Eighteenth Canadian Geotechnical Colloquium: Limit States Design For Foundations. Part I. An overview of the foundation design process

Abstract

This paper examines the foundation design process in terms of level of safety associated with current state-of-practice, sources of uncertainty and how they are handled, importance of engineering judgement and experience, and the role of codes of practice. Working stress, limit states, reliability-based design approcahes are described and discussed in terms of their historical development, fundamental bases and differences, advantages, and limitations. Limit states are conditions under which a structure no longer performs its intended function. Limit states design considers seperately the two classes of ultimate and serviceability limit states using partial factors of safety. The European factored strength approach and the North American factored overall resistance approach are compared and discussed. The factored resistance approach is a logical extension of working stress design and has the significant advantage that it reflects not only uncertainty in strength, but also in theoretical models, site conditions, construction tolerances, and failure mechanisms. The partial load and resistance factors are interrelated and are a function of characteristic values. A consistent, rational basis for the selection of the geotechnical characteristic value is required. The use of a conservatively assessed mean value is recommended, and an approach for its interpretation is presented. Key words: limit states design, working stress design, characteristic value, partial factors, factored resistance, load and resistance factor design.