are introduced below the slabs to reduce the spans. The type of floor may either consist of individual slab units on the steel girder or continuous slabs on the other girders. In the first type, the reinforcement is provided on the underside of the slab whereas in the second type the reinforcement is provided at the top of the slab over the beams the part of which is also taken down to the bottom portion. In case of fire-resistive type of construction, it may be necessary to protect the steel beams from fire by 300

F L O O R I N G

A N D

R O O F I N G

encasing them in concrete or any other protective covering (See Figure. 4.9)

Figure 4.9 R.C.C slab and Steel beam floor The construction procedure is same as for R.C.C. slab except the additional use of steel girders below the slab. 4. R.C.C. Beam and Slab Floor. For larger spans and heavy loading conditions or in situations where intermediate walls are not provided to reduce the span, R.C.C. beam and slab floor construction is becoming very common for most of the buildings. In this type of floor, beams and slabs are designed as rectangular sections and the slabs are supported on beams. In monolithic construction, the beams and slabs are cast together as shown in Figure 4.10. 301

F L O O R I N G

A N D

R O O F I N G

Figure 4.10 R.C.C beam and slab floor The beam used in monolithic construction are known as Tbeams because a part of the floor slab helps in resisting the compression developed in the beam. The projecting part of the beam below the slab is called the rib of the beam. There floors of R.C.C. beam and slab type, are constructed either as one-way slabs supported on two sides and with main reinforcement in one direction only, or as two-way slabs supported on four sides and with reinforcement in two directions.'

302

F L O O R I N G

A N D

R O O F I N G

The construction procedure of this type of floor is same as discussed for R.C.C. slab floor except the type of centering or formwork required for it [Refer Chap. 24 for Form work]. N.B. Guiding Principles of Economic Design and Construction of R.C.C. Floors. The following principles should be well observed during the design and construction of R.C.C. floors: i) The slabs and beams should be kept continuous as far as possible. ii) The floor panels should be made either square or nearly square. The span should be kept shortest possible. In case the floor plan is rectangular in shape, with width less than 4 metres then the width should be treated as span and main reinforcement should be provided in that direction only. This generally occurs in case of apartments.

303

F L O O R I N G

A N D

R O O F I N G

5. Flat Slab Floor (or Beamless Slab Floor). This floor is made of R.C.C. slab which is directly supported on columns without the use of beams or girders. The flat slab floors are generally used where heavy loads are to be carried and head-room is limited. These floors are generally" used for buildings such as warehouses, factories, mills, theatres, public building, etc. The columns supporting the flat slabs are usually circular in section and tops of columns are invariably flared or tapered. This flared or tapered portion is called capital. The portion of the slab, symmetrical with the column, is sometimes thickened in order to give additional strength to the capital and in turn to the slab. This thickened portion is termed as a drop panel. (Figures. 4.11).

304

F L O O R I N G

A N D

R O O F I N G

Figure 4.11 details of Flat slab Floor Flat slabs are generally thicker (12 mm or more) in case of beam and slab floors but are more economical as when the floor loads are heavy, panels are square or nearly square and column spacing ranges between 5 to 8 metres. They are specially

useful

under

the

following

conditions

circumstances: i) Where large number of panels (atleast three rows of panels) of almost equal dimensions are required in each direction, i.e., length-wise and breadth-wise. ii) Where large clear spaces are required iii) Where the head room is limited. 305

F L O O R I N G

A N D

R O O F I N G

iv) The ratio of length to breadth of panels should not be more than 4 : 3. v) The length and breadth of any two adjacent panels should not differ by more than 10% of the greater of the two. vi) The end spans should not be more than the interior spans. The reinforcement in flat slab floors is provided by different systems but the following two are commonly used: i) The two-way system. ii) The four-way system. In two-way system of reinforcement, the reinforcement is carried from column to column and at right angles to one another. The area left in this system is regarded as supported on four sides. In the four-way system of reinforcement, the reinforcement is placed in four directions and consists of two bands of main steel from column to column and other two bands placed diagonally across the panel from column to column.

306

F L O O R I N G

A N D

R O O F I N G

Chief Advantages of Flat Slab Floors i) The formwork required for slabs is very simple and cheap ii) The floor has a flat ceiling which gives a good appearance. iii) Due to flat ceiling, gives better lighting facili-ties and also affords the convenience of hanging pipes or shafting, which is an added advantage in factories. iv) For the same clear head room, there is saving in the storey height, and hence the cost. v) For heavier loads, thinner section of the slab is needed, particularly when four way system is used. vi) As already mentioned flat slab floors are specially suited where live loads are heavy (more than 490 kg/m) when large clear spaces are required and when e columns spacing is between 5 to 8 metres.

