Background WRI analysis and design is performed in accordance with the design methodology presented in Design of Slab-On-Ground Foundations - A Design, Construction & Inspection Aid for Consulting Engineers, prepared by Donald Snowden for the Wire Reinforcement Institute and the Concrete Reinforcing Steel Institute. This guide was first published in 1981. An update was published in 1996. The update did not change the design method in any material fashion. The document is available for free at http://www.wirereinforcementinstitute.org. The WRI method is a variation of the BRAB design method, and uses the same support index and assumed support conditions presented in the BRAB report. The major modification is the use of a cantilever length adjustment factor and the use of the cantilever lengths to directly calculate shear, moment and deflection. Design Methodology Similar to the other three design methods implemented in SlabWorks™, the WRI manual provides a method of predicting the maximum moment, shear and deflection occurring in a slab-on-grade. However, diverging from the other design manuals, the WRI manual does not provide any design guidance beyond development of design loads and deflections. For example, the manual does not define a maximum allowable deflection (although L/480 is suggested), does not define maximum beam spacing and does not define minimum steel ratios. Similar to the BRAB report, the WRI method determines moments, shears and deflections in the slab by assuming that the slab is partially supported by the underlying soils. The soil is assumed to be rigid in the areas of support (inelastic support conditions), and the interface is assumed to transfer both tensile and compressive stresses. Two design conditions are considered (see BRAB Figure 2), loss of support at the edges (center lift/edge settlement) and loss of support at the interior (edge lift/center settlement). The design cantilever used by WRI is based on the same support index used by BRAB (Appendix 2). The support index is a function of the climatic rating and the effective plasticity index of the soil. However, in the WRI method the support index directly establish the cantilever length independent of slab length. Therefore, the WRI method predicts larger cantilevers in short slabs and shorter cantilevers in long slabs relative to the BRAB method.  Figure 1: WRI Basic Cantilever Length. The cantilever length is then reduced by a factor K, which is based solely on slab length. This allows for a shorter cantilever length as the slab length decreases.  Figure 2: WRI Cantilever Adjustment Factor. Once the support index is found, the design moment, shear and deflection are found as:  Figure 3: WRI Design Equations. Effective PI In the WRI design method, the effective PI is first calculated as a weighted, effective PI using the BRAB methodology (Page 65 of the BRAB manual). The Effective PI is then modified by two correction factors, an over-consolidation coefficient (Figure 4) and a slope correction factor (Figure 5). The derivation of these correction factors is not discussed in the WRI manual. If the user wishes for these adjustment factors to be ignored in the analysis, an unconfined compressive strength of 6000 psi and a slope of 0% should be entered. This provides coefficients of 1.0 for both correction factors.  Figure 4: Over-Consolidation Coefficient  Figure 5: Slope of Natural Ground vs. Slope Correction Coefficient |