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Some useful guidelines for RCC slab design

In this exclusive civil engineering tutorial, you will get some useful guidelines for designing any RCC slab.

A) Effective span of slab – It should be least of the two

1) L = clear span + d (effective depth)
2) L = Center to center spacing among the support

B) Depth of slab: The depth of slab is influenced by bending moment and deflection criterion. The trail depth is achieved with the following :-

Effective depth d = Span/((L/d) Basic x modification factor)

To get modification factor, the percentage of steel for slab is taken from 0.2 to 0.5.

The effective depth d of two slabs is also taken as cl.24.1,IS 456 on the condition that short span is 3.5m and loading class is <3.5KN/m2.

Categories of supports: Fe-250 – L/35, Fe-415 – L/28

Continuous support: Fe-250 – L/40, Fe-415 – L/32

The following thumb rules are commonly applied :-

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)

Load On Slab: The load on slab contains dead load, floor finish and live load. The loads are measured according to unit area (load/m2).

Dead Load = D x 25 kN/m2 (Here D denotes thickness of slab in m). Floor finish (taken as) = 1 to 2 kN/m2. Live load (taken as) = 3 to 5 kN/m2 (based on the occupancy of the building)

Nominal Cover
For mild exposure – 20 mm
For moderate exposure – 30 mm

When the diameter of bar does not go beyond 12 mm or cover is decreased by 5 mm. For main reinforcement up to 12 mm diameter bar and for mild exposure, the nominal cover is 15 mm.

Least reinforcement: The reinforcement in either direction in slab should not remain under :-

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

Distance of bar: The maximum distance of bars should not surpass. Main steel – 3d or 300 mm which is lower. Distribution steel – 5d or 450 mm whichever is lesser

Here, d denotes the effective depth of slab. The least clear spacing of bars should not be under 75 mm (desirably 100 mm). Highest diameter of bar. The highest diameter of bar should not go over D/8, here D denotes the total thickness of slab.

Some useful guidelines for RCC slab design