STRUCTURAL DESIGN FOR ARCHITECTURE

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

Reinforced concrete structures

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

the flexibility of the slabs. 139