SITE SPECIFIC SEISMIC HAZARD ANALYSIS FOR BUILDING CODE COMPLIANCE

The International Building Code (IBC) allows for a 20% reduction in seismic design ground motion intensity if the design ground motions are determined from a site-specific probabilistic analysis rather than from the National Earthquake Hazard Reduction Program (NEHRP) maps or website or other approved published source. This reduction in ground motion intensity may not only reduce seismic demand on a structure, but in some cases may also move a structure into a lower, less rigorous Seismic Design Category. In fact, in areas of low seismicity, a 20% reduction in ground motion intensity may even eliminate the need for a seismic analysis for code compliance. However, the code provides no guidance on what constitutes an acceptable site-specific analysis. In some instances, the design engineer has merely downloaded the seismic sources and activity rates used to generate the NEHRP maps, run them through a probabilistic seismic hazard analysis (not surprisingly getting the same values as shown on the NEHRP maps), and used this as a justification for reducing the NEHRP values by 20%.

One justification for allowing a 20% reduction in ground motion intensity for a site-specific analysis is that because the site –specific analysis uses the best (most current and detailed) available information on local and regional seismicity, there is less uncertainly associated with the site-specific analysis compared to NEHRP map values. Therefore, a lower recurrence interval is required for the ground motions from a site-specific analysis to provide the same level of reliability as the NEHRP map values. Based upon this justification, without justification that the downloaded source and activity rate data used to generate the NEHRP maps is the best available information, simply repeating the NEHRP analysis would not appear to satisfy the spirit or intent of the code. A case history is presented to illustrate these issues.