Jac Timbrell - ARC 401
Construction Technology Summative Assessment
Page 1 of 12
Jac Timbrell - ARC 401 d.
Using the Good Building Guide for Sizing Simple Foundations in myCourse, select a
suitable floor construction and choose and size a foundation for the rear wall for a firm clay soil type. Explain your selection and the process used to establish the foundation size (width and concrete thickness) and depth below ground level. Assume that floor spans are up to 4.5m and roof spans are up to 9m. Show all the dimensions on your drawing along with full construction details of the floor and rear wall including damp proofing, insulation etc. e.
The roof detail shown is a ‘cold roof’; explain what this implies with reference to
temperature, condensation and ventilation. Draw an alternative detail enabling the roof space to be used for accommodation and explain how the approach to temperature, condensation and ventilation differs from a cold roof construction. Note: this is a pitched roof so reference to flat roof technology is irrelevant. f.
Explain, with the aid of drawings, how and why the following would have to be
constructed in a different way if they have to function as the separation between two flats: a.
The first floor
b.
The junction between the first floor and the external wall
c.
The junction between the floor and an internal wall backing onto another flat
You need to select and give reasons for a single specific solution; simply copying information from the Building Regulations is not sufficient. You are recommended to refer to the Building Regulations Part E accessible from the Planning Portal and Robust Details available through library e-resources, Construction Information Service. g.
This part of the site backs onto a new business park. Draw a section through the
wall of one of the new industrial/retail units from foundation level to roof level showing both structure and cladding (including the eaves) and explain the rationale behind your construction decisions. Ground conditions here are the same as in ‘d’ above. You may make any assumptions you wish about the usage of the unit; these assumptions should be made clear in your explanation (for example a retail unit would probably require higher quality finishes than a workshop).
Page 2 of 12
Jac Timbrell - ARC 401 (d) In order to choose an appropriate foundation type, foundation width and foundation depth, a site investigation must be the first step in the process evaluating the ground conditions and potential hazards (such as pre-existing pipelines, sewers or old foundations). At this site the soil has been determined as a uniform firm-clay type soil, with no hazards present. For this reason a spread foundation, (economically a trench fill foundation), would be most appropriate with a concrete slab floor. (V.Bird, Foundations tutorial 1, page 24). To then determine the foundation width for the rear walls several steps must be taken. Firstly, using table 1b of the “Good Building Guide for Sizing Simple Foundations� (due to the ground floor being a ground supported concrete slab and floor and roof spans or 4.5m and 9m respectively) the load category of the load bearing walls must be calculated, in this case the walls fall into load category C, as I have chosen to use a concrete beam and block first floor. The next step is to determine the minimum foundation width using the information gathered about the soil type and wall load category. Using table 4 in the GBG 39 document, The minimum foundation width for load category C, in a firm clay soil type is 450mm. After the foundation width is determined, the foundation thickness can be designed. In this case I would use a 300mm cavity wall, meaning a 75mm width overhang of foundation on either side of the wall. Shear failure in foundations is caused when the load of the building, directed down onto the narrow (300mm) load bearing walls, shears off a triangulated 45o section of the foundation. This is counteracted by making the foundation thickness equal to or more than the value of the overhanging foundation on either side of the wall (75mm in this case). However it is a requirement that the minimum foundation thickness is 150mm. The minimum depth of a foundation is 750mm, however due to the clay soil type with a long settlement time, I have decided that a trench foundation of 900mm depth, (J.Merriman, ground and foundations 4 lecture, page 31) 450mm width, and due to health and safety aspects such as narrow worker access and risk of collapse a 600mm foundation thickness is more appropriate than the minimum 150mm.(GBG ) If the site however was at risk from large water volume changes (from removed vegetation for example), the clay soil would be susceptible to heave I would recommend using a suspended concrete beam ground floor, this in turn would change the size of the foundation greatly, making the load category of the rear wall category E and the minimum foundation width of 750mm. A compressive material layer such as Claymaster could also be used around the foundation to reduce heave pressure. However, due to the assumption that this is not the case from the question, I decided to use a cheaper, ground supported concrete slab instead. Below is an annotated, dimensioned sketch of the foundation, rear wall and floor described.
