Stiffened Slab-On-Grade on Shrink-Swell Soil: New Design Method

Stiffened slabs-on-grade are one of the most economical solutions for 1- to 3-story buildings on shrink-swell soils. They consist, for example, of 1-m-deep, 0.3-m-wide beams resting on the soil, spaced 4m apart in both directions covered by a 0.1-m-thick slab-on-grade. The challenge is to optimize the beam depth, width, and spacing for the slab to be stiff enough that the movement of the soil at the edges of the slab will not cause undue bending of the foundation and the structure above it. Indeed, shrink-swell soils are affected by the weather and/or by the irrigation at the edges of the foundation and will swell and shrink as a result. This article presents a new method to design stiffened slabs on grade such that the differential movement divided by the length over which this movement takes place is within tolerable limits. The method gives the beam depth necessary to achieve such stiffness as well as the bending moment and shear force to be resisted. The method is automated in a spreadsheet called TAMU-SLAB.