STRUCTURAL DESIGN FOR ARCHITECTURE

136

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.

Structural Design for Architecture

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