BASIC 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	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.


Steps to be followed in the design of slab :

1. Assuming suitable bearings (not less than 10cm), find the span of the slab between the centers of bearings.
2. Assume the thickness of slab (take 4 cm per metre run of the span).
3. Find the effective span which is lesser of (i) distance between centres of bearings, and (ii) clear span and effective depth.
4. Find the dead load and the live load per square meter of the slab.
5. Determine the maximum bending moment for a one meter wide strip of the slab.

The maximum bending moment per meter width of slab,

Where, w = total load intensity per square meter of the slab.

1. Equate the balanced moment of resistance to the maximum bending moment

Find the effective depth ‘d’ from the above equation.

1. Calculate the main reinforcement per metre width

For M15 concrete, lever arm = 0.87 d

Spacing of bar =

CONTINUOUS SLAB :

Suppose a slab is supported at the ends and also at intermediate points on beams, the maximum sagging and hogging moments to which the slab is subjected to due to uniformly distributed load, can be computed as follows:
Let

= intensity of dead load per square metre

= intensity of live load per square metre.


Bending moment due to dead load and live load may be taken as follows (IS: 456 – 2000)
 

	At middle of end span	Over support	At middle of interior support	Over interior support
BM due to dead load
Bending moment due to live load