307

F L O O R I N G

A N D

R O O F I N G

4 Topic

Types of Pitched Roofs or Sloping Roofs Pitched roofs are those which have the decks or surfaces with considerable slope for covering the building structure. These roofs are generally lighter than flat roofs and are constructed either in wood or steel. As already mentioned, such roofs are most suited in regions of heavy rainfall and snowfall. Broadly speaking, these pitched roofs are classified into the following three categories: 1. Single Roofs, 2. Double or Purlin Roofs, and 3. Triple-membered or Framed or Trussed Roofs.

308

F L O O R I N G

A N D

R O O F I N G

1. Single Roofs. These roofs consist only of common rafters which are secured at the ridge and wall plates. The various forms of this type are as follows: i) Lean-to-roof or Shed or Verandah roof or Pent roof or Aisle roof, ii) Couple roof, iii) Couple close roof, and iv) Collar-beam roof or collar tie roof. 2. Pitched Roofs Pitched roof is generally regarded as the cheapest alternative for covering a structure. Pitched roofs are almost always constructed in wood or steel. In areas of heavy snowfall steeper slopes 1 to 1 1/2 or 1:1 are provided to reduce the snow load on the roof.

309

F L O O R I N G

A N D

R O O F I N G

Technical Terms The various technical terms used in timber pitched roof construction are given below. Span: The clear distance between the supports of beam or roof truss. Ridge: It may be defined as the apex of the angle formed by the termination of the inclined surface at the top of a slope. Rise of roof: It is the vertical distance between the wall plate and the top of the ridge. Pitch of roof: The inclination of the sides of a roof to the horizontal is termed as the pitch of the roof. Eaves: The lower edge of the inclined roof surface of a pitched roof is termed as Eaves. Ridge Piece: It is a horizontal timber piece provided at the apex of a roof truss.

310

F L O O R I N G

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R O O F I N G

Valley: It is the acute angle or a gutter formed by the intersection of two slopes in a pitched roof Hip: It is a ridge formed by the intersection of two sloped surfaces having an exterior angle greater than 180째. Gable: It is the triangular portion of the end wall of a sloped roof formed by continuing the end wall up within the roof. Thus the gable has two slopes with ridge in between. Common Rafters: These are inclined wooden members laid from the ridge to the eaves. Purlins: These are horizontal members of wood or steel, used to support common rafters. Cleats: These are short sections of wood or steel, nailed to the rafters of the truss for supporting the purlins.

311

F L O O R I N G

A N D

R O O F I N G

Types of Pitched Roofs Pitched roof can be broadly divided into the following different types. 1. Lean-to- roof 2. Coupled roof 3. Couple close roof 4. Collar roof 5. Scissors roof 6. King post roof truss 7. Queen post roof truss 8. Mansard roof truss

312

F L O O R I N G

A N D

R O O F I N G

1. Lean-To-Roof This is the simplest type of sloping roof, in which rafters slope to one side only. It is also known as pent roof or Aisle roof. The wall to one side of the room (or verandah) is

taken

higher than

the wall (or pillars) to the other

side. A wooden wall plate is supported either on a steel corbel or a stone corbel, which are provided at 1 m centre to centre.

Lean to roof Figure 4.12

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The wall plate (or post plate) is embedded on the other side, to the wall or pillars. The difference in elevation between the two wall plates is so kept that the desired slope is obtained. Usual slope is 30째. The common rafters are nailed to wooden wall plate at their upper end, and notched and nailed to the wooden plate at their lower end. Sometimes, iron knee straps and bolts are used to connect the rafters to the post plate. Eaves boards, battens and roof coverings are provided. This type of roof is suitable for maximum span of 2.5 m. These are provided for

sheds,

out-houses

attached

main

building,

verandahs, etc. 2. Coupled Roof Couple roof consists of two inclined rafters. These rafters are nailed to a common ridge piece at their upper ends. Their lower ends are nailed to the wooden wall plates which are fixed on the top of the wall on either side. The rafters are spaced at suitable intervals. Battens are fixed over the rafters. Over the battens

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suitable roof covering is fixed. This type of roof is suitable up to a span of 3.6 m.

Couple Roof Figure 4.13 3. Couple Close Roof It is very similar to the couple roof but the feet of the rafters are joined by a tie beam. This tie beam keeps the rafters in position and prevents them from spreading and thrusting out of the wall. This type of roof can be used for spans upto 4.5m. For large spans a

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vertical rod called king rod is provided connecting the ridge piece and the center of the tie.

Couple Close Roof Figure 4.14 4. Collar Roof In this type of roof a tie beam is fixed near the middle of the rafters. The collar roof is similar to the couple close roof but the only difference is that the tie is placed at a height of half to one third rise of the roof. The tie beam

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also serves the purpose of ceiling joists when false roofing is to be provided.

