IS SPECIFICATION FOR RCC SLAB DESIGN

RCC SLAB DESIGN SPECIFICATION :

Guidelines for RCC slab design for depth of slab, loads on slab, reinforcement guide for one-way and two-way slabs have been tried to present here. Following are the RCC Slab Design and Detailing guidelines.

1. Effective Span Of a Slab :

Unless otherwise specified, the effective span of a slab  shall be taken as follow.

a) Simply supported slab : Effective span of slab shall not be more than of the two. 

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

b) effective cantilever length : Effective length shall be taken as its length to the face of the support plus the half the effective depth except where it forms the end of the continuous slab where the length up to the centre of the support shall be taken.

c) Bearing for a slab : The length of bearing to support a slab on the wall may be from 100 mm to 150 mm.

2. Depth Of Slab:

The depth of slab depends on bending moment and deflection of a slab. The trail depth can be obtained for different types of slab by using :

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

• for one way simply supported slab :

Effective depth d= Span /20 x modification factor).

• for one way continuous slab (both end cont.)

Effective depth d= Span /23 x modification factor).

• For one way continuous slab  (one end cont.)

Effective depth d= Span /26 x modification factor).

• For cantilever slab :

Effective depth d= Span /26 x modification factor).

• For obtaining modification factor, the percentage of steel for slab can be assumed from 0.2 to 0.5%.

• For two way slab, the shorter of the two spans should be used for determining the span to effective depth ratios. Two way slab of shorter span up to 3.5 m the span to overall depth ratios given below may generally be assumed to satisfy vertical deflection limits for loading class is up to 3.5 KN/m²

Types of support	Fe-250 (mild steel)	Fe-415 (high strength steel)
Simply supported	L/35	L/28
Continuous support	L/40	L/32

Or, the following rules can be used:

• Two way simply supported slab d=(L/20) to (L/30)

• Two way restrained slab d=(L/30) to (L/32)

3. Load On Slab:

The following types of load working on a slab is given as :

• Dead load
• floor finish 
• and live load

• Dead load : It is the self weight of building can be calculated by the formula 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)

4. Reinforcement For Slab As Per IS 456: 2000

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.

• 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) Cover To Slab Reinforcement : 

The minimum cover to outside of the main bars shall not be less than  15 mm or diameter of main bars.