DESIGN OF SLAB
Slabs are generally designed on the assumption that they consists of a number of beams of breadth ‘one meter’.
Effective Span
The effective span of a simply supported slab shall be taken as the lesser of the following:
 Distance between the centers of bearings,
 Clear span plus effective depth
Thickness of Slab
The following table gives the maximum values of the ratio of span to depth.
Type of slab
 Ratio of span to depth

Simply supported and spanning in one direction  30 
Continuous and spanning in one direction  35 
Simply supported and spanning in two directions  35 
Continuous and spanning in two directions  40 
Cantilever slabs  12 
Reinforcement
Minimum reinforcement in either direction shall be 0.15 percent of total crosssectional area.
Main reinforcement which is based on the maximum bending moment shall not be less than 0.15 per cent of the gross sectional area. The pitch of the main bars shall not exceed the following:
 Three times the effective depth of slab, and
 45 cm.
Distribution bars are running at right angles to the main reinforcement and the pitch shall not exceed
 Five times the effective depth of slab, and
 45 cm.
The diameter of main bars may be from 8 mm to 14 mm. for distribution bars, steel 6 mm or 8 mm are generally used.
Cover of Reinforcement:
The minimum cover to outside of main bars shall not be less than the following:
 15 mm and
 Diameter of the main bar.
Steps to be followed in the design of slab
 Assuming suitable bearings (not less than 10cm), find the span of the slab between the centers of bearings.
 Assume the thickness of slab (take 4 cm per metre run of the span).
 Find the effective span which is lesser of (i) distance between centres of bearings, and (ii) clear span and effective depth.
 Find the dead load and the live load per square meter of the slab.
 Determine the maximum bending moment for a one meter wide strip of the slab.
The maximum bending moment per meter width of slab,
Where, w = total load intensity per square meter of the slab.
 Equate the balanced moment of resistance to the maximum bending moment
Find the effective depth ‘d’ from the above equation.
 Calculate the main reinforcement per metre width
For M15 concrete, lever arm = 0.87 d
Spacing of bar =
CONTINUOUS SLAB
Suppose a slab is supported at the ends and also at intermediate points on beams, the maximum sagging and hogging moments to which the slab is subjected to due to uniformly distributed load, can be computed as follows:
Let = intensity of dead load per square metre
= intensity of live load per square metre.
Bending moment due to dead load and live load may be taken as follows (IS: 456 – 2000)
 At middle of end span  Over support  At middle of interior support  Over interior support 
BM due to dead load  
Bending moment due to live load 