Figure 4.15 5. Scissors Roof In this type of roof two tie beams are provided so that the roof have the appearance of scissors

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6. King-Post Roof Truss A king-post truss shown in figure. 4.16 consists of the following components:

(i) lower tie beam, (ii)

two

inclined principal rafters. (iii) Two struts, and (iv) a king post. The principal rafters

support the purlins.

The

purlins support the closely-spaced common rafters which have the

same slope

the principal

rafters. The

common rafters support the roof covering as usual.

King Post Truss Figure 4.16

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The spacing of the king-post truss is limited to 3 m centre to centre. The truss is suitable for spans varying from 5 to 8 meters. The lower, horizontal, tie beam receives the ends of the principal rafters, and prevents the wall from spreading out due to thrust. The

king-post

prevents the tie-beam and

the principal

rafters in inclined direction, prevent the sagging of principal rafters. Ridge beam is

provided at the apex of the roof to

provide end support to the common rafters. The trusses are supported on the bed blocks of stone or concrete, embedded in the supporting walls so that load is distributed to the greater area. The principal rafter is jointed to the tie beam by a 'single abutment and tenon joint' or by a 'bridle jointâ&#x20AC;&#x2122;. The joint is further strengthened by a wrought iron heel strap, would round the joint. The head of each strut is fixed to the principal rafter by an 'oblique' mortise and tenon joint. The king-post is provided with splayed shoulders and feet, and is tenoned into the upper edge of the tie beam for a sufficient 319

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distance. It is further strengthened by mild steel or wrought iron strap. At its head, the king-post is jointed to the end of the principal rafters by tenon and mortice joint. The joint is secured by means of a three-way wrought or mild steel strap on each side. Purlins, made of stout timber or placed at right angles to the sloping principal rafters, and are secured to them through cogged joint and cleats. Cleats, fixed on principal rafter, prevent the purlins from tilting. The common rafters may be connected to eaves board or to pole plate at the other end. Pole plates are horizontal timber section which runs across the top of tie beams at their ends, or on principal rafters near their feet. They thus run parallel to purlins.

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7. Queen-Post Truss A queen-post truss differs from a king-post truss in having two vertical posts, rather than one. The vertical posts are known as queen-posts, connected

the tops of

by a horizontal piece, known

which

are

as straining

beam. Two struts are provided to join the feet of each queen-post to the principal rafter. The queen-posts are the tension members. The straining beams receives the thrust from the principal rafters, and keeps the junction in stable position. A straining sill is introduced on the tie the queen-post t counteract

be a between

the thrust from inclined

struts which are in compression. In absence of the straining sill, the thrust from the strut would tend to force the foot of the queen-post

inwards. Purlins, with cleats,

are provided as in the king-post truss. These trusses are suitable for spans between 8 to12 meters.

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The joint at the head of queen-post is formed due to the junction of

two member (queen-post).

The head of

the queen-post is made wider, and the head of the principal rafter and the end of the straining beam are tenoned into it. The joint is further strengthened by fixing a 3-way strap of wrought - iron or steel on each face. Similarly the feet of queen-post is widened to receive the

tenon of the inclined

abutment and tenon

strut,

joint'. The

forming a 'single queen-post

then

tenons into the tie beam. The joint is further strengthened by stirrup straps and bolts.

Figure 4.17

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Steel Roof Trusses When the span exceeds 10m, timber trusses become heavy and uneconomical. Steel trusses are more economical for large spans. However, steel trusses are most commonly used these days, for all spans-small are large, since they are: 1) more economical 2) easy to construct or fabricate, 3) Fire proof, 4) more rigid and 5) permanent. Steel trusses are fabricated from rolled steel structural members such as channels, angles, Tsections and plates. Most of the roof trusses are fabricated from angle- sections because they can resist effectively both tension as well as compression, and their joining is easy. In India, where timber has become very costly (except in hilly region), steel trusses have practically superseded timber trusses.

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Some forms of steel trusses are shown below:

Figure 4.18 steel Trusses

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Roof trusses are used in workshops, industrial buildings, auditoriums etc., to support roofing materials which consist of sheets. The roof trusses may be supported on walls, columns or girders. Purlins run over the trusses generally at panel points and support roofing materials. Wind bracing is used between trusses to take wind forces and to stiffen trusses in longitudinal direction. The span of the truss is fixed from the dimension of the area to be kept free of columns. The slope of the roof called pitch is governed by the roofing, material selected and such requirements as ventilation and light. Fink type of truss is suited to large pitch. Howe and pratt trusses for medium pitches. Where natural lighting is required north light trusses and saw-tooth roof trusses are used. Generally the purlins are kept at the panel points.

