Limit States Design (LSD) for Shallow and Deep Foundations

Abstract

IntroductionFoundation design consists of selecting and proportioning foundations in such a way that limit states are prevented. Limit states are of two types: Ultimate Limit States (ULS) and Serviceability Limit States (SLS). ULSs are associated with danger, involving such outcomes as structural collapse.SLSs are associated with impaired functionality, and in foundation design are often caused by excessive settlement. Reliability-based design (RBD) is a design philosophy that aims at keeping the probability of reaching limit states lower than some limiting value. Thus, a direct assessment of risk is possible with RBD. This evaluation is not possible with traditional working stress design. The use of RBD directly in projects is not straightforward and is cumbersome to designers, except in large-budget projects. Load and Resistance Factor Design (LRFD) shares most of the benefits of RBD while being much simpler to apply. LRFD has traditionally been used for ULS checks, but SLS's have been brought into the LRFD framework recently (AASHTO 1998).Load and Resistance Factor Design (LRFD) is a design method in which design loads are increased and design resistances are reduced through multiplication by factors that are greater than one and less than one, respectively. In this method, foundations are proportioned so that the factored loads are not greater than the factored resistances.In order for foundation design to be consistent with current structural design practice, the use of the same loads, load factors and load combinations would be required. In this study, we review the load factors presented in various LRFD Codes from the US, Canada and Europe. A simple first-order second moment (FOSM) reliability analysis is presented to determine appropriate ranges for the values of the load factors. These values are compared with those proposed in the Codes.For LRFD to gain acceptance in geotechnical engineering, a framework for the objective assessment of resistance factors is needed. Such a framework, based on reliability analysis is proposed in this study. Probability Density Functions (PDFs), representing design variable uncertainties, are required for analysis. A systematic approach to the selection of PDFs is presented.Such a procedure is a critical prerequisite to a rational probabilistic analysis in the development of LRFD methods in geotechnical engineering. Additionally, in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be consistent with the methods assumed in the development of the LRFD factors. In this study, a methodology for the estimation of soil parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in traditional methods.The primary objective of this study is to propose a Limit States Design method for shallow and deep foundations that is based on a rational, probability-based investigation of design methods. FindingsThis research was able to deve...