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Geotechnical Problem Solving by John C. Lommler

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5.2

Geotechnical Load and Resistance Factor Design

5.2.1 Limit State Design – General

Starting in 2007 the Federal Highway Administration (FHWA) required that all bridges built with federal funds (or partially funded by federal funds) had to be designed by the LRFD design procedures included in the LRFD Bridge Design Specifications (currently AASHTO, 2010). This included geotechnical LRFD design procedures. To my knowledge this is the first time that geotechnical LRFD was required to be used in the United States.

AASHTO's LRFD design approach used reliability theory (as introduced in Section 5.1) to quantify uncertainty in bridge loadings and resistances. In the AASHTO code there are many load combinations requiring analysis, and load factors are given to increase or decrease these loadings as required by reliability analyses.

The four general limit states considered by AASHTO are described as service, strength, extreme, and fatigue limit states.

Geotechnical/foundation designs generally involve the first three limit states. The service limit state is an evaluation of load-settlement performance of foundations.

Geotechnical strength design evaluates when a structure's loading exceeds the foundation soil's strength. Unlike settlement and deflection analyses, which consider soil movements and strains under service loading (i.e., everyday loading cases), strength analysis considers loadings that are highly unlikely, but are statistically possible at the upper end of loading cases. ...

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