Minimum thickness for two way slab construction designed as per methods provided by ACI 318-11 Code namely, direct design method and equivalent frame method. The minimum thickness of slab is specified by ACI 318-11, section 9.5.3.
Deflection of two way slab could be calculated by using available methods to make sure that the deflection will not be problematic and will not exceed serviceability limitations.
Minimum Thickness of Two Way Slab as per ACI 318-11 for Deflection ControlThe deflection of two way slab is based on a number of parameters for example the flexural stiffness of the slab which is the function of slab thickness. The flexural stiffness of the slab is increased with increasing the thickness of the slab; as a result the deflection of the two-way slab is declined. In order to prevent excessive deflections and avoid calculation of two way slab deflection which is a complicated procedure, ACI 318-11 restricts the minimum thickness of two way slab by applying three empirical limitations. It will not be necessary to carry out deflection computation if these limitations are met. Finally, slab thicknesses smaller than those obtained from empirical limitations can be employed if deflection calculation is within specified limits which are determined by Code. ACI Code limitations for minimum thickness of two way slab are discussed in the following sections. 1. For afm larger than 0.2 but not larger than 2, the slab thickness must not be less than the values of equation-1:
Figure-1: Portion of slabs to be included for moment of inertia calculation, edge beam (left side) & internal beam (right side)
Figure-2: Dimensions of internal and external slab for moment of inertia calculations2. For (afm) larger than 2, the thickness of the slab must not be less than the value of equation-2:
Table-1: Minimum thickness of slabs without interior beams
- For slabs without drop panel 125 mm.
- For slabs with drop panel 100 mm.