Following AASHTO design recommendations, civil engineering designs of shallow bridge foundations must change from allowable stress design (ASD) to load resistance factor design (LRFD) specifications regarding limit states concerning allowable bearing stresses and tolerable settlement magnitudes. Two LRFD criteria (limit state and service state) can be used to develop design charts of applied foundation stress (factored bearing capacity) versus footing effective width at various adopted settlement. In the presented methodology, the bearing capacity is evaluated using limit plasticity theory, settlement values are predicted using elastic continuum solutions, and geoparameter evaluations are based on site-specific data input using in situ field data from either standard penetration tests (SPT) or cone penetration tests (CPT). The focus is on shallow foundations resting on sands, granular soils, and geomaterials exhibiting drained behavior given the relatively small use of shallow foundations on clays for bridge structures in the U.S. since these should be dealt with on case-by-case basis because of additional concerns regarding time-rate-of-settlement, undrained distortion, creep, and other concerns. Two case studies are presented for comparing the in situ measured load-displacement curves with the LRFD recommended design values obtained using in situ tests.
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