New 2020 NEHRP Provisions for Seismic Design of Diaphragms

摘要

New provisions for seismic design of diaphragms have been developed for inclusion in the 2020 NEHRP Provisions, focusing on wood diaphragms and bare steel deck diaphragms. The new provisions have their basis in consideration of the ductility, displacement capacity and overstrength of diaphragms, a departure from past practice where diaphragm design forces were primarily tied to the R-factor of the vertical elements of the seismic force-resisting system. The resulting diaphragm design methods are believed to provide more rational approaches to diaphragm design, and to provide improved earthquake performance with little or no added cost. Designers are encouraged to consider use when these methods are applicable. One group of new provisions codifies the rigid wall-flexible diaphragm (RWFD) methodology, published in FEMA P-1026, Seismic Design of Rigid Wall-Flexible Diaphragm Buildings: An alternative Method (FEMA, 2015a), as well as Part 3 of the 2015 NEHRP Provisions (FEMA, 2015b). The RWFD design method incorporates the long-understood concept that the seismic response of RWFD buildings is governed by the response of the diaphragm much more than the vertical elements. The second group of new provisions incorporates bare steel deck diaphragms into the alternative diaphragm design method, developed in the NEHRP 2015 update and found in ASCE 7-16 (ASCE, 2017) Section 12.10.3. The steel deck diaphragm provisions were developed with significant new information from the Steel Diaphragm Innovation Initiative (SDII) Project and the Advancing Seismic Provisions for Steel Diaphragms in Rigid Wall-Flexible Diaphragm Buildings (RWFD) Project. Proposals moving these provisions through the NEHRP update process were developed through the work of Issue Team 9 (IT9), and significant interaction with the researchers involved in the SDII and RWFD Projects. This paper provides an overview of the new diaphragm seismic design provisions.