FACULTY OF INFORMATICS AND DESIGN DEPARTMENT OF ARCHITECTURAL TECHNOLOGY
CONSTRUCTION AND DETAILING 2
PRINCIPLES OF STRUCTURE IN BUILDINGS
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PRINCIPLES OF STRUCTURE IN BUILDINGS (Construction and Detailing) These notes deal with the broad principles related to framed structures in concrete, steel and timber. In simplest terms the main loads to consider are caused by GRAVITY and WIND, and then there are multiple lesser loads and impacts. Can you think of these and list some in verbal and diagrammatic form. Under GRAVITY one has to cater for live loads and dead loads, each of these acting vertically downwards; hence vertical and straight columns are most common. WIND loads are mainly horizontal but are also multi-directional, creating BOTH positive and negative loads. In combination these all impose oblique and twisting forces which have to be carefully handled by a professional structural engineer.
REINFORCED CONCRETE THE BEAM Concrete is made because of its COMPRESSIVE strength. Steel however has tremendous tensile strength - far greater than that of concrete. From the diagrams below it will quickly be seen that a combination of steel and concrete gives the best result.
The above beam in concrete would fail in its lower parts because of its strength in compression, not tension. To overcome this failure, reinforcing steel is placed in the lower portion of the beam.
Steel used to be hooked to anchor it in the concrete but for a number of years RIBBED reinforcing has been used creating an adequate anchor.
In cross section some steel is placed near the top with stirrups at frequent spacing along its length, to hold the beam together against compressive (crushing) forces. The size, number and arrangements of bars depends upon the load
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Reinforcing must have adequate cover - at least 40mm so that: a. the bar has good grip to a body of concrete around it and b. there is no danger of moisture causing corrosion and spalling
A projecting beam supported at one end is called a cantilever. The top of the beam is therefore in tension and the reinforcing placed accordingly.
Compression in the beam at A is much greater than B. Why? This is where the maximum bending moment is. The force of the bending moment is a combination of the load and its distance from the point of support. On a see-saw, greater leverage is exercised by moving to the very end. A large weight at the centre has no effect. The above beam may, because of the greater compression at A be modified as below:
Look for example of this shape. There are some just outside the lecture room windows in the engineering building.
For the beam which spans across columns as below, where would the main reinforcing be placed? Add it in yourself, after discussion with your mentor in the workplace.
With the above arrangement, engineers also provide reinforcing against shear failure.
This diagram gives a visual explanation of the shear principle. Failure, however often occurs at the dotted lines. Reinforcing is added at right angles to these lines to resist against failure.
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If a column to a four-storey building rests on a square concrete base as below, where should the reinforcing be placed? Add it in yourself, after discussion with your mentor in the workplace.
Between beams, we have suspended floor or roof slabs which are reinforced using just the same principles in compression, tension and reinforcing. They are arranged most economically to suit the particular project according to the basic principles below. Further detail will be covered in your third year of study.
A simple slab with smaller spans between supports will need no beams. This may apply in small buildings.
The "mushroom" is very economical, and used for large areas of space with no complications.
A development of the mushroom slab. The flat rib allows longer spans in the one direction.
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Typical of many buildings up until the last decade, the concept is simple in that beams pick up slab loads and convey them to columns. The slab is thinner than the previous examples.
The slab between these ribs can be very thin. It is beginning to resemble the hollow slab if you think of a thin slab on the underside as well.
The coffer or waffle slab provides an ideal combination of slab and beam, with a very reasonable overall depth for spans.
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THE COLUMN Columns transmit loads from the building to the foundations at given points. Intermediate walls do not carry any loads from higher levels. Because we have observed that gravity pulls VERTICALLY downwards, columns are normally vertical and straight. If you think of a brick, stone or other column which is very tall and thin, and time will cause, with increasing the load, for it to buckle or crush.
The same applies in concrete, and engineers refer to the SLENDERNESS RATIO of a column, where the ratio of width or thickness to height becomes structurally dangerous. If reinforcing is added, as in a beam, a given column will take much more load without buckling or crushing. The diagrams indicate the effects, the right-hand example above being stable because of its thickness.
In this example where the top and bottom of the column is rigidly fixed, buckling takes place in the centre, and in fact is more restrained, or stable.
In each of these cases, the one side of the column, as for a beam, is in tension and the other in compression. However, no one knows which direction it will buckle, and reinforcing is evenly and symmetrically placed.
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An oblong column is shaped thus to take overhead beams on the line of its longer axis OR to suit surrounding constructional elements e.g. a wall thickness of 230mm. The stirrups prevent failure by crushing.
SOME ADDITIONAL POINTS OF INTEREST Have you noticed that a major difference between concrete and steel structures is in bracing? Concrete columns and beams are much thicker than their counterparts in steel, so the joints between them as structural elements are far more rigid and secure. The terms "joints" is not really correct - "junction" being more appropriate - because they become MONOLITHIC, i.e. a single strong mass.
