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Commentary Appendix A DEVELOPMENT OF MAXIMUM CONSIDERED EARTHQUAKE GROUND MOTION MAPS FIGURES 3.3-1 THROUGH 3.3-14
BACKGROUND The maps used in the Provisions through 1994 provided the Aa (effective peak acceleration coefficient) and Av (effective peak velocity-related acceleration coefficient) values to use for design. The BSSC had always recognized that the maps and coefficients would changewith time as the profession gained more knowledge about earthquakes and their resulting ground motions and as society gained greater insight into the process of establishing acceptable risk. By 1997, significant additional earthquake data had been obtained that made the Aa and Av maps, then about 20 years old, seriously out of date. For the 1997 Provisions, a joint effort involving the BSSC, theFederal Emergency Management Agency (FEMA), and the U.S. Geological Survey (USGS) was conducted to develop both new maps for use in design and new design procedures reflecting the significant advances made in the past 20 years. The BSSC’s role in this joint effort was to develop new ground motion maps for use in design and design procedures based on new USGS seismic hazard maps. The BSSC appointed a15-member Seismic Design Procedure Group (SDPG) to develop the seismic ground motion maps and design procedures. The SDPG membership was composed of representatives of different segments of the design community as well as two earth science members designated by the USGS, and the membership was representative of the different geographical regions of the country. Also, the BSSC, with input from FEMAand USGS, appointed a five-member Management Committee (MC) to guide the efforts of the SDPG. The MC was geographically balanced insofar as practicable and was composed of two seismic hazard definition experts and three engineering design experts, including the chairman of the 1997 Provisions Update Committee (PUC). The SDPG and the MC worked closely with the USGS to define the BSSC mapping needsand to understand how the USGS seismic hazard maps should be used to develop the BSSC seismic ground motion maps and design procedures. For a brief overview of how the USGS developed its hazard maps, see Appendix B to this Commentary volume. A detailed description of the development of the maps is contained in the USGS Open-File Report 96-532, National Seismic-Hazard Maps: Documentation, June1996, by Frankel, et al. (1996). The USGS hazard maps also can be viewed and printed from a USGS Internet site at http://eqhazmaps.usgs.gov. The goals of the SDPG were as follows: 1. To replace the existing effective peak acceleration and velocity-related acceleration design maps with new ground motion spectral response maps based on new USGS seismic hazard maps. 2. To develop the new ground motionspectral response maps within the existing framework of the Provisions with emphasis on uniform margin against the collapse of structures. 3. To develop design procedures for use with the new ground motion spectral response maps.

PURPOSE OF THE PROVISIONS The purpose of the Provisions is to present criteria for the design and construction of new structures subject to earthquake ground motions inorder to minimize the risk to life for all structures, to increase 317

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the expected performance of higher occupancy structures as compared to ordinary structures, and to improve the capability of essential structures to function after an earthquake. To this end, the Provisions provide the minimum criteria considered prudent for structures subjected toearthquakes at any location in the United States and its territories. The Provisions generally considers property damage as it relates to occupant safety for ordinary structures. For high occupancy and essential structures, damage limitation criteria are more strict in order to better provide for the safety of occupants and the continued functioning of the structure. Some structural and nonstructural...