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4.19 Steel trusses

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5 Topic

Shell Structures A structural shell is a curved surface structure. It is generally capable of transmitting loads in more than two directions to support. These structures are highly efficient structurally when they are so shaped, proportioned, and supported that they transmit the loads without bending or twisting. A shell is defined by its middle surface halfway between its outer surface and inner surface. Depending up on the geometry of the middle surface, shell may be classified as : i) a dome ii) barrel arch iii) cone and iv) hyperbolic paraboloid

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A thin shell has relatively small thickness compared to other dimensions. It should not be so thin that the deformation would be large compared with thickness. The shell shearing stresses normal to the middle surface should be negligible. Thin shells usually are designed so that normal shears, bending moment and torsions are very small except for relatively small portions. Translation Shells Shell of translation is a type of shell obtained by moving a vertical curve parallel to itself along another vertical curve usually in a plane at right angles to the plane of the sliding curve. A cylindrical shell is a special case of it where the sliding curve is a straight line. The sliding and the stationary curves may have any geometrical shape.

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Figure 4.20 Elements of a Cylindrical shell (Single barrel)

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Figure 4.21 Multiple Barrel Type Shell Hyperbolic paraboloid is obtained by sliding a vertical parabola with upward curvature on another parabola with downward curvature in a plane at right angles to the plane of the first. Here the curvature of two sections at right angles will be in opposite directions, up in one and down in the other. This surface is generally called a saddle surface. There are different ways in which saddle surfaces can be supported. These surfaces are generally designed with small rises so as to pro-duce fairly flat roofs. If cut by planes parallel to the two parabolas, the edges will be parabolic and the supporting structure must be parabolic. 331

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Ruled Surfaces Shells This type of shell is obtained by moving a straight line so that its ends lie on two fixed vertical curves. These vertical curves can be of similar types or of different types. When two curves are of similar nature then cylindrical shell is obtained. If one of the curves is circular, elliptical, etc. and the other a horizontal straight line parallel to the base line of the vertical curve, then the shell so formed is called a 'conoid'. Such shells also have two opposite curvatures and have saddle surfaces. A cone is a special case of a conoid in which the horizontal line is a point and the other curve is circular. Hyperbolic paraboloid is also a ruled surface. The form work for these ruled surfaces can be made by straight wooden strips. Shell design relies heavily on the designer's experience and judgement. The designer should consider the type of shell, material of which it is made, and support and boundary conditions, and then decide whether to apply a bending theory in full, use an approximate bending theory, or make a rough estimate of the effects of bending and torsion.

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These structures are becoming popular for industrial buildings, hangers and other large buildings because they provide uninterrupted space without columns. Thin shell structures result in reduction of dead weight and in turn the economy. Generally, concrete is a very good material to take compressive force if used for the construction of a shell whose major portion is subjected to compression. The centering used for the construction of a shell structure can be re-used repeatedly. This process results in economy. These structures re-quire very low maintenance cost. The amount of steel used is also lesser. The large space with lesser number of columns provides better ventilation. Shell structures are better in appearance and provide good reflecting surfaces. These structures require less time for their construction. Most of the shell roofs are of single curvature. They consist of either long or short barrels. In case of short barrel shell roofs, the shell has a long variable radius. It is because of the reason that the shell is supported on long span with arch ribs which are closely placed. The span of the shell between arch ribs is small compared with the span of the supporting ribs. In case 333

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of long barrels the cylinder itself becomes the supporting member between the columns. The length of the cylinder is generally kept greater than its radius of curvature. The shell structure to be used in factories should have north light arrangements. These shells should have edge beams at the ends and arrangement is made for lighting in the sloping walls of the roof. This type of construction is very economical Cantilevered roof type shells may also be used for industrial structures. In this arrangement the shell roof is cantilevered out from a series of columns. Ribs introduced across the length of shells for spanning this type of structure increases its rigidity. Of late, special shaped shells have been developed which include the double curved shells and shells in the form of spherical triangle. C.B.R.I., Roorkee, has developed a twin Cteisphon shell which is of corrugated type. In these shells the depth of the corrugated determines the rigidity of arch ring. The depth of the corrugated must be increased as the span increases. It has been found that for spans up to 10 m, depth of the 334

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corrugated should be 20 cm. On the other hand, for a span of 25 m 60 cm depth of corrugation is required. In order to cover larger areas, multi span corrugation shell may be used. This type of construction is found to be more economical and is recommended for small low cost houses. The main feature of the formwork for shell roofs is to utilize identical members which are so designed that repeated use of the same form becomes possible (See Figures. 16.25 and 16.26). Tubular steel fittings form work have been patented for its use in the shell construction. Folded Plate Structures These structures are comprised of a series of thin planer elements or flat plates connected to one another .along their edges. These structures are generally used on long spans specially for roofs. These types of constructions derive their economy from the girder action of the plates and the mutual support they give to one another. These plates may be continuous over their supports longitudinally. There may be several plates in each bay transversely (See Figure. 4.21). 335