Furthermore, at the junctions, the reinforcing bars are designed to take bending moment loads, giving a structurally continuous frame. For very tall buildings, the equivalent of bracing may be introduced in the form of diaphragms or other. Another point to observe in multi-storey buildings is that at higher levels the columns remain the same size as the lower levels. Clearly the load is much lighter. It pays to keep a standard size because of formwork and finishes such as skirting, cornices and power trunkings. Cost savings are therefore achieved by reducing steel and cement. The upper levels are consequently made of stone and sand, with a little cement and steel added. The converse applies of course to the lower levels.
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WHY IS CONCRETE SO POPOLAR? Concrete, due to its fluidity may be moulded to an infinite variety of shapes, the limit being the cost of the formwork. Various forms are therefore relatively easy to make. Pipes and services maybe cast in. The finish of concrete, if properly detailed keeps its good appearance for a long time. Its inherent quality lasts indefinitely and in fact it becomes harder and stronger with age. Buildings, traditionally built of stone and brick which last a long time, quite naturally accommodate concrete aesthetically. Concrete is also a very natural material being made largely of sand and stone. One of the greatest advantages is resistance to corrosion.
THE ADVANTAGES OF FRAMED STRUCTURES The major change in buildings at the beginning of the 20th century was due to the steel and concrete frame replacing the masonry or brick load-bearing structure. The frame, with steel or reinforced concrete beam could span greater distances and thus larger openings were feasible, whether internally or in the envelope. The greater advantage however is that since no walls are load-bearing, they may be placed anywhere - providing great flexibility in planning. Furthermore walls may be of any material, provided they perform the basic functions such as privacy, resisting weather and thermal variations. Lighter partitions result in enormous structural savings - column and foundation sizes as well as the quantity of reinforcing. There is a great deal more to concrete which you will learn in your third year.
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SMALLER STEEL STRUCTURES The principles governing steel structures are the same as for concrete, but the method of using those principles is in some cases different and in overall APPERANCE very different. This is because steel is a very different type of material, and is very strong structurally and has extremely useful tensile qualities. Remember that there are almost innumerable qualities of steel, and it is simply manufactured to suit the desired function. The better the tensile qualities, the higher the cost. Because of its strength, it reads as a much lighter and elegant material than concrete. By its nature it is flexible, which has both positive and negative implications. Its depth-to-span ratio is far lower than for concrete. In a number of instances, steel structurally is less costly than timber.
STRUCTURAL SHAPES/SECTIONS
A flat steel bar will easily bend under an imposed load
A steel angle will resist a moderate load. However the vertical leg or flange, under compression will tend to buckle.
To resist such buckling, another flange may be added, giving a CHANNEL
To provide yet more strength, the I beam or Hsection beam is used very commonly, and due to its symmetry looks neat and attractive. The vertical is the web and it holds to top and bottom flange in places and prevents them buckling. Can you begin to see any similarity with the reinforced concrete beam? The flanges are positioned where all the work has to be done. Yet a further shape is available and that is the T-section, not as common as the above three. Note that all the above sections have a fairly 'heavy' cross-section compare with the light-gauge section which follow. The above are referred to as HOT ROLLED sections, referring to their process of manufacture. Engineers specify not only Sizes of section but also WEIGHT (kg per metre), to suit the loads to be carried.
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For columns, the I or H section is most commonly used. For trusses, a combination of sections is used. For shorter beam spans, the I section is used. Light gauge sections These are referred to as cold-rolled (or cold-formed) pressed or light gauge. An example is seen nearly every day in the form of household door jambs, in painted steel. These shapes are designed for lighter loads and smaller spans, and most commonly as roof purlins and vertical sheet cladding rails (or girts) for industrial buildings. NOTE that various gauges are available and the engineer chooses the most economical for the job. They are bent to from a flat sheet.
For drawing purposes NOTE that hot rolled sections are drawn in outline (and hatched at larger scales) wheras l.g sections are drawn in ONE single thickish line, with ROUNDED corners, as above. The former range from about 8 to 20mm thick and the later average 2mm. They MUST be easily distinguished on your drawings. Do not forget the tip on the l.g shapes.
COMMON STRUCTURAL FORMS We have seen that the I or H beam is used commonly for beams of shorter spans. Below is illustrated a direct extension of the I beam in castellated form and from there, trusses and other means of spanning space.
The next principle in steel structures is extremely important, and you need to understand it well because it will be needed repeatedly in the field of building for the rest of your life. It is the principle of bracing, or triangulation. We noted for concrete that joints between columns and beams were robust and monolithic. In steel, because of smaller joints, and also because of the flexibility of steel, trusses have to be braced. The principle was taught in your first year. A rectangular arrangement of members, even with weaker joints or pivoted joints will stabilise with a brace.
