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Often structural elements are made from more than one type of structural material to form composite structural elements. These combinations are made to exploit the different quali-ties of the materials to produce an element that performs better than one made from only one material. The usual combination is of a relatively cheap material such as concrete or masonry with a relatively expensive material such as steel. In these elements, the concrete or masonry carries the compressive stresses and the steel carries the tensile stress.

132 Building Structures: From Concepts to Design

By far the most common form of composite construction for building structures is rein-forced concrete. Reinrein-forced concrete, together with structural steelwork, is widely used throughout the world for a great variety of structures both large and small. Because con-crete has no useful engineering tensile strength, the steel, usually called reinforcement or re-bar, is placed in areas of the structural element where calculations predict tensile stresses. This is where diagrams like Fig. 4.18 are useful. This shows that there are tensile stresses along the bottom of the beam near the centre and these slope near the ends of the beam. So, to make a reinforced concrete beam, there would be longitudinal reinforcement in the bottom of the beam and sloping reinforcement towards the ends.

Fig. 4.65

Historically the sloping bars shown in Fig. 4.65 were used, but it is now more usual to resist the sloping tensile stresses by vertical reinforcement. This vertical reinforcement is bent into rectangles called links. The reinforcement is made into a cage by tying the bars together with wire.

Fig. 4.66

The cage is placed into a mould or form and the wet concrete is poured around the rein-forcement thus forming a composite element. As the concrete dries it shrinks and ‘grips’ the reinforcement. To aid this grip the reinforcing bars are often made with a rough pattern.

Fig. 4.67

Advanced concepts of stress 133 When the composite beam is bent by a moment, the principle of push/pull forces (see Fig. 3.43) still applies but because concrete cannot resist tensile stresses, the pull force is resisted by tensile stresses in the reinforcement.

Fig. 4.68

For the steel and concrete to act compositely the steel must not slip in relation to the con-crete. This is the same effect as that shown in Fig. 4.30 for horizontal shear stresses. The relative slip is resisted by horizontal shear stresses on the face of the reinforcement, these stresses are often called bond stresses.

Fig. 4.69

The behaviour of reinforced concrete when subjected to shear forces is very complex but, as can be seen from the principal stress diagram (Fig. 4.18), in areas of high shear there are diagonal tensile stresses. In unreinforced concrete these would cause cracks at right angles to the lines of tensile stresses.

Fig. 4.70

The role of the diagonal reinforcing bars or links is to provide tensile strength across these lines of tensile force.

134 Building Structures: From Concepts to Design

Fig. 4.71

Although in theory concrete only needs reinforcement in areas of tensile stress, except for very minor elements, it is usual to provide a complete cage of reinforcement. The rein-forcement that resists the tensile stresses is called the main reinrein-forcement and the other reinforcement is called nominal reinforcement. For the portal frame shown in Fig. 2.27, loaded on the cross-beam, the bending moments cause tensile bending stresses in both the cross-beam and the columns.

Fig. 4.72

Here the main reinforcement is placed in areas of tensile stress, but for practical reasons whole cages of reinforcement would be used for the cross-beam and the columns.

Fig. 4.73

It is not always possible to provide continuous reinforcement in areas of tensile stress, so reinforcing bars are ‘joined’ by lapping. The bars are laid next to each other in the mould and the concrete is poured around both bars. The force is transmitted from one bar to another by bond (shear) stresses in the surrounding concrete.

Advanced concepts of stress 135

Fig. 4.74

By lapping bars, parts of the reinforced concrete structure can be cast in a preferred sequence. In the case of the portal frame, the sequence would be foundations, columns and then the cross-beam. Bars would be left projecting from each part to be lapped with the reinforcement of the next part.

Fig. 4.75

Whilst reinforced concrete is the most common form of composite construction, structural steelwork and reinforced concrete can also be combined to form structural elements. This form is frequently used in spanning structures where the floor slab, a two-dimensional rein-forced concrete element, is also used as part of the main beams, acting compositely with the one-dimensional structural steel elements.

