Structural forms precast forms

4.4.3.1 Introduction

In precast concrete construction the structural components are cast in a location which is different from that which they will finally occupy in the structure. They are normally manufactured in a precasting factory which is remote from the site but they may sometimes be cast on the site. The advantage of the latter is that it removes the constraints imposed by

the need to transport the components to the site. In either case the main advantage of precasting is the achievement of higher quality control, which results in higher strength, better durability and better surface quality than is possible with equivalent in situ concrete. It also reduces the time required to construct the building on site.

The particular advantages and disadvantages of precast concrete are summarised below.

Advantages

1 Almost all of the advantages of in situ reinforced concrete are obtained. The mater- ial has relatively high strength in compres- sion, flexure and tension and is therefore suitable for all types of structural element and for skeleton-framework arrangements. It is also durable and fire resistant, which facilitates the exposure and expression of the structure.

2 The precasting of concrete allows better quality control to be achieved. The material is therefore stronger than equivalent in situ concrete and has a better standard of surface finish. Structural elements can therefore be more slender and are less likely to require the application of a finishing material to bring them to a satisfactory visual standard.

3 Precasting allows greater complexity of the form of individual components to be achieved. This characteristic can be

exploited in several ways. In heavily serviced buildings, in which a large number of service ducts are required, it allows precast concrete columns and beams of complex cross-section to be used as frame elements and as service ducts. Precast concrete frames have therefore been fairly widely used for building types, such as hospitals and laboratories, in which the provision of services is a major factor in the design. The precasting technique has also been widely used for the manufacture of proprietary components with complex cross-sectional

shapes. ₁₄₁

13 The most famous building in which this technique was exploited was perhaps the Highpoint flats by Lubetkin and Arup.

4 Precasting of components leads to a much simpler site operation than is possible with in situ concrete. The building can therefore be erected more quickly with fewer tem- porary structures.

Disadvantages

1 One-off precast concrete systems tend to be more expensive than in situ equivalents. This favours their use for large-scale projects where economy of scale reduces the differ- ential. It is claimed by the precast concrete industry, however, that the overall building cost is reduced if a precast structure is used instead of in situ concrete, due to the simpler and shorter site operation.

2 Inflexibility. Precast concrete units must normally be manufactured some time in advance of their being transported to the site and installed in a building to give time for the concrete to gain adequate strength. (Sometimes the highest load to which a component will be subjected will occur during transportation or installation.) Late changes to the design of the building cannot therefore be readily accommodated.

3 Standardisation. Another aspect of inflex- ibility is that, because precast structures are assembled on site from components which are fairly large, it is convenient to maintain the overall geometry of the structure in as simple a form as possible.

There is considerable incentive to

standardise components so as to obtain the maximum re-use of moulds and to facilitate erection. Thus, while precasting offers the possibility that individual elements can have complex cross-sections and profiles, the overall form of the building must normally be relatively simple and repetitive. This is a disadvantage which precast concrete shares with steel.

4.4.3.2 Precast frame structures

Planning principles for precast concrete frames are the same as for in situ concrete frames: floors are normally of the one-way-spanning

type, in which case a rectangular beam- column grid is used but two-way-spanning floors, on a square column grid, are also possi- ble. Normally the structure consists of individ- ual beam, column and slab units which are erected on site in a similar way to a steel frame (Fig. 4.56); the joints between elements can be of the hinge or rigid type, depending on the detailing. If hinge-type joints are used (Fig. 4.57), additional bracing components are required and these can take the form of infill walls, either in situ or precast, or diagonal bracing. Where the beam-column joints are rigid the frames are self-bracing. Frequently in precast frame construction the joints between the individual units do not coincide with the beam-column junctions (Fig. 4.58). The units then have a fairly complex geometry which makes transport and stacking on site more difficult; the advantage is that it makes pos- sible the achievement of a self-bracing frame without the need for site-made joints which are of the rigid type.

Fig. 4.56 Layout of a basic precast concrete framework.

(a) Typical plan arrangement in which a beam-column frame supports a one-way-spanning floor slab.

