RCC Slab Design Guidelines:
a) Effective span of slab:
Effective span of slab shall be lesser of the two
1. L = clear span + d (effective depth )
2. L = Center to center distance between the support
b) Depth of slab:
The depth of slab depends on bending moment and deflection criterion. the trail depth can be obtained using:
- Effective depth d= Span /((L/d)Basic x modification factor)
- For obtaining modification factor, the percentage of steel for slab can be assumed from 0.2 to 0.5%.
- The effective depth d of two way slabs can also be assumed using cl.24.1,IS 456 provided short span is ?3.5m and loading class is <3.5KN/m2
Type of support
Or, the following thumb rules can be used:
- One way slab d=(L/22) to (L/28).
- Two way simply supported slab d=(L/20) to (L/30)
- Two way restrained slab d=(L/30) to (L/32)
c) Load on slab:
The load on slab comprises of Dead load, floor finish and live load. The loads are calculated per unit area (load/m2).
Dead load = D x 25 kN/m2 ( Where D is thickness of slab in m)
Floor finish (Assumed as)= 1 to 2 kN/m2
Live load (Assumed as) = 3 to 5 kN/m2 (depending on the occupancy of the building)
Detailing Requirements of RCC Slab as per IS456: 2000
a) Nominal Cover:
For Mild exposure – 20 mm
For Moderate exposure – 30 mm
However, if the diameter of bar do not exceed 12 mm, or cover may be reduced by 5 mm. Thus for main reinforcement up to 12 mm diameter bar and for mild exposure, the nominal cover is 15 mm.
b) Minimum reinforcement: The reinforcement in either direction in slab shall not be less than
- 0.15% of the total cross sectional area for Fe-250 steel
- 0.12% of the total cross-sectional area for Fe-415 & Fe-500 steel.
c) Spacing of bars: The maximum spacing of bars shall not exceed
- Main Steel – 3d or 300 mm whichever is smaller
- Distribution steel –5d or 450 mm whichever is smaller Where, ‘d’ is the effective depth of slab. Note: The minimum clear spacing of bars is not kept less than 75 mm (Preferably 100 mm) though code do not recommend any value.
d) Maximum diameter of bar: The maximum diameter of bar in slab, shall not exceed D/8, where D is the total thickness of slab.