Fig. 4.76

To achieve composite action, the slab is joined to the top of the floor beams by what are known as shear connectors. These are pieces of steel, usually in the form of studs, welded to the top of the beam.

136 Building Structures: From Concepts to Design

Fig. 4.77

The concrete slab is cast around the shear connectors and these prevent the slab and the top of the steel beam moving in relation to each other. This allows horizontal shear stresses to develop between the slab and the steel beam as is explained on pages 95–96.

Fig. 4.78

Now the floor beam is the steel beam and the concrete slab. By the addition of the shear connectors, the concrete slab becomes part of the compression flange.

Fig. 4.79

Advanced concepts of stress 137 Another form of composite construction is to pre-stress materials such as concrete (or less often masonry). This is a technique that causes stress in structural elements before they are loaded. Like the addition of reinforcement, the purpose of pre-stressing is to add tensile strength to elements made of materials that can only resist compressive stresses. The prin-ciple can be illustrated by stressing together some match boxes with an elastic band. This pre-stressed element can now act as a beam.

Fig. 4.80

Here the stretched elastic band causes compressive stresses between the match boxes before there are stresses due to beam action. When the lateral load is applied it is resisted by internal push/pull forces that cause tensile and compressive bending stresses (see Fig.

3.40). Provided the numerical size of the pre-stressed compressive stress is equal or greater than the bending tensile stress the match boxes, with the pre-stress, will act as a beam. The stresses due to pre-stress and bending can be combined as shown in Fig. 3.83.

Fig. 4.81

Exactly the same principle is used to make pre-stressed concrete elements. The pre-stress is caused by tensioning the steel reinforcement and this can be done in two ways called pre-tensioning and post-pre-tensioning. For pre-pre-tensioning the steel reinforcement is tensioned by jacking against strong points fixed to the ground, then the concrete is poured around the tensioned reinforcement. When the concrete has hardened the jacks are released.

138 Building Structures: From Concepts to Design

Fig. 4.82

When the jacks are released the stretched reinforcement tries to shorten. But, because the hardened concrete has shrunk around tensioned reinforcement (see Fig. 4.68), it prevents the reinforcement shortening and by doing this goes into compression. So in the case of pre-tensioned pre-stressed concrete there are bond (shear) stresses between the (tensioned) reinforcement and the concrete before any load is applied.

Fig. 4.83

Concrete can also be pre-stressed after it has hardened, this is called post-tensioning. The concrete element is made with a hole through it, this hole is usually called a duct. The rein-forcement to be stressed is then threaded through the duct, in many cases the reinrein-forcement is put in the duct before the concrete is cast. When the concrete is hard enough the rein-forcement is tensioned by jacking it against the end of the concrete element. This causes tension in the reinforcement and compression in the concrete. When the required tension force has been obtained, the reinforcement is ‘locked off and the jacks removed. There are several methods of locking off and these depend on the proprietary method being used.

In the pre-tensioned method, the force in the tensioned reinforcement is transferred to the concrete along the whole length of the element but in the post-tensioned method, the force is transferred at the jacking points. This can sometimes require special end details to make sure the concrete is not over-stressed locally.

Fig. 4.84

Advanced concepts of stress 139 In the same way as eccentric applied loads cause axial stresses and bending stresses (see Fig. 3.75), if the tensioned reinforcement does not go through the centre of area of the sec-tion then the pre-stress will not be a constant axial stress. This can be an advantage as it can increase the size of the compressive stress in the part of the element that will have tensile stresses due to the applied load.

Fig. 4.85

The idea is that the stress distribution due to pre-stress is completely reversed under maximum applied lateral load. This means that, for a simple beam element, the stress at the top is zero due to pre-stress, and the stress at the bottom due to pre-stress and applied load is zero.

Fig. 4.86

The effect of the eccentrically applied pre-stress is to apply a moment to the element and this moment can be used to counteract the effect of the bending moment caused by the self-weight of the element. For concrete elements, the self-weight is a significant part of the total load. By careful adjustment of the pre-stress force and its position, the stress distribution due to pre-stress and self-weight can be made triangular, with zero stress at the top face.

