Basic elements

Beams

Reinforced concrete beams are normally either rectangular in cross-section or combined with the slab which they support to form T- and L- shaped cross-sections (Fig. 4.37). The normal span range for reinforced concrete beams is 4.5 to 10 m. Spans of up to 20 m are occasionally used but depths of around 1.5 m and upwards are required for this, and a large volume of concrete is therefore involved. Spans greater than 20 m are possible but other types of structure will normally perform better in this span range. The depth which is required for beams depends on the span and the load which is carried. It is frequently determined from deflection rather than from strength requirements and in the normal span range a depth of around one twentieth of the span is required for a simply supported beam and one twenty-sixth of the span for a beam which is continuous across a number of supports. The breadth of a rectangular beam is usually around one third to one half of its depth.

Columns

Columns are constructed with a range of cross- sectional shapes, the most common being square, rectangular and circular. The primary reinforcement in columns is longitudinal reinforcement which contributes to the resis- tance of the compressive load and therefore reduces the required size of the cross-section. Transverse reinforcement, in the form of links, is also provided to give lateral support for the longitudinal reinforcement to prevent a burst- ing-type compression failure (Fig. 4.38). As in all compression elements, the principal factor which determines the dimensions which must

Fig. 4.37 T- and L-beams. Where reinforced concrete

beams support a reinforced concrete slab the elements are normally cast together and act compositely to form T- and L-beams.

Fig. 4.38 Reinforcement in a column. Columns are

provided with longitudinal reinforcement, to increase their compressive strength, and links to prevent buckling failure of the very slender reinforcing elements.

be adopted for columns is the need to avoid high slenderness, which would make the column susceptible to a buckling type of failure. The slenderness ratio of a concrete column is its effective length12 divided by its

least width and the British Standard (BS 8110, 'Structural Use of Concrete') recommends a maximum value of 60; 30 should be regarded as a practical maximum for most forms of construction, however, and to achieve reason- able load-carrying capacities, slenderness ratios in the range of 20-25 are normally adopted. Reinforced concrete walls perform in a similar way to columns and are made to conform to the same slenderness ratio require- ments. In the case of walls the slenderness ratio is the effective height of the wall divided by its thickness.

Slabs

Reinforced concrete slabs can be either one- way- or two-way-spanning structures. The behaviour of a one-way-spanning slab is similar to that of a rectangular beam, and these elements can in fact be regarded simply as very wide beams. The primary reinforcement consists of longitudinal bars placed parallel to the direction of span on the tension side of the cross-section (that is the lower part in the mid- span position of slabs and the upper part near the supports where slabs are continuous over a number of supports).

Secondary reinforcement, in the form of straight bars which run at right angles to the direction of the span, is provided to control shrinkage and to distribute the effects of concentrated load (Fig. 4.39). The reinforce- ment for a solid slab normally takes the form of a mesh in which the bars of the primary and secondary reinforcement are welded together at the points where they cross. Shear stresses in slabs are very low, except close to columns, and primary reinforcement is not normally provided to resist shear.

Fig. 4.39 Reinforcement in a slab. Slabs are provided

with mesh reinforcement close to the top and bottom surfaces. If the bars are of equal thickness and spacing in both directions the slab will be capable of spanning simul- taneously in both directions (two-way-spanning slab). If the degree of reinforcement in one of the directions is greater than in the other (larger bars at closer spacing) then the slab will span more effectively in that direction (one-way-spanning slab). In the latter case reinforcement is nevertheless provided in the non-span direction to distribute concentrated load and to control shrinkage of the concrete.

The depth of one-way-spanning slabs is normally approximately one thirtieth of the span for a simply supported slab and one thirty-fifth of the span for a slab which is continuous over a number of supports. The economic span for a solid slab is in the range 4 m to 8 m but this can be extended by using a ribbed form in which a proportion of the concrete in the lower half of the cross-section is removed. This type of slab is economic in the range 6 m to 12 m. If pre-stressing is applied the maximum spans are increased to

13 m for solid slabs and 18 m for ribbed slabs. A two-way-spanning slab spans simultane- ously in two directions and must be supported on beams, walls or rows of columns around its perimeter or at its corners. It should ideally have a square plan and the primary reinforce- ment is a square mesh of longitudinal bars; no secondary reinforcement is required. Two-way slabs are statically indeterminate structures and allow a more efficient use of material than

one-way slabs. Similar span-to-depth ratios are ₁₃₁

12 Effective length is based on the distance between points at which the column is supported laterally by other parts of the structure. Lateral support normally occurs at each storey level.