Page 3 of 12
Jac Timbrell - ARC 401 (Fig.1)
A- 150mm Concrete Slab B- Insulation C- 65mm Screed D- Skirting Board E- Block Inner Leaf F- Insulation G- Brick Outer Leaf H- DPC I-
Ground Level
Page 4 of 12
Jac Timbrell - ARC 401 J- Foundation K- Lean mix Concrete L- Compacted Hardcore M- DPM
(e) The term “cold roof� describes a roof area in which the loft insulation is located between and on top of the joists just above the ceiling, meaning that the loft space is uninsulated. (V. Bird, Roofs tutorial 2, 2013, Slide 4). Due to the probable build-up of condensation in this roof type, where the warm internal air meets cold external air, cold roofs must be correctly insulated. As mentioned before the insulation is located between the joists, and due to stricter building regulations noted in approved document L1a, insulation is now additionally placed on top of the joists to comply with reduced emission rates. This is usually in the form of Quilt insulation (V. Bird Roofs tutorial 2, 2013, Slide 7). Cold bridging can occur in uninsulated areas where cold and warm elements meet, therefore insulation must continue over the timber wall plates and down to the cavity wall insulation to protect the vulnerable eaves of the roof structure. This ensures a continuous band of insulation leaving no gaps for cold bridging to occur, as recommended in section 5.9, Approved document L1A of building regulations. Ventilation is required in cold roof situations in order to allow air to freely enter and leave the loft space which prevents the accumulation of water vapour. (www.superglass.co.uk/product_solutions/roofs_and_celings/pitched_roofs/). In order to do this a few adjustments must be made. Soffits with vents and crossflow ventilators (also known as eaves panel ventilators) are used. Crossflow ventilators are hollow boards that run parallel to the rafters and are used to stop the roof insulation around the eaves blocking the passage of air from the soffit vents to the roof space, they also help direct air upwards towards the tiles and roof ridge, which may also aid ventilation with the use of a dry ridge system which creates a gap between the ridge tiles and roof tiles instead of the traditional mortar sealed ridges. The alternative to a cold roof is known as a warm roof, in this case the insulation is located along and above the rafters, creating a warm internal loft area. (V. Bird, Roof Detail Lecture 2013, slide 16). Fig.1 is an annotated drawing of the cross section of a warm roof, showing the different components and layers needed to achieve limited heat loss and limit condensation. The warm side of the insulation is subject to condensation; therefore a vapour
Page 5 of 12
Jac Timbrell - ARC 401 barrier (H, Fig 2) is advised in order to stop water vapour gathering in the insulation, which could lead to cold bridging and potential damage to the insulation. As well as insulation between the rafters (G), insulation is fixed on top of the rafters (F). (Roofing Technical Review, RIBA Publishing, page 20) This insulation would be in rigid board form and fixed in place with counter battens running parallel to the rafters (E), the reason for this is to allow for ventilation (50mm) between the insulation and underlay and to let any water to flow down to the eaves. (Roofing Technical Review, RIBA Publishing, page 21). The underlay (D) is then fixed onto the counter battens. In order to release any build up of interstitial condensation in the insulation layers, the underlay would be vapour permeable to allow water vapour out, but not let liquid water through. (V. Bird, Roof Detail Lecture 2013, slide 25). (Fig.2) Sketch of warm roof described.
A - Guttering B - Roof Tiles C - Battens (perpendicular to rafters) D - Breathable Membrane/Underlay E - Counter Battens (parallel to rafters) F - Still Board Insulation (held in place by counter battens) G - Insulation (between rafters) (Continues down to cavity insulation.) H - Vapour Barrier I - Roof Joists
Page 6 of 12
Jac Timbrell - ARC 401 J - Loft Space K - Timber Wall Plate L - Cavity Closer M - Strap/Tie N - Block work Inner Leaf O - Cavity Insulation (up to roof insulation) P - Brick Work Outer Leaf Q - Fascia (with air vent) R - Soffit (Vent- less)
(f) Any new building containing separate dwelling must comply with Approved document E of the building regulations for sound insulation. Seperating walls between separate dwelling must have sound insulation for a minimum of 45dB, separating floors must have a minimum protection from 45dB of airborne sound and a maximum of 62dB from impact sound (Table 0.1a, page 11, Approved Document E).
