Concrete Floor Slabs Design Requirements, Equations and Calculator

Civil Engineering Design Knowledge

Floor slabs on grade should be analyzed for adequate structural live loads. Since floor slabs maybe designed for moving and extreme live loads, the design should be checked for stationary live loading conditions as well. The following equation may be used to find allowable loads. 

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Formula Floor Concrete Slab


w = the maximum allowable distributed stationary live load, pounds per square foot

s = the allowable extreme fiber stress in tension excluding shrinkage stress and is assumed to be equal to one-half the normal 28-day concrete flexural strength, pounds per square inch

k = the modulus of subgrade reaction, pounds per cubic inch

h = the slab thickness, inches

E =  the modulus of elasticity for the slab ( typically 4.0 x 106 pounds per square inch)

257.876 = Conversion constant

Further safety may be obtained by reducing allowable extreme fiber stress to a smaller percentage of the concrete flexural strength.  The design should be examined for the possibility of differential settlements which could result from non-uniform subgrade support. Also, consideration of the effects of long-term overall settlement for stationary live loads may be necessary for compressible soils.

Reference: Army Corps of Engineers, TM 5-809-12/AFM 88-3, Chap. 15