DESIGN OF SLAB (IS:456-:2000)

STEPS FOR DESIGN OF SLABS

Slabs form floors and roofs of building. They are generally assumed to carry uniformly distributed loads. Slabs are generally designed on the assumption that they consists of a number of beams of breadth ‘one meter’. The following steps are used in designing a slab :

Effective Span

The effective span of a simply supported slab shall be taken as follow

• L = clear span + d (effective depth )
• L = Center to center distance between the support.

Thickness of Slab

 Maximum values of span to depth ratio for slab.

Type of slab	Ratio of span to depth (L/D)
Simply supported slab spanning in one direction (one way slab)	30
Continuous slab spanning in one direction (one way slab)	35
Simply supported slab spanning in two directions (two way slab)	35
Continuous slab spanning in two directions(two way slab)	40
Cantilever slabs	12

Reinforcement

The reinforcement for a one way slab consists of main bars and distribution bars (temperatures bars). These two sets of bars form a mesh of reinforcement.

a) Main Bars : This is calculated by equating moment of resistance and max. bending moment. 

When Fe 415 grade steel is used,

• The minimum reinforcement shall not be less than 0.12% of the gross sectional area.

• The diameter of main bars may be 8 mm to 10 mm is used or shall not exceed one eight of the thickness of the slab.

For Fe 250 grade steel is used, 

• The minimum reinforcement shall not be less than 0.15% of the gross sectional area.

• The diameter of main bars may be 10 mm to 12 mm is used.

b) Distribution Bars : these are reinforcement provided running at right angles to the main bars in order to distributes the load and the temperature and shrinkage stresses.

• When Fe 415 grade steel is used, the minimum reinforcement shall not be less than 0.12% of the gross sectional area. The diameter of distribution bars may be 8 mm is used.

• For Fe 250 grade steel is used, the minimum reinforcement shall not be less than 0.12% of the gross sectional area. The diameter of distribution bars may be 6 mm to 8 mm is used.

c) Spacing : the gap between the two consecutive reinforcement bars is called spacing.

• maximum spacing of main bars : shall not exceed 3 times the effective depth of the slab or not exceed 300 mm.

• maximum spacing of distribution bars : shall not exceed 5 times the effective depth of the slab or not exceed 300 mm.

note : The minimum spacing of bars shall not be less than 75 mm for satisfactory concreting 

d) Clear Cover

The minimum cover to outside of main bars shall not be less than the following:

• 15 mm and

• Diameter of the main bar.

Steps In Design Of Slab :

1. Assume the suitable bearing (not less than 100 mm) find the span between centre of bearing.

2.  Assume the thickness of slab (take 40 mm per meter run of the span. 

3. Find the effective span which is lesser of

• distance between centres of bearings.
• clear span and effective depth.

4. Find the dead load and the live load per square meter of slab.

5. 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.

6. Equating the moment of resistance to the maximum bending moment and find the effective depth required for flexural strength consideration.

7. calculate the main reinforcement per metre width of the slab.

for M15 concrete lever arm = 0.87d

8. Spacing of bar =