5.3

LRFD Spread Footings

5.3.1 LRFD and ASD Spread Footing Analyses – An Overview

Geotechnical LRFD analyses of spread footings are basically analyses of ultimate bearing capacity for the strength limit state and footing settlement for the service limit state. As we discussed earlier in Section 5.2, three primary loading conditions need to be analyzed to satisfy life-cycle loading analyses: service loadings for every day conditions, strength loadings for unlikely, but possible, overloading conditions that may result in structural failure, and extreme loadings representing once in a life-time events.

Current spread footing design practice is a mixture of both allowable stress design (ASD) and strength design (LRFD). Structural engineers estimate a range of wall and column loadings and give these loadings to the geotechnical engineer in the document requesting a geotechnical investigation. If building or bridge loadings are not given in the request for services document, the geotechnical engineer calls the structural engineer and asks for them. Many geotechnical engineers don't seem to be aware that the loadings received from structural engineers are service loadings. If the structural engineer is a young rather inexperienced engineer, he or she often gives the geotechnical engineer factored loadings when requested to provide building loadings. Using factored loadings to analyse footings and then applying a factor of safety (ASD) to the design, results in designing a footing with ...

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