Combined rupture mechanisms in shallow foundations

Conventional ultimate limit state (ULS) shallow foundation design is typically based on a simplified analysis that fails to consider the possible existence of a combined structural and geotechnical failure, which is shown here to significantly affect the limit load. Neglecting this occurrence may lead to unsafe design, whereas a full analysis can be beneficial for the dimensioning. With the emphasis on separate SLS and ULS design in modern design codes such as Eurocode 7 (EN 1997-1, 2004), this paper explores unsafe loading scenarios and the benefits to be gained from a rigorous ULS design based on combined failure. For the sake of simplicity, a long foundation slab subjected to three different loading conditions is analysed using elastic, elasto-plastic and rigid-plastic methods and the results compared for a range of foundation strengths and stiffnesses. It is found that the limit load may be significantly influenced by plastic hinges in the structure and for each load condition it is possible to derive a curve relating ultimate load to plastic bending moment representing the ultimate limit state of the foundation.