Structural forms cast-in-situ forms 1 Introduction

Reinforced concrete cast-in-situ forms are used principally for multi-storey buildings in which a frame-type structure is required. The form of the structure is determined by the same factors as influence the design of all structures, which are the need to provide effective resistance to

Fig. 4.45 Four basic types of reinforced concrete struc-

ture.

(a) Two-way-spanning ribless flat-slab. (b) One-way-spanning ribbed flat-slab.

(c) One-way-spanning slab supported on rigid frame. (d) One-way-spanning slab supported on loadbearing walls.

both gravitational and lateral load, and to achieve reasonable economy in the use of material. Although the mouldability of

concrete theoretically gives the designer a wide choice of frame geometries, the need to minimise costs normally favours the use of rectilinear arrangements which require simple patterns of reinforcement and formwork. The variety of multi-storey forms which are used in

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Table 4.1 Span ranges and size requirements for the four basic types of in situ reinforced concrete structure

Structure type Span range (m) Span/Depth Column/wall Reinforced Pre-stressed Reinforced Pre-stressed slendemess

One-way-span slab 4X6 to 8X11 8X11 to 8X14 25 36 15 to 20 Two-way-span slab: solid 4X4 t o 6 X 6 6X6to10X10 25 to 30 30 to 35 15to20 coffered 6X6 to 18X18 8X8 to 20X20 35 30 15 to 20 Beam/Column frame:

one-way-span slab 3X6 to 6X12 6X12 to 8X15 slab 36 36 15 to 20 beam 15 to 20 20 to 25

two-way-span slab 4X4 to 8X8 8X8 to 15X15 slab 36 36 15 to 20 beam 15 to 20 20 to 25 Loadbearing wall: one-way-span slab 3 to 12 36 15 to 20 two-way-span slab 3X3 to 12X12 36 (b) (a) (c) (d)

practice is nevertheless considerable and they are categorised here into the four basic types of one-way-spanning flat-slab structures, two- way-spanning flat-slab structures, beam-and- slab structures and loadbearing-wall structures (Fig. 4.45).

All four basic types are beam-and-post arrangements and can be considered to consist of a floor deck system supported on a vertical structure of either columns or walls. Their properties, span capabilities and sizing requirements are summarised in Table 4.1.

In all cases, the design of the floor deck is determined principally by the requirements of gravitational load and this in turn dictates the pattern of the column or wall grid which is provided to support it. One-way-spanning floor systems are best carried on rectangular grids while two-way systems perform better on column or wall grids which are square. In either case the grid is normally kept as regular as possible for reasons of economy but it need not be perfectly regular.

The principal effect of lateral loads is upon the design of bracing systems. In many cases this will have no influence on the overall form of the structure because the beam-column arrangement will be self-bracing due to the high level of structural continuity which is possible with reinforced concrete. Some reinforced concrete structures require bracing walls for stability, however, and where these are necessary the internal planning of the building is affected.

Only the most basic, regular forms of each type of structure are described here to give an indication of the general arrangements and span ranges for which they are suitable. Often these basic forms are manipulated and distorted to produce more complex structural geometries (see Section 2.5).

4.4.2.2 One-way-spanning flat-slab structures In this system the floor slab spans one way between rows of columns (Fig. 4.46). The arrangement is also known as ribbed-slab because the slab is frequently given a ribbed cross-section in order to improve its efficiency by removing concrete from the tensile side of

Fig. 4.46 One-way-spanning flat-slab system. The ribbed

version depicted is used at the long span end of the span range for improved efficiency. For short spans, one-way- spanning flat-slabs have a simple rectangular cross- section.

the cross-section (Fig. 4.46). The ribs follow the span direction of the slab. The voids between the ribs are stopped short of the strip of slab between the columns and this solid area acts as a beam spanning between the columns. As this is a one-way-spanning system the column grid is rectangular with the slab spanning parallel to the long side of the rectangle. The normal span range i s 4 m X 6 m t o 8 m X l l m but the maximum span can be increased to 8 m X 14 m if pre-stressing is used. The span/depth ratio is normally around 25 but can be as high as 36 in pre-stressed versions. Ribbed-slab structures are braced either by rigid frame action or by walls acting as diaphragm bracing in the vertical plane.

The particular advantages of the ribbed-slab system are the relative simplicity of the construction and high structural efficiency, which allows relatively long spans to be achieved with a small volume of concrete and a small depth of structure.

4.4.2.3 Two-way-spanning flat-slab structures In two-way-spanning slab structures the floor deck system consists of a two-way-spanning flat plate of reinforced concrete which is supported directly on a grid of columns (Fig. 4.47). The column grid is normally square to

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Drop panel

structure is highly statically indeterminate and the resulting structural continuity allows the necessary strength and rigidity to be achieved with great efficiency. The economic span range for a solid slab is 4.5 m to 6 m which is increased to 10 m if pre-stressing is used. The span/depth ratio is typically in the range 25 to 30 for reinforced slabs and 30 to 35 for pre- stressed slabs. An indication of typical slab depths is given in Table 4.2.

