THE SIGNIFICANCE OF SITE RESPONSE EFFECTS ON PERFORMANCE BASED DESIGN

3 SUMMARY This paper illustrates how site response effects influence the design forces when using performance based design. This is done by showing how the relative force levels vary between the various performance design requirements. Two cases for sites on rock, one in a region of low to moderate seismicity the other in a region of high seismicity are discussed initially. This situation is then used as a reference for comparison when considering other soil conditions. To illustrate how the performance design objectives are influenced by site response effects three soil profiles are used, two being dense sand sites, typical of Class D as defined by UBC (1997) and one being a soft clay site, typical of Class E. We have calculated the effects of site using bedrock ground motions matching three return periods specified in FEMA-273 (1997). These are the 2%, 10% and 50% chance of being exceeded in a 50 year design life corresponding to 'no collapse', 'life safety' and 'immediate occupancy' respectively. In this way the influence of site response on the design process when using performance based design methods is clearly illustrated. For the rock and the soil sites conforming to Class D the results are as expected with more stringent 'no collapse' force levels being significantly larger than the 'immediate occupancy' force levels. The Class E site however shows that 'no collapse' ground motion is similar in regions of low to moderate seismicity to that of regions of high seismicity. It follows the design of structures on Class E sites in highly seismic regions will often be controlled by the 'immediate occupancy' requirements.