Monday, June 4, 2018

Reinforced Concrete Slab Design and Detailing Guide IS456: 2000


Reinforced concrete slab design and detailing guidelines for depth of slab, loads on slab, reinforcement guide for one-way and two-way slabs as per IS 456:2000 have been tried to present here.
Following are the RCC Slab Design and Detailing guidelines:

Reinforced Concrete 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 supportFe-250Fe-415
Simply supportedL/35L/28
Continuous supportL/40L/32
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)
concrete-slab

Detailing Requirements of Reinforced Concrete 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.

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