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Figure 4.26 Folded Plate Structure At the edges or folds the folded plate structures may be capable of transmitting both moment and shear or only shear. These structures have two-way action in transmitting loads to its supports. The transverse elements act as slabs spanning between plates on either side. The plates then act as girders in carrying the load from the slabs longitudinally to the support, which must be capable of resisting both horizontal and vertical forces. The design of the structures becomes relatively simple if the plates are hinged along their edges. In order to further simplify the design, it is assumed that the 336

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plates are steeply sloped. Beam theory may be applied for analysis if the span is sufficiently long with respect to other dimensions. So far the different methods available for design take into account the effects of plate deflections on the slabs. The different assumptions generally made are 1. The material is elastic, isotropic and homogeneous. 2. The longitudinal distribution of all toads on all plates is the same. 3. The plates carry loads transversely only by bending normal to their planes and longitudinally only by bending within the planes. 4. Longitudinal stresses vary linearly over the depth of each plate. 5. Supporting members, diaphragms, frames and beams are infinitely stiff in their own planes and completely flexible normal to their own planes. 6. Plates have no torsional stiffness normal to their own planes. 7. Displacements due to forces other than bending moments are negligible 337

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6 Topic

Roof Covering For Pitched Roofs and Their Selection Roof covering is a material covering provided over the form work of roof structure, ie., roof-deck, to safeguard the roof against the weather elements such as rain, sun rays, wind action , snowfall etc., and sometimes to give it is a decorative appearance also. It should be noted that the roof covering does not share the load in the building. It is rigidly fixed with the roof deck by means of various types of fitting and fixtures. There are several types of roof-coverings, but only those which are commonly adopted in India for pitched roofs are given below: 1. Thatch covering, 2. Shingles, 338

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3. Tiles, 4. Asbestos-cement sheets, 5. Galvanized corrugated iron sheets and 6. Slates. The above roof coverings will now be described in the following pages in their serial order: 1. Thatch Covering. This form of covering is extensively used in sheds, low-cost houses and village buildings. It is considered suitable for rural areas because it forms the cheapest and the tightest material as a roof covering. Moreover, it is simple to construct and keeps the building cool. Thatch covering made from straw or reed is used. The framework for supporting the thatch consists of round bamboo rafters spaced at 30 cm apart and tied with split bamboos or bamboo reepers laid at right angles to the rafters. The thatch is tightly secured to the framework or battens with the help of ropes or twines dipped in tar. Sometimes fire-resisting properties are imparted to thatch 339

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by soaking it in specially prepared fire-resisting solutions, but prove to be very costly. For adequate drainage of rainwater, the thatch covering should be atleast 15 cm thick and laid with a slope of 45 degrees. However, thatch is easily combustible, harbours rats, absorbs moisture, rots or mud gives out a foul odour. On account of these defects, it requires frequent renewals and is not recommended to be used in good work. The life of a thatched roof is not more than 15 to 20 years. 2. Shingles. The use of wood shingles as a roof covering is generally restricted to hilly areas where wood is easily and cheaply available in abundance. Wood shingles are nothing but the sawn or split thin pieces of wood resembling slates or tiles. These sawn shingles, which are obtained from the well seasoned timber, are dipped in creosote to impart preservative qualities. To form continuous roof covering, wood shingles are laid in a similar manner as slates and tiles, described later. Shingle 340

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strips are driven on rafters and shingles are nailed on their top. Generally, at the eaves, two courses of shingles are laid directly one over the other. Shingles are commonly obtained in lengths varying from 30 to 50 cm and in width varying from 5 to 25 cm. 3. Tiles. The use of tiles is one of the oldest methods of roof covering. The tiles are named according to their shape and pattern, and they are manufactured in the similar manner as bricks. The clay tiles are of various types, such as flat tiles, pan tiles, pot tiles or half round country tiles and patent tiles such as Mangalore, Allahabad tiles, Sialkot tiles (i.e., corrugated tiles). Sometimes, cement concrete tiles have also been used but their use is limited on account of high cost and the difficulties in their manufacture. Clay tiles have been widely used as a roof covering for residential buildings because of the following advantages.

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Advantages of Clay Tiles as a Roof-Covering i) Clay tiles, being non-conductors of heat, prevent the buildings from extreme changes of temperature outside and keep them cool. ii) These tiles provide quite a durable roof covering when made of well-burnt good materials. iii) They are quite strong and pleasing in appearance. iv) If properly selected and laid, they have good resistance against fire and moisture penetration. v) These tiles provide a very economical roof covering with aesthetic values and hence are used for urban and rural houses. However, these tiles suffer from the limitation of being heavy in weight. The weight of the roof covering is further increased as the rafters are kept closer to reduce the span of wooden members and to throw off the rain-water. The average weight of a tile roof is about 75 kg/m2. In casting tiles, the precaution should be taken to see that they are made impervious and uniform in size and shape.

