Development of the next generation of seismic design value maps for the 2020 NEHRP Provisions
During the period January 2015 through August 2018, a joint committee of U.S. Geological Survey (USGS) representatives and National Institute of Building Sciences Building Seismic Safety Council (BSSC) volunteers and staff formed a committee to conduct Project 17. The purpose of Project 17 was to formulate recommendations for the rules by which next-generation seismic design value maps, derived from USGS national seismic hazard models, will be developed for adoption by the 2020 National Earthquake Hazard Reduction Program Recommended Provisions for Seismic Regulations of New Buildings and Other Structures (NEHRP Provisions), ASCE 7-22 and the 2024 International Building Code. Two similar projects, Project 97 and Project 07, had been conducted in the past. Each of these projects established, for a period of approximately ten years, the rules by which design ground motion values referenced by the building codes would be developed both by USGS and by private consultants engaged in site-specific studies. Project 17 was originally commissioned in response to issues identified in adopting the 2014 edition of the USGS national seismic hazard model and the design procedures that reference them for use, including the NEHRP Provisions, building codes and referenced standards. Specific issues included: the engineering profession’s discontent with the fluctuating design values portrayed by successive map editions; discovery that the standard spectral shape referenced by the design provisions did not adequately represent ground motion amplitude and spectral character on some sites; and a change in seismologic characterization of the possible size of earthquakes originating on various faults and source zones. Project 17 was funded by the Federal Emergency Management Agency (FEMA), and supported by the USGS with some collaborating experts. An initial planning committee met throughout calendar year 2015 to identify key issues to be considered and to develop a work plan for addressing these as part of the 2020 NEHRP Provisions update cycle. The planning committee recommended an effort of approximately 30-months duration during which the USGS would develop draft design maps based on the rules proposed, to allow evaluation and refinement of the recommendations. A Project 17 Committee (P17C) was empaneled and four task subcommittees were formed, each charged with evaluating one of the key issues identified in the planning effort: Stabilizing mapped values; Definition of Acceptable Risk; Development of multi-period spectral parameter data; and, Definition of procedures for computing deterministic caps, should it be necessary to continue use of such caps in development of the maps. A fifth task subcommittee was formed in 2017 to look at ways to stabilize the seismic design category as an extended effort to stabilize mapped values. The P17C met three times per year throughout 2016, 2017 and 2018 to resolve these issues and develop recommendations for an updated technical basis and procedures to be followed in preparing next-generation seismic design value maps for inclusion in the NEHRP Provisions. The P17C documented these in the form of draft proposals for revision of the NEHRP Provisions. In August 2018, the P17C passed these recommendations to the Provisions Update Committee (PUC) for completion, development of consensus and adoption as appropriate.