Fig. 4.47 Two-way-spanning flat-slab system. _{The critical stresses in solid slabs are shear}

stresses which are very high in the vicinity of the supporting columns. The strength of the floor can be increased by thickening the slab locally at these points by a system of 'drop' panels or by using 'mushroom-type' column caps (Fig. 4.48); alternatively the slab can be strengthened in shear in the vicinity of the columns by the use of extra reinforcement in the form of steel shearheads (Fig. 4.49). These variations allow heavier loads to be carried or higher span-to-depth ratios to be achieved. The use of shear heads also allows voids for services ducts to be located close to the columns.

Fig. 4.48 Column caps and drop panels. The column cap

and drop panel can be used independently or together to increase the shear strength of flat-slabs in the vicinity of columns where high shear forces require a greater thick- ness of slab.

Table 4.2 Overall slab thickness and column widths for two-way-spanning flat-slab structures

Column spacing Slab thickness Column width (square column (mm) (mm) grid) (m) Solid Coffered 4 150 - 200 5 175 - 200 6 200 300 250 7 250 300 250 8 275 300 250 9 300 400 300 10 400 300 12 - 500 400 14 - 500 500 16 - 600 600 18 - 700 700

Fig. 4.49 Steel shearheads, which are cross-shaped

arrangements of steel plates joined by welding, are an alternative method to column caps or drop panels for increasing the shear strength of flat-slabs in the vicinity of columns.

The maximum size of column grid of this type of structure can be increased to around

18 m for reinforced slabs and 20 m for pre- stressed slabs if a coffered system is used to reduce the weight of concrete present in the underside of the slab. The span/depth ratios for coffered slabs are around 25 for

reinforced slabs and 30 for pre-stressed slabs.

The two-way-spanning slab system is de- pendent, to a large extent, on structural conti- nuity for its strength. It performs best (i.e. allows the highest ratios of span-to-depth) with column grids which have at least three bays in each direction and in which the varia- tion in the sizes of the bays is kept to a minimum. The efficiency of the system is such, however, that slab thicknesses are approxi- mately the same as those in frame structures of equivalent span, i.e. a system in which the slab is supported on downstand beams.

Fig. 4.50 Typical arrangements of vertical-plane bracing

for flat-slab structures, which should be as symmetrical as possible, are shown here.

Where a thin, solid slab is used its stiffness can be insufficient to allow rigid-frame action to develop with the columns in response to lateral load. Extra bracing must therefore be provided and in situ concrete walls are usually incorporated into the structure for this purpose. These must be arranged in two mutually perpendicular directions and can usually be accommodated around lift or stair towers (Fig. 4.50). In the long-span part of the range, that is for spans greater than around

10 m, the depth of the coffered slab which is required will be greater than 350 mm and the slab is usually sufficiently stiff to allow rigid- frame action to develop between the floor and columns, in response to lateral load; in these cases no additional bracing is required.

The high degree of statical indeterminacy which is associated with the two-way-spanning slab allows greater flexibility in the column grid than is possible with frame structures. The small construction depth of flat-slab structures, compared to frame structures, also facilitates the easy accommodation of a services zone and will usually result in a lower overall storey height

Fig. 4.51 Plan arrangements for reinforced concrete

beam-column frames.

(a) One-way-span slab spanning between parallel beams. (b) Two-way-span slab on a grid of beams.

Due to the simplicity of the formwork and the high level of structural efficiency which is achieved as a result of the high degree of stat- ical indeterminacy, two-way-spanning slab structures of both the plain and the coffered types provide very economical support systems for multi-storey buildings in which large wall- free areas are required. They are particularly suitable where the imposed load is high and uniformly distributed but are less suitable where concentrated loads are high, for example in buildings in which machinery has to be housed. They are also unsuitable for buildings in which the benefits of structural continuity are limited, either because large breaks in the continuity of floors occurs (Fig. 2.14) or because the building has a compli- cated geometry in plan and section (Fig. 4.15). In such cases a frame structure is normally required.

4.4.2.4 Frame structures

The distinctive characteristic of a frame is that it consists of an arrangement of beams and columns which supports slab floors. There are two basic types of reinforced concrete frame, the distinguishing feature being whether the floor is a one-way-spanning or a two-way- spanning system (Fig. 4.51).

In the frame with a one-way-spanning floor the floor-slab spans as a continuous system

Fig. 4.52 Beam-column frame with one-way span slab.

One-way span slabs are normally carried on parallel arrangements of beams supported by a rectangular column grid. No beams are provided in the direction of the slab span.

across a series of beams which are supported individually on columns. The beams act with the slab to form flanged T- or L-beams (Figs 4.37 and 4.52). Linking beams, running in the same direction as the slab, are not normally provided between the columns because the slabs themselves provide an adequate struc- tural connection. The whole structure of beams, columns and slabs is cast in stages to form a continuous monolithic unit. Typical

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5m to 10m

Table 4.3 Span range and principal dimensions of reinforced concrete frame structures

Slab span Beam span Slab thickness Beam depth Column width

from top of slab)

(m) (m) (mm) (mm) (mm) 3 4.5 125 350 200 4 6.0 150 420 250 5 7.5 175 520 275 6 9.0 200 670 275 7 10.5 225 780 275 8 12.0 275 900 300 9 13.5 300 1060 300

dimensions for this type of arrangement are given in Table 4.3.