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The method of laying or fixing clay tile is given below: i) Flat or Plain Tiles.

Before laying or fixing any roof

covering, a certain amount of ground work is required to be done. For this, the common rafters are laid usually at a spacing of 20 to 30 cm and then battens or reepers are fixed across the rafters at a spacing of about 6 cm. The tiles are finally laid over this with sufficient over-lap on sides and edges as shown in Figure. 4.22. Flat or plain tiles are manufactured in rectangular shapes (size, 25 x 15 cm to 28 x 18 cm) in thicknesses varying from 9 to 15 mm. Tiles are not perfectly flat but have a slight camber, usually 5 to 10 mm. These tiles have two small projecting nibs and two or more nail holes at one end of their surface. These nibs and holes help to fix the tiles on the battens of the roof truss. The tiles should be laid at proper gauge and overlap, both at sides and edges, as it is important for their strength, durability and imperviousness.

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Special tiles are made for the under course at eaves, top course at the ridges, for hips and for valleys, to avoid cutting. Flat tiles, ceiling tiles or boarding may be used below the top covering of tiles for keeping out cold and heat.

Figure 4.22 Details of Allahabad Roofing Tiles or half round country tiles with flat base 344

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ii) Curved Pan-Tiles. These tiles are shorter, less curved, heavier, stronger and more durable than the pot-tiles. These tiles are moulded flat first and then the required curvature is given. These tiles may be made of clay or of asbestos cement. These tiles are about 30 to 35 cm long and are about 20 to 25 cm wide.

Figure 4.23 Details of Mangalore roofing tiles

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Figure 4.24 Details of Sialkot tiles on a roof The curved pan tiles are fixed over battens in a similar manner as the plain tiles. These tiles have less corrugations and are laid with laps of 15 cm and 10 cm at ends and at the sides respectively, as shown in Figure. 4.24. The last row of tiles near the eaves is laid in lime or mud mortar, and is further secured in position by an iron rod on its top, tied down to the battens below it. Lead flashing is liberally used in the formation of hip and valley gutters in pan tiling.

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iii) Pot Tiles or Half-Round Country Tiles. These tiles are laid on a ground work consisting of boarding or closely drived battens (size 5 cm x 1 cm). A layer of felt or matting can be provided on this before laying the tiles. These tiles are very commonly used for rural houses as they offer a very cheap roof-covering. Further economy is achieved by replacing battens in ground work by split bamboos. These tiles are laid in pairs of under tiles (i.e., concave upwards) and overtiles (i.e., convex up-wards) with a proper overlap of atleast 8 cm on all the sides. Generally, there are two varieties of such tiles. In one variety, the under tiles are flat with a broad head tapering towards the tail, while the overtile has a wider tail and the narrower head but segmental in section (Refer Figure. 15.28, Allahabad Tiles). In another variety, both overtiles as well as undertiles are semicircular and tapering by about 4 cm from bottom to top as shown in Figure. 15.27. All these tiles are laid on the framework in alternate positions in a course, the first with round side upwards and the second with hollow side on the top. The last row of tiles, near the 347

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eaves and the ridge, the valley and the hip, is laid in lime mortar. Sometimes, country tiles are laid in two layers, one over the other, and the roof is then called a 'Double tiled roof'. This type of roof requires heavy sup-porting timbers of greater strength than the usual ones. Occasionally, the tiles are provided with holes near the head so that they can be tied down by wire in case of steep slopes or when subjected to strong wind action. (iv)

Patent Tiles or Interlocking Tiles or Corrugated Tiles. These tiles are generally rectangular in plan, with surface corrugations so arranged that the corrugations of tiles fit in or interlock with those of other tiles. These roofing tiles have been patented by some companies in India and accordingly they are named as Mangalore tiles, Allahabad tiles, Sialkot tiles, etc. These interlocking tiles, which are machine-made, provide a lighter roof covering with a decent ap-

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pearance. Moreover, these tiles are available in different forms and designs in India. In ordinary works, the ground work for these tiles consists of battens only. In superior type of construction, the tiles are laid on hoardings, covered with a protective coat of tar or felt. Boarding is directly nailed to the purlin and tiles are on battens nailed on the boarding. Fir ridges and hips, the tiles moulded into special shapes are used. They are laid dry and finally pointed with cement mortar. But the valleys are formed with the aid of lead flashing laid over boarding. If, there is a possibility of tiles to be blown away by strong wind action, then lowermost row (i.e., at eaves) should be screwed down to the battens, or secured by wires through holes drilled in them. These patent tiles afford very strong, durable and economical roof coverings for pitched roofs in India. 349

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4. Asbestos Cement Sheets, i.e., A.C. Sheets. Asbestos cement is a material which consists of portland cement and asbestos fibres (about 15%). Roof coverings made of this material are cheap, tough, durable, watertight, fire resisting and light in weight. Asbestos cement sheets do not require any protective paint and cannot be eaten away by vermin. Almost all varieties of roof coverings are now made with asbestos cement. On account of these properties, A.C. sheets roof covering is commonly adopted for factories, workshops, offices, garages, cinemas and residential buildings. A.C. roof coverings have added advantages of low maintenance cost and high speed of construction. However, A.C. sheets roof coverings have also some drawbacks, viz., condensation in buildings when located in colder climates, low heat insulation and low aesthetic values.

