Damage-based seismic reliability concept for woodframe structures

In the United States the majority of modern residential and a portion of commercial structures are woodframe construction which typically consists of dimension lumber sheathed with 1.22 x 2.44 m (4 x 8 ft) OSB or plywood panels. This paper presents the results of a study on the development of a damage-based seismic reliability model for light-frame wood structures subject to earthquake load. It is presented in conceptual form with a simple example for a symmetric one-story woodframe building. The concept presented here provides the basic methodology to calibrate a seismic damage model and compute the structural reliability using a damage-based limit state function. The illustrative example necessitated a small experimental program and the use of existing software. The mechanistic damage model chosen for the example expresses damage as a linear combination of the maximum displacement during an earthquake simulation and the hysteretic energy dissipated by each shearwall within a structure. In order to make the model mechanistic, it was regressively calibrated based on the results of static and dynamic tests conducted on 2.44 x 2.44 m (8 x 8 ft) wood shearwalls. The fastener spacing on the perimeter of the shearwall sheathing panels was chosen as the design variable of interest, hence all damage model parameters were developed as a function of their spacing. Following calibration, a damaged-based limit state for reliability analysis is developed that enables one to calculate the structural reliability index provided against a prescribed damage level. The result for an example limit state is presented and compared to Federal Emergency Management Agency transient drift performance specifications for collapse prevention.