Eighteenth Canadian Geotechnical Colloquium: Limit States Design For Foundations. Part II. Development for the National Building Code of Canada

The geotechnical engineering profession in Canada is in the process of evaluating limit states design (LSD) for its incorporation into codes of practice for foundation engineering to provide a consistent design approach between geotechnical and structural engineers. This paper describes the work carried out for the initial development of LSD for foundations in the National Building Code of Canada. A load and resistance factor design approach, based on a factored overall geotechnical resistance, is used. The resistance factors for the ultimate limit states of bearing capacity and sliding of shallow and deep foundations are derived from a direct calibration with working stress design (WSD) and from a reliability analysis. The resistance factors derived from both approaches are consistent with each other and provide a reasonably constant reliability index of about 3.0 to 3.5. A relationship is presented that relates the reliability index to a global factor of safety and resistance factor. Design examples are provided that show that the proposed LSD produces designs that are comparable with those produced by traditional WSD. The importance of serviceability limits states is discussed, and the items that require further study and research work to refine code calibration are identified. Key words: limit states design, reliability index, code calibration, resistance factors, foundations, ultimate limit states.