Buildings with Rigid Walls and Flexible Roof Diaphragms. II: Evaluation of a New Seismic Design Approach Based on Distributed Diaphragm Yielding

A seismic collapse evaluation study of buildings with rigid walls and flexible roof diaphragms (RWFD) presented in the companion paper indicates that this type of structure, as designed to current seismic design provisions in the United States, does not satisfy FEMA P695 performance criteria for Risk Category II Buildings. The seismic performance of RWFD buildings is often characterized by large deformations and yielding in the roof diaphragm rather than in the vertical elements of the seismic force-resisting system (SFRS). In this paper, a new seismic design approach is proposed to account for flexible roof diaphragm response. The proposed approach relies on distributed yielding in the roof diaphragm as the predominant inelastic response under extreme ground shaking. This is obtained by strengthening the end diaphragm regions, thereby allowing yielding to spread deeper into the diaphragm. The basic steps of the design approach and its limitations are described. A validation study of the proposed design procedure is conducted for 17 RWFD building archetypes with wood roof diaphragms in accordance with the FEMA P695 methodology. The results of this study indicate that the proposed seismic design approach, produces a system that meets the probability of collapse requirement for a Risk Category II building of the FEMA P695 methodology under maximum considered earthquake ground motion for RWFD buildings with wood roof diaphragms.