# What should be the minimum thickness for slab ceiling

Least thickness for two way slab construction designed as per methods provided by With adherence to ACI 318-11 Code, both direct design method and equivalent frame methods are used to determine the least thickness for two-way slab construction design. The least thickness of slab is indicated by ACI 318-11, section 9.5.3.

Deflection of two way slab should be measured with existing methods to keep the deflection problem free and the deflection will not go beyond serviceability limitations.

Least Thickness of Two Way Slab as per ACI 318-11 for Deflection Control.

The deflection of two way slab depends on several parameters like the flexural stiffness of the slab that is the function of slab thickness. The flexural stiffness of the slab is raised if the thickness of the slab is increased; consequently the deflection of the two-way slab is diminished.

To resist unnecessary deflections and avoid calculation of two way slab deflection that is a difficult method, ACI 318-11 controls the least thickness of two way slab with the use of three empirical limitations. It is not essential to accomplish the measurement of deflection if these limitations are fulfilled.

Lastly, slab thicknesses that remains under those acquired from empirical limitations can be applied if deflection calculation remains within specified limits which are obtained by Code.

1. For afm greater than 0.2 but not greater than 2, the slab thickness should not be less than the values of equation-1:

Where:
h: Minimum slab thickness
ln: clear span measured in long direction face to face of column or face to face of beam for slabs with beams

Ratio of clear span in longer direction to clear span in shorter direction

afm: Average value of (af) for all beams on the sides of a panel
af: the proportion of flexural stiffness of the beam section to the flexural stiffness of the slab bounded laterally by centerline of the panel on each side of the beam. Computation is done as per the following equation:

Where:
Ecb and Ecs: concrete modulus of elasticity of beam and slab respectively that is normally the same
Ib and Is: Moment of inertia of beam and slab respectively.

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