Reliability-based Strength Factors for the Geotechnical Design of Foundations

摘要

Most current standards for designing structures and foundations prescribe the Load and Resistance Factor Design where strength factors reduce nominal capacities on account of uncertainties in material and geometrical properties and inaccuracies in procedures to estimate nominal capacities. This paper examines reliability-based derivation of strength factors for the geotechnical capacity of foundations using data from full scale load tests developed by EPRI and other sources. We consider the use of single and multiple strength factors for designing drilled shafts, direct embedded foundations, and shallow footings, placed in soil and/or rock. Studied types of loading are uplift, compression, and flexural and twisting moments. Design procedures are assessed based on their bias and coefficient of variation when compared to full scale load tests. A Bayesian approach is suggested to incorporate judgment for cases with limited a-mount of test data. In addition to standard design procedures, we consider methods recently formulated by the authors for drilled shafts. In particular, we consider FAD Tools, a computer system recently developed by DiGioia, Gray & Associates, LLC and released by EPRI, for designing drilled shafts and direct embedded foundations for single poles and H-Frames. Strength factors are recommended for these and other types of foundations studied by the authors and statistical parameters of the full-scale test data are provided. Finally, we illustrate the impact of different options on the size of foundations under representative prescribed loads.