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The distance from A to C is greater than A to B. The tie or brace therefore prevents or restrains this tendency. If movement tended to be in the other direction i.e. to the left, a second cross stay would be appropriate. Two ties would be lighter and perhaps less costly than one stiff or rigid brace. For timber, this would be appropriate. The application to rectangular trusses of the above principle would then be as below. Work out the tendency of movement first THEN arrange braces to act in tension. NOTE the change in direction of diagonals in the centre. T = tension, C = compression
You will also see trusses as below where the triangles provide a rigid overall structure
Note on the previous diagrams where the downward arrows happen to be. If they were between the nodal points (or junctions, joints) the member would tend to deflect and an unwanted pattern of forces would be present. The top cord would also tend to buckle under compression. The purlins, which carry the roofing, are therefore placed AT THESE JOINTS where the load is distributed to 3 members at once.
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The lattice principle is also used to make up columns and quite often is continuous with the truss to make up a PORTIAL FRAME
A common application of the portal frame however is found with Hsection STANCHIONS (columns) and rafters.
Remember that engineers have to design roof trusses to take an upward lift as well. This is due to wind and because of the weight of the structure, is not as strong as the downward force. Furthermore, should there be freak storm winds; it is assumed that the sheeting or cladding will come away first, leaving the structure behind. So it should be designed. Before leaving bracing, note how steel framed buildings have additional bracing to prevent movement or deflection in the overall structure. Bracing is limited to a few bays only, keeping the whole structure stable.
When visiting steel structures of this nature, make a point of LOOKING for bracing members. These are often added to economise on the size of the element being braced, i.e. if the brace were absent, the element would be larger to withstand its deflection in buckling, twisting and other forms of failure. Page 12 of 15
To conclude this section on the principles of steel structures, you will have observed that steel sheet used for roofing and cladding is profiled (corrugated). This is doing the same thing as the I beam, where steel is placed at the top and bottom of a given depth to stabilise it along its length. The deeper the profile, the greater the span. The thicker the sheet, the greater the span.
For any particular job in hand, the engineer, for the most ECONOMICAL result arranges or designs the most suitable combinations. Greater spans are possible with heavier structural elements, but they then cost more. The OPTIMUM solution therefore applies. For steel, concrete or wood, the principles then are to construct the envelope of a THIN spanning material, which rests on a secondary (and perhaps also a tertiary) support system, which rests on the main structural frame, which rests on columns, which rest on foundations, which rest on the earth. (which is tied to the sun by gravity!) see next page
Fibre cement and concrete roof tiles have strength based on the depth of profile, just as for steel. Find an example of an architecturally designed steel structure. It must be a small scale building, for instance House Sue by Derick de Bruyn or some of the residential work by Pieter Venter. Explain diagrammatically which of the principles discussed above has been incorporated to make the building structurally sound.
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For a given problem where cost is a priority, the engineer has to design an optimum combination of spans, depth, shapes and gauges. As in concrete, the lattice principle may be used in two directions (in its simplest format) to give a space frame, having some similarity to the waffle slab. As a very rough guide the depth-to-span ratio of a, solid I beam is 1:50, a castellated beam 1:40 and a lattice or space frame 1:16-20. For concrete use 1:12-15 for the most conventional systems. DISCUSS steel in TENSION in structures; ditto timber. Discuss other sections e.g. tabular. List abbreviations e.g. rsj, hss - see AJ In conclusion, whether to choose steel for structural purposes as against other materials will be governed to some extent by its inherent qualities which have advantages and disadvantages. The advantages With shop fabrication, quick site erection is possible. Structural elements occupy less space. Large span-to-depth ratios are possible, with a lighter overall weight than concrete. The last two points make steel an elegant and aesthetically appealing material, especially when painted, perforated or shaped. The material is excellent for recycling, either in its original sectional form or melted down and re-formed. The disadvantages Without proper finishing in a good paint, or equivalent, corrosion from moisture is a problem, especially in a humid climate such as have in Durban. Re-coating fairly frequently is a serious maintenance cost problem. A fire with sufficient heat in close proximity will weaken steel and cause it to collapse under load. It may be covered in heat-resistant foam or cast in concrete. Under normal climatic thermal variations, movement is considerably more than that for concrete or brick. Not a serious drawback, but steel can be quite noisy. Why?
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SOME ADDITIONAL ASPECTS ON TIMBER STRUCTURAL PRINCIPLES In your first year you will have learned a good deal about timber trusses for houses and other small buildings. Find information about composite structures such as laminated beams, trusses with external integrated webs, walling and "skin" systems. List standard sizes of laminated timber beams and how far each of these can span. Compare this to a list of “normal” timber beams of similar sizes.
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