Report - PEER - University of California, Berkeley

examination of the risk implications of basing structural design on the ground motionswith a return period of 475 years, although the estimates of losses are based entirelyon expert judgment rather than modeling, and the tone is very much one of assessingand judging the chosen design basis as being reasonable and at least as stringent as thedesign basis in use at the time.The formulation and arguments presented in ATC 3-06 represented an importantlandmark and are laudable when viewed in a historical context. With time, however,the issues have been re-visited and examined in the light of improved understandingof seismic hazard and the relationships between ground-motion intensities andstructural damage. These considerations have led to the adoption of 2% in 50 years —a return period of 2,475 years, considered to be more closely related with theprobability of structural collapse — as the design level of hazard in the 1997 NEHRPguidelines and in IBC 2000 (Leyendecker et al., 2000) as well as in the 2005 editionof the Canadian seismic code (Heidebrecht, 2003). However, the seismic design codesof nearly every other country in the world, regardless of differences with the USA interms of seismicity and construction practices, have adopted — generally without anyclear risk-based rationale — the 475-year return period as the basis for the groundmotions considered in design. A notable exception to this is the 1986 Costa Ricancode, which provides maps of PGA for return periods of 50, 100, 500 and 1,000 yearsand allows the designer to calculate the appropriate return period explicitlyconsidering the importance, the design life and the ductility of the structure. Althoughmost codes are based on a single hazard map, such performance-based considerationsare actually present in most codes through the use of importance factors that increasethe spectral ordinates for structures required to perform above simple life-safetycriteria under the 475-year ground motions. The factors essentially result in safetyimportantstructures being designed for longer return-period ground-motions. Theshortcomings of this approach are obvious: firstly, it assumes that the variation ofground-motion amplitude with return period is constant throughout the countrycovered by the code, and secondly, it assumes that designing for life-safety underground shaking with a longer return period will ensure that the structure remainsoperational under the 475-year return period.2.2 Design Response Spectra2.2.1 Horizontal Spectral ShapesIn the majority of current seismic design codes around the world the elastic responsespectrum of acceleration is constructed by anchoring a spectral shape defined for eachsite class to the design peak ground acceleration (PGA). Apart from the lack ofgeophysical or engineering significance of PGA, this approach has the significantdrawback that the shape of the response spectrum changes only with the site class,even though it is well established that the spectral shape is strongly influenced byearthquake magnitude and, to a much lesser extent, by source-to-site distance. As aresult, the spectrum will often not be of uniform hazard (McGuire, 1977). The471