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 .5KN/m2 Type of support Fe-250 Fe-415 Simply supported L/35 L/28 Continuous support L/40 L/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) 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.
Posted on: Sat, 15 Mar 2014 08:24:56 +0000
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