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Asbestos-cement roof coverings are supplied in flat, corrugated and ribbed sheets in various sizes. Ribbed sections are available with ribs at a spacing of 30 or 40 cm. The A.C. sheets are fixed at a very low cost as they can be cut, nailed, sawn or screwed easily where desired. A.C. sheets are obtained in the following three types, but in various lengths from 1 to 3 metres, rising in 15 cm increments: i) Everite big six corrugated A.C. sheets, ii) Everite standard A.C. sheets, and iii) Turnall Trafford A.C. tiles. The particulars of these three tiles are given in Table 4.1.

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Table 4.1 Particulars of Asbestos Cement Sheets Types

Standar

Laid

Thickn

Side laps

No. of

Pitch

Dept

A.C.

width

ess

in cm

Corrug-

in cm

h in

sheets

length

in mm

metres

ations

metres i) Everite

1 to 3

big

m in 5

corrugate

corrugati

rises

six A.C.

1.05 m

6 mm

5 cm or

7½

5.5

2.5

sheets ii) Everite

1 to 3

standard

1.05 m

6 mm

10 cm or 1½

A.C.

corrugati

sheets

iii)

1.2 to 3

Turnall

10½

1.09 m

6 mm

10 cm or

'4' but

5.0

m in 15

then

Trafford

corrugati

with

A.C. tiles

rises

alterna te flat portion s

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Figure 4.25 Details of 3 Types of A.C Sheets a) Everite big six type b) Everite Standard type c) Turnall Trafford Type 353

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Fixing of A.C. Sheets The smoother surface of the sheets is kept up-wards. The purlins are laid at a spacing of 1 to 1.25 m centre to centre. In fact, in an economical design, the spacings of the purlins and the length of the sheets are so worked out and adjusted that the end joints of the sheet come on a purlin. The sheets are fixed to purlins, from top of corrugations through the 11 mm dia. holes (having dia. 3 mm greater than the dia. of the screw) drilled to receive the screws or bolts. Generally, coach screws (8 mm in dia. and 11 cm long) with wood purlins, and crank bolts with steel purlins, are employed. The coach screws are driven into the wooden purlins along with cup washers of asbestos and lead to assure the water tightness. Similarly, hook bolts with cup washers are used for keeping the A.C. sheets in position with steel purlins. Every sheet is secured in position at six points, two at the head, two at the bottom and two at the intermediate, i.e., at the purlin.

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The ridge is formed with the aid of a pair of ridge tappings each overlapping the other. These adjustable ridge cappings should preferably be secured to the ridge purlin by the same kind of bolts which are used for fixing sheeting. 'J' hook bolt should be used for angle purlins and crank bolts should be used for purlins of timber, rolled steel joists, channel sections, etc. At the eaves, suitable eaves filler pieces are used to fit in the respective corrugations of the A.C. sheets. so as to check the formation of under-draft into the roof and infiltration of birds, etc. The following points should be considered while fixing the A.C. sheets: i) The end of the A.C. sheets marked as Top' should be laid pointing towards the ridge. ii) A.C. sheets are usually laid with an end lap 15 cm, but this value may be slightly changed to suit the purlin spacing. If roof slopes are less than 211/2 degrees, then this lap value should be increased accordingly. 355

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iii) The side laps for 'Big six', 'Trafford' and 'Standard' types are

kept

Â˝

corrugation,

corrugation

and

1Â˝

corrugations respectively. This side lap varies from 5 cm to about 12 cm. iv) The holes for fixing the screws or bolts to purlins are drilled through the tops (i.e., crowns) of the corrugations with their diameter 3 mm bigger than the dia. of the screw or bolt. This provides an allowance for expansion of A.C. sheet due to temperature changes. v) While fixing A.C. sheets the purlin spacing and length of sheets should be examined to ensure that they provide specified overhang at the eaves and proper laps as laid. The unsupported overhang at the eaves in no case should be greater than 30 cm. vi) With the screws and bolts, the lead cupped and asbestos washers or alternatively, galvanized iron and bitumen washers are employed to ensure the water-tightness of the joints. Normally, after laying 10 or 12 A.C. sheets, the nuts of the screws or bolts are made sufficiently tight but not very tight.