Frames of this kind can extend to a large number of bays in each direction and are best planned on a rectangular column grid with the slabs spanning parallel to the short side of the rectangle. The normal span range is 3.5 m X 6 m to 6 m X 12 m for reinforced concrete and this can be extended to 8 m x 15 m if pre- stressing is used. The most economic grid ratios vary between 1:1.5 at the short-span end of the range to 1 : 2 for longer spans. The span/depth ratio is around 36 for the slab and 15 to 20 for the beam.

So far as resistance to lateral load is

concerned the one-way-spanning frame is rigid and self-bracing in the plane of the

beam/column frames, due to the relatively high stiffness of the beams and columns and the rigid joints which exist between them, but is not stable in the direction of the span of the slab, because the stiffness of the slab is normally insufficient for effective rigid-frame action to be possible (Fig. 4.53). Additional bracing is therefore necessary and this is normally provided in the form of in situ concrete walls which act as vertical-plane diaphragm bracing (Fig. 4.51). These are constructed by simply extending pairs of columns to fill the space between them. As with other types of vertical-plane bracing it performs best if it is disposed around the building in a symmetrical arrangement and can usually be conveniently located at stairs and service ducts. The need to position the verti-

cal-plane bracing correctly is a factor which affects the internal planning of buildings which have this type of structure.

Where a two-way-spanning slab is used in conjunction with a grid of beams it is neces- sary that the slab spans should be more-or- less the same in each direction. The column grid must therefore be more-or-less square (Fig. 4.51). The two-way-spanning floor struc- tures have higher degrees of statical indeter- minacy than one-way systems and this allows thinner slabs and shallower beams than the equivalent one-way system; it also increases the maximum economic span for a solid slab to about 8 m. Because frames with two-way- spanning floor systems have beams running in two mutually perpendicular directions they are completely self-bracing and do not require any

Fig. 4.53 Bracing of one-way-span frame systems by rigid

joints is ineffective in the direction of the slab span due to

Fig. 4.54 Plan arrangements for loadbearing-wall struc-

tures of reinforced concrete.

structural walls for stability. This is sometimes the reason why a frame, as opposed to a flat- slab structure is adopted.

The frame types which have been described here are the most basic forms. Considerable variation from these is possible although this will normally increase the cost of the structure. The simplest variation is the displacement of individual columns from a strictly regular grid, in order to accommodate some aspect of the space-planning of the interior. If the displace- ment is kept within one quarter of the span it can be accommodated easily by strengthening the structure locally. Another common vari- ation is a small change in the level of a floor over a short area of the plan; this too can be easily accommodated in in situ reinforced concrete, as can the ramps and stairs which are required for access. More significant varia- tions from the standard forms are illustrated in Figs 2.8, 2.10 and 2.14.

Fig. 4.55 Plan arrangement for reinforced concrete

loadbearing-wall structure with one-way-spanning floor slabs. Note that this conforms to the parallel-wall arrange- ment which is typical of loadbearing wall-structures in all materials.

4.4.2.5 Loadbearing-wall structures

The planning principles for reinforced concrete loadbearing-wall structures are similar to those which are used for masonry structures

although the arrangement of walls is normally simpler because the greater flexural strength of concrete eliminates the need for local stiffen- ing to combat buckling. Three plan-forms are commonly used: cross-wall and spine-wall, with one-way-spanning floors, and cellular, with two-way-spanning floors (Fig. 4.54 and 4.55). The spacing between the walls is kept as uniform as possible so that the slab spans are equal and maximum benefit is obtained from continuity. The most economic spans are around 5 m to 6 m. In cross-wall and spine- wall structures, in which the loadbearing walls

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Plan showing layout of rooms

Plan showing structural elements Non-structural face panels

Non-structural partitions 1m 6.9 m 5m 2.5m 2.5 m 3.7 m 2.5 m 2.5 m 5m

are parallel to one another, it is necessary to brace the structure with a limited number of walls running in the orthogonal direction; these can normally be located around stairs and service cores.

The advantages of reinforced concrete over masonry for this type of structure is that it allows the structural plan to be simpler and gives much more planning freedom generally: it is, for example, possible to omit some of the loadbearing walls at occasional locations and bridge the gap by reinforcing the wall above so that it acts as a deep beam.13 The disadvantage

of reinforced concrete is that it is almost invariably more expensive than the equivalent masonry structure.

4.4.2.6 Summary of cast-in-situ forms

\n situ reinforced concrete structures perform best in circumstances in which imposed loads are high and they are therefore used principally to support multi-storey commercial and indus- trial buildings and only rarely as the structures for low-rise domestic-type buildings or single- storey buildings.

The main alternative to reinforced concrete for the multi-storey building is steel and the particular advantages of reinforced concrete are that it allows complex and irregular forms to be achieved more easily than with steel and that it is both durable and fire resistant, which eliminates the need for finishing materials.