The design of industrial and logistic building's slab-on-grades is a complex exercise. The design needs to consider the different loading types and configurations (uniform or alternated loading, racks, live loadings…) together with the relative positions from the hinged constructions joints to the loads, whose position and intensity can vary during the life of the structure. The non-uniform stress reaction distribution in the soil reinforced with rigid inclusions creates an additional stress in the slab with a different pattern than the ones of the loads and of the joints. The optimization of the design of the slab becomes a complex problem with three different intertwined patterns (loading, joints, and rigid inclusions) that can move relative to one another with usually no typical symmetry conditions. Existing code of practice dedicated to slab-on-grades are only able to consider uniform soil conditions and the typical size of those structures forbid the modelling of the full extent of the slab. Through the decomposition of this complex problem into the sum of three unit variable-separated problems, this paper presents a simple and comprehensive method to take into account all the parameters of the equation. This method is a powerful solution which is easy to use while allowing for the precise optimization of the design of slab-on-grades. The approach has been validated and calibrated with an extensive number of finite element calculations and has been integrated in the French ASIRI national research program in France.