(a) The separating floor between two dwellings must be properly constructed and insulated to limit both airborne and impact sound. In this case, I would use a floor consisting of a screed floating floor, resilient layer, concrete first floor and ceiling. (V. Bird, Floors Tutorial 1, slide 6) The first floor this building would consist of CUBE6速 150mm concrete t-beams with 5mm diameter steel wire reinforcement, supported by masonry inner leaf walls at each end (http://www.cube6.co.uk/beam-and-block.htm). Then 100mm deep polystyrene infill blocks would then be placed between these beams to create the base of the first floor. This base would provide a more lightweight, sound insulated solution to traditional floor constructions. On the underside of this base, lengths of timber would be fixed to the concrete beams to create a flat ceiling, on which plasterboard/plaster can be applied. The void between the floor base and ceiling could be filled with 80mm Isowool速 to provide additional sound insulation. Above the concrete base, a resilient layer would be sandwiched between the floor base and a screed floating floor. The resilient layer is made of a softer material (such as rubber) and absorbs and dampens some of the sound which is travelling through it, it is especially effective against impact sound waves. For this floor, I would use a recycled rubber resilient layer, such a Versalayer速. This 10mm thick recycled rubber layer has been proved to comply with Approved Document E for sound insulation and replaces the need for the 25mm mineral wool layer usually specified. This product comes in sheet form and must have a minimum 50mm overlap between sheets. (http://www.euroform.co.uk/versalayer_applications_concrete_beam_block.shtml) A layer of screed would then be added on top of this resilient layer to provide finish the floor. It is important that the resilient layer laps up the walls to contain the screed as shown in the diagram below (F), a skirting board can then be put in place to secure and hide the resilient layer. A flexible sealant should then be applied around the edge of the floor where the resilient layer meets the walls.
Page 7 of 12
Jac Timbrell - ARC 401 (b) As well as sound travelling through the floor between two dwellings, airborne sound can travel through the cavity of an external wall and into the dwelling above or below the sound source. This sound transmission which bends round the corners of the separating floor is known as flanking sound transmission (V. Bird, Floors Tutorial 1, slide 8). To limit the amount of sound transmission it is important that the cavity is not bridged by the concrete floating floor specified above. A cavity stop is used to absorb airborne sound waves spreading through the cavity wall of one dwelling to another, and example of this is Timloc Acoustray速. This is recommend by RIBA as it provides sound insulation, fire protection between dwellings and is a functional cavity tray. (http://www.ribaproductselector.com/Product.aspx?ci=7504&pr=AlumascTimlocAcoustray) Alternatively, the whole cavity could be completely filled with insulation; however this could lead to water bridging problems. (c) With reference to part E of building regulations I would use a type 2 wall, a masonry cavity wall. The two wall leafs would be of 100mm thickness each and will be connected by butterfly wall ties to minimise impact sound transfer. The cavity would be 50mm wide and filled with sound proofing insulation. In this case I would use rock mineral wool insulation in the cavity, ROCKWOOL ENERGYSAVER速 is an example of this type of insulation, it would be filled between the cavities to provide sound insulation in a similar way the resilient layer does on the first floor. I would then recommend using staggered plasterboard on each external face of the blocks finished smooth with plaster. At each end of the internal wall I would use cavity stops to limit airborne sound transmission travelling to the external wall cavities or any other internal party wall cavities.
Page 8 of 12
Jac Timbrell - ARC 401 (Fig. 3) Sketch of floor and external wall sound insulation.