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vii) A.C. sheets may be laid from left to right or vice versa, but it is preferred to lay them, commencing from the end opposite to the direction of prevailing wind and rain. Mitring or cutting of sheets becomes necessary where four corners of the sheets are required to be joined or fitted properly without gaps. Laying of the A.C. Sheets. The details of laying A.C. corrugated sheets from left to right are described below: i) The laying of A.C. corrugated sheets is always commenced from the eaves. So, in the first row, i.e., eaves course, the first sheet is laid uncut while the remaining sheets in the bottom row should have top left-hand corners cut or 'mitred'. ii) While laying the second or other intermediate rows, the first sheet in each row should have bottom right-hand corner cut, while all other sheets should have both top left-hand and bottom right-hand corners cut or mitred. iii) In laying last, i.e., top-most row, all the sheets excepting the last one, should have the bottom right-hand corners cut. The last sheet is always laid uncut. 357

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N.B. 1. Whenever the sheets are required to be laid from right to left, the whole procedure of cutting the sheets described above is reversed. 2. It should be ensured that top edges over eaves must extend by 7.5 cm beyond centre line of wood purlins or 4 cm beyo-nd the back of steel purlins. Recently Fibre-concrete Roofing (FCR) technology is gaining widespread attention as a low cost roofing material. FCR tiles are claimed to be less expensive, have less self-imposed weight and, in general, have less intricate quality control. FCR mix is comprised of 1 part of cement to 3 parts of sand with a fibre (15 to 25 mm) of 1 percent by weight of total mix. Only a small amount of water is required just enough to produce a plastic or workable paste. The popular roofing fibres can be classified into three main groups e.g., mineral fibres of asbestos, animal and vegetable. Out of these, asbestos is the most popular. Vegetable fibres are easily available and are most appropriate for low-level technology production of FCR. The coir from coconut husk, stem fibre such as jute, and leaf fibre such as sisal are the most common examples of 358

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vegetable fibres which have been used with success. In general, the selection of suitable fibres should aim at avoiding fibres which are: excessively stiff, oily or greasy, easily impregnated by chemicals which have adverse effects on cement, e.g., sugar, and susceptible to large dimensional changes from wet to dry state. A simple test for the suitability of fibers involves chopping up a sample of the fibre and mixing it in a sand-cement mortar 100 times the weight of the fibre. The resulting concrete is allowed to see overnight. If the fibre pieces protruding from the concrete can be easily pulled out or if the concrete surrounding a particular location of fibre is discoloured or powdery, the fibre is unsuitable. The main function of fibre in the concrete is to resist segregation of the fresh mix during moulding and to prevent the formation of shrinkage cracks during the initial setting and curing stages.

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FCR production includes transformation of the fresh concrete matrix into tiles or sheets. This involves spreading a quantity of the fibre concrete on a polythene sheet which is placed on a vibrator for compaction. Thereafter, the sheet of concrete is lifted and cast on a mould. The next stage is the normal concrete curing process which is done by transferring the set FCR tiles off the moulds into a curing tank for about seven days. UNCHS in 1987 has given a comparison between FCR sheets and tiles which states that FCR sheets are relatively more costly to produce than the tiles because for the same area of roof coverage the sheets tend to consume more raw material inputs. A cement mortar of one part cement to three parts sand is a typical mix for FCR tiles, while a mix of 1 : 1 is normal for the sheets. The table below give a brief comparison between sheets and tiles. Sheets are normally supported on 75 x 50 mm purlins spaced at 850 mm on centres.

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Tiles rest on 50 x 25 mm battens at 400 mm on centres. Table

4.2

Summary

Basic

Production

Data

for

Comparison between FCR Sheets and Tiles Product size Thickness

Effective Weight

Cement

(Dimensions)

cover

per m

content

per m

10mm

0.62m

32 kg

9.0 kg

15 kg

6mm

0.08m

20 kg

0.4 kg

5kg

Sheets 1000 x 780 mm Tiles 500 mm

250

The volume of timber used is therefore less for the tiles than for the sheets by about 25 per cent.

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A metre long FCR sheet weighs approximately 20 kg while a tile 0.5 metre long weighs 1.62 kg. For this reason, the FCR sheets are cumbersome and delicate to handle in the production process. The large size of the sheet is a disadvantage in quality control for a rather small-scale manual production technology and it is a more demanding task to lay the sheets over the roof structure. If three purlins are not placed accurately in a straight line, a single long sheet (A) will not be supported correctly in the centre and could break. This degree of accuracy will not be so important if two separate sheets (B) are installed.

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END OF THE BOOK

Building Technology â&#x20AC;&#x201C; Flooring and Roofing

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