(b) and (c) Slab units can be of rectangular or ribbed cross- section depending on the span.

142

Slab span

Beam span _(a)

Table 4.4 Span range and principal dimensions of basic precast concrete frames

Slab span Beam span Slab depth Beam depth (m) (m) (mm) (mm) 4 6.0 140 450 5 7.5 140 600 6 9.0 150 700 7 10.5 190 800 8 12.0 190 1000 9 13.5 190 1150 10 15.0 250 1300 11 16.5 250 1400 12 18.0 250 1500

It is normal for precast concrete frameworks to have a regular, rectilinear form so as to

maximise the standardisation of components but it is possible to adopt irregular grids. Typical element sizes for the normal span ranges of rectilinear frames are given in Table 4.4.

Columns are normally rectangular in cross- section but other shapes can be provided if this is necessary to accommodate irregular beam layouts or for architectural effect. The most favoured beam cross-section is the inverted T, as this facilitates the carrying of simple slab types. More complex shapes are used to accommodate changes in floor level.

Precast floor slabs are normally of the one- way-spanning type and are either solid, hollow-core or with a T-profile (Fig. 4.56c). All of these are ideally suited to a rectangular beam layout. Rhomboid plan-forms are fairly straightforward to produce but where the plan shape is highly irregular the section of floor involved is cast in situ.

Where vertical-plane bracing is provided by structural walls these can be precast units and can be used to provide support for floors as well as to resist lateral load. They are normally arranged as bracing cores around lift or stair wells. A reasonable number of bracing walls must be provided in two orthogonal directions and these should be arranged as symmetrically on plan as practicable.

4.4.3.3 Hybrid in situ and precast forms Hybrid structures, in which both in situ and

Fig. 4.57 Joints in precast concrete frameworks. All of the

joints shown are capable of transmitting shear and axial force only. They are therefore hinge-type joints.

precast concrete are used, allow the advan- tages of both forms of construction to be realised. The precast components will bring the benefits of factory production (high strength and efficiency, durability, good appearance, complex element cross-sections, dimensional accuracy, rapid erection) and the in situ parts allow complex or irregular overall forms and structural continuity between elements to be easily achieved.

The in situ and precast components in hybrid

Fig. 4.58 Where rigid joints are required in precast

arrangements, so that the structure is self-bracing, this is frequently accomplished by positioning the junctions between the elements at different locations from the beam-to-column connections.

Fig. 4.59 Hybrid in situ/precast structure. In this structure

precast concrete perimeter units are used in conjunction with an in situ concrete framework. The superior standard of finish which precasting allows is exploited to allow the units to be exposed on the exterior of the building. [Photo: British Cement Association]

ways. In the first of these the structure consists of a mixture of element types each of which is either precast or in situ (Fig. 4.59). The ratio of precast to in situ components can vary widely. At one extreme an in situ framework of beams and columns, with rectangular shapes of cross- section, might be combined with precast stair and ribbed-slab elements whose more compli- cated profiles are more easily achieved under factory conditions. At the other extreme, the use of in situ concrete might be confined to the making of continuous joints in a structure in which all of the principal elements were precast.

A second type of hybrid structure is one in which the individual elements are formed by a combination of precast and in situ concrete acting compositely. With this type of arrange- ment the precast parts of the structure are invariably used as permanent formwork on which the in situ parts are cast (Fig. 4.60). Thus a ribbed slab may be formed from a slender

precast soffit of complex shape on to which an in situ top is cast. Beams of complex shape can be formed in the same way. Composite hybrid structures are particularly well suited to the realisation of the advantages of both precast and in situ concrete.

The general arrangements of hybrid struc- tures are similar to those for precast forms. They are either beam-column frameworks or flat-slab arrangements with column grids which are either square or rectangular depend- ing, respectively, on whether the floor slabs are two-way or one-way-spanning systems.

Fig. 4.60 In this hybrid struc- ture precast beam and slab units act compositely with an in situ concrete topping slab.

4.4.4 Curved forms and structures of