ABCDEFG-
Brick External Leaf Insulation Block Internal Leaf Skirting Board Flexible Sealant Resilient Layer overlap Resilient Layer
Page 9 of 12
Jac Timbrell - ARC 401 HIJKLMNO-
Screed Floating Floor Ceiling Ceiling void (Optionally Filled with Mineral Wool) Battens Piping (With Mineral Wool Coating) Polystyrene Blocks Concrete Beams Cavity stop (With Cavity Tray)
(Fig.4) Sketch of separating wall soundproofing
ABCDE-
Plaster Rock Mineral Wool Insulation Block Leafs Cavity Stop Staggered Plasterboard
Page 10 of 12
Jac Timbrell - ARC 401
(g) I have made the assumption for part g that the industrial building which backs onto the housing development is a large factory belonging to an aerospace company. The company manufactures large aircraft parts in this building and therefore requires large uninterrupted internal floor area and a very large roof span. Based on this assumption a portal frame would be the most suitable option for this construction. This type of industrial frame takes advantage of stiffness at the knee joint to create large roof spans without the need for trusses and internal vertical bracing. (J Merriman, structures lecture, slide 35) Portal frames are very common in construction; in fact 50% of construction steel used in the UK is used in the construction of portal frames, which are used commonly for industrial, retail and storage purposes (http://www.steelconstruction.info/Portal_frames). (Fig. 5) is an annotated drawing of the portal frame used in this instance. As the ground conditions are similar to the housing site, A large concrete slab foundation would be used (note: not shown in diagram), this will provide a solid base for heavy loads and help protect the floor structure from cracking due to potential differential settlement of the soil. The columns of the portal frame would sit on this foundation and be pinned in place. I have decided to use a dado wall on the lower section of the column for aesthetic reasons, This would be supported by a steel frame fixed to the internal face of the wall connected to the column of the portal frame. Above this wall I will use corrugated steel cladding, fixed to the frame with horizontal side rails. These side rails also provide structural bracing to the columns. At eaves level the columns are connected to rafters and haunches. The haunch supports the heaviest loaded region of the rafter and is the key of the portal frames stability. Located at the lowest point of the haunches are torsional restraints, these horizontal plates are welded to the column to stop the column flanges compressing from the roof load (Column design and stability, http://www.steelconstruction.info/Portal_frames). The rafters are braced horizontally by purlins and on these purlins the roof cladding will be fixed. Similar to the wall cladding this will consist of corrugated steel sheets. To avoid rust affecting the purlins, a wood piece would be wedged between the purlin and the ridge of the corrugated sheet where the fixing would take place. A cold rolled eaves beam acts as both the topmost side rail and lowest purlin. This guides the roof cladding over the wall cladding and into the guttering system. As this industrial unit has no need to create an internal climate, no insulation is required, however ventilation would be provided at eaves level to release any excess moisture and heat.
Page 11 of 12
Jac Timbrell - ARC 401
(Fig.5)
Page 12 of 12
Jac Timbrell - ARC 401
A- Guttering B- Eaves Beam (cold rolled) C- Roof Cladding D- Purlins (fixed to cladding) E- Rafter F- Knee joint (haunch) G- Eaves Connection H- Torsional restraint I- Column J- Cladding K- Side Rails L- Column M- Dado Wall N- Wall Support O- Holding down bolts P- Base Plate Q- Bedding space R- Foundation S- Anchor Plate T- Pile U- Cladding
Page 13 of 12
Jac Timbrell - ARC 401 Bibliography and References. All sketches drawn by myself, scanned, then edited on Microsoft paint. Lecturers Verity Bird and Jeffrey Merriman (Good Building Guide for Sizing Simple Foundations, GBG39, Bre press, 2001) (Roofing Technical Review, RIBA Publishing) (Approved Document A, Building Regulations) (Approved Document C, Building Regulations) (Approved Document E, Building Regulations) (Approved Document L1A, Building regulations) all available on Planning Portal (www.superglass.co.uk/product_solutions/roofs_and_celings/pitched_roofs/) (http://www.cube6.co.uk/beam-and-block.htm). (http://www.euroform.co.uk/versalayer_applications_concrete_beam_block.shtml)
(http://www.ribaproductselector.com/Product.aspx?ci=7504&pr=AlumascTimloc-Acoustray) (http://www.rockwool.co.uk/products/u/2011.product/9841/building-insulation/rockwoolenergysaver%C2%AE) (http://www.steelconstruction.info/Portal_frames)
Page 14 of 12