A minimum load safety factor of 1-5 is employed for the design but, when many repetitions of loading are to be imposed over a particular section of floor, a fatigue failure may occur. To avoid this type of failure, the Portland Cement Association suggests that a stress ratio of 0-5 may be safely used for any number of repetitions of loading. The stress ratio is the ratio of the induced tensile stress in the loaded floor to the tensile strength of the concrete. As the stress in the floor is proportional to the load applied, the reciprocal of the stress ratio may be used as a load safety factor.
This repetition of loading occurs on floors when vehicles are travelling along an aisle; consequently a load safety factor larger than the minimum of 1- 5 should be applied to these loads. In this instance, one repetition of loading is considered to be a vehicle travelling laden and returning unladen along the aisle.