Secondary reinforcement to distribute the effect of concentrated load

Main reinforcement in base of slab resisting positive bending moment at mid-span

main reinforcement in top of slab over supports

Fig. 4.40 Coffered slab.

The coffered slab is a two- way-spanning flat-slab structure (i.e. a slab which is supported directly on columns). The coffers improve the efficiency by removing concrete from the tensile areas where it contributes little to the structural performance [Photo: Pat Hunt].

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used but the maximum economic spans are higher; solid slabs are used for spans of up to 8 m and this can be extended to 16 m if the weight is reduced by removing some of the concrete on the tension side of the cross- section, and to 20 m if pre-stressing is used. A system of intersecting ribs, which form a square grid, is then created on the soffit of the slab; the resulting form is said to be a coffered or 'waffle' slab (Fig. 4.40).

Stairs

Stairs are designed as reinforced concrete slabs whose depths are equal to the waist thickness of the stair, and which are normally an integral part of the structure (Fig. 4.41). The requirements in respect of the ratio of span to depth which is specified are the same as for one-way-spanning slabs.

Precast components

The precasting technique, in which concrete components are manufactured in a factory and transported subsequently for erection on site, allows higher quality control to be achieved than is possible with in situ concrete. Among the advantages which this offers are higher concrete strength for given mix proportions, better quality of finish, greater dimensional accuracy and the economies of scale which are associated with the factory process. The factory method of manufacture also allows more complicated shapes of cross-section to be achieved than are possible with in situ concrete. This, in turn, makes possible higher levels of structural efficiency and can also fa- cilitate the use of structural elements, such as beams and columns, as ducts for services. Precast concrete has therefore been widely

Fig. 4.41 Reinforced concrete stairs are simply one-way-

spanning slabs which span between the landings.

Fig. 4.42 Precast concrete flooring units. Precasting of

concrete allows higher levels of quality control than are possible with in situ casting, which allows greater strength to be achieved from a particular set of mix proportions. This permits the use of reduced thicknesses for a given load-carrying capacity and the use of more complex cross- sections. The efficiency of the units depicted here is improved by the introduction of voids into the core.

used in the context of heavily serviced build- ings where the combination of structure and services is desirable.

A wide variety of precast concrete propri- etary components is currentlyavailable ranging from simple rectangular-cross-section beams or slabs (Fig. 4.42) to complete framing systems for buildings (Fig. 4.43). In cases where no proprietary system is available, bespoke elements are used provided the scale of the project is such as to justify the setting up of the necessary production process.

Among the disadvantages of precasting are the difficulties associated with connecting elements together satisfactorily. Precasting also favours the adoption of building forms which are regular and repetitive because this simplifies the erection process and allows maximum advantage to be taken of

economies associated with the mass produc- tion of identical units. The restrictions on form which are associated with steel frame- works are therefore also a feature of precast concrete structures.

Fig. 4.43 Precast concrete framework. The precasting

technique can be applied to all types of structural element and makes possible the construction of complete precast frameworks.

Precast components are frequently

combined with in situ concrete and this allows the advantages of both forms of construction to be enjoyed (Fig. 4.44). The use of in situ concrete at the junctions of precast frameworks can solve the problems associated with joint-

Fig. 4.44 Composite in situ and precast concrete. Precast

units of complex shape are used here in conjunction with in situ concrete to form a composite floor deck with an efficient 'improved' cross-section.

fit' problem and the provision of structural continuity. In situ concrete can also be used to allow more complex or irregular overall forms to be adopted.

4.4.2 Structural forms - cast-in-situ forms