Buildings with Rigid Walls and Flexible Roof Diaphragms. I: Evaluation of Current U.S. Seismic Provisions

Buildings having rigid walls and flexible roof diaphragms (RWFD) are a type of building construction widely used for light industry in the United States; they incorporate rigid in-plane concrete or masonry walls and flexible in-plane wood, steel, or hybrid roof diaphragms. In this first of two companion papers, the probability of collapse of this type of building designed to current code provisions in the United States, given an MCE earthquake event, is evaluated according to the FEMA P695 methodology for a large set of representative building archetypes. The results of the study indicate that current code provisions for this type of building do not satisfy the collapse objective requirements of FEMA P695 under maximum considered earthquake ground motions. This is because the analysis provisions are based on assumed yielding of the walls rather than the roof diaphragm. Also, current code provisions considerably underestimate the period of RWFD buildings. To assist with creating provisions that take into account large flexible diaphragm deformations, a semi-empirical formula to estimate the fundamental period of RWFD buildings that accounts for roof diaphragm flexibility is derived in this paper. A new proposed seismic design approach for RWFD buildings is developed and evaluated in the second companion paper.