Fig. 4.87

140 Building Structures: From Concepts to Design

When the maximum live load is applied, the stress diagram is reversed as before. It is usual for the maximum compressive stresses to be the maximum allowable for the concrete.

By using pre-stress in this way the concrete member can be used more effectively than a reinforced element because the maximum stress in a reinforced element has to include both self-weight and live load. Because of this the pre-stressed element can carry higher loads or alternatively be shallower for the same load than a reinforced element.

The pre-tensioning method means that the pre-stressing reinforcement has to be straight, but there is no restriction on the shape duct that can be cast into a concrete element. This means that the post-tensioning method allows the position of the pre-stressing force to be varied along the element. As the lines of the principal tensile are rarely straight, the ducts can be cast into the concrete along these lines.

Fig. 4.88

This has the advantage of putting compressive stresses into the element which directly counteract the tensile stresses caused by the applied load.

The method of pre-tensioning is ideally suited to the production of standard pre-stressed elements made in pre-casting yards. These types of elements are used extensively as floor spanning members or beams, usually called lintels, over openings in masonry walls. The method of post-tensioning is slower so its use in building structures is relatively rare and is limited to unusually large elements in major buildings. It is used extensively for large bridges. Often these are made from a number of pre-cast units that are stressed together exactly like the matchboxes (see Fig. 4.80).

Composite action can also be used to increase the ‘size’ of a beam in a masonry wall. This is done by making the masonry that is built on top of the concrete beam act with the beam.

Fig. 4.89

This is similar in principle to the action between the concrete slab and the steel beam shown in Fig. 4.78. Here the horizontal shear stresses that are essential for the composite action are resisted by the mortar joints, the beam and the masonry units (bricks or blocks).

Advanced concepts of stress 141 Many other examples of composite action could be given but the essential point is to understand the role played in the total structural action of the element by the different materials. This is done by understanding how each part of the element is stressed when it acts as part of a load path.

As buildings are constructed by joining together a variety of elements, walls, floors, windows, stairs, etc. it is important to be sure that loads do go down the chosen load paths and not into non-loadbearing elements that are not capable of carrying the loads. This is really composite action in reverse. For example, the portal frame when loaded will deflect.

If the portal frame is glazed as part of the building design, the glazing would try and pre-vent the portal frame from deflecting. This means that the glazing is acting compositely with the portal frame to ‘make’ a two-dimensional wall element.

Fig. 4.90

Unless the glazing is designed to be part of the load path it may fail as it tries to carry a share of the load due to composite action. In these cases special ‘soft’ joints or other devices must be introduced to prevent the composite action. These are exactly opposite to the idea of the shear connectors shown in Fig. 4.77. In this case the joint between the glazing and the portal frame must be designed so that the portal frame can deflect without loading the glazing.

4.8 Summary

This section shows how structural elements act when they are loaded as part of a load path.

This behaviour has been characterised by the stress distribution at each point of the ele-ment. These stresses are caused by the structural actions, axial bending and shear forces described in Chapter 3. The stress distributions in this section have been obtained by using the Engineer’s theory. These assumptions have been used by several generations of structural designers. Whilst the Engineer’s theory is still widely used, non-elastic theories are now also used. These theories are outlined in Chapter 6.

Part of the skill of designing structures is the prediction of the stress distribution in each element as it acts as part of a load path (or paths!). The accuracy of prediction will vary depending on the stage of the design process. For instance the exact size of elements may not need to be calculated at preliminary stages. However, it should be clear to the structural designers that the proposed types of elements, shells, slabs, I beams etc., will act

142 Building Structures: From Concepts to Design

effectively as their part of the load path. This is clarified if the stress distribution is known in principle. For instance if load-bearing walls are used at different levels and they cross at angles, then the whole wall will not be effective (see Fig. 3.31).

Fig. 4.91

Or again if an element is acting as a beam, then an I section is better than a + section (see page 73) and it might be worthwhile to vary the depth (see Fig. 3.57).

The central point is that structural design is not the result of a logical process but the result of an imaginative concept. For this concept to be successful it must be informed by a conceptual understanding of how the imagined structure will behave.