Seismic Reliability of Low-Rise Nonsymmetric Woodframe Buildings

Seismic responses and reliability of a one-story L-shaped woodframe building tested at the CSIRO Structures Laboratory and a hypothetical two-story L-shaped building based on the test house were investigated using a three-dimensional hysteretic frame model under bidirectional ground motions. Parameters of the degrading and pinching hysteresis used for the shear walls were identified from whole house testing. The SAC ground motions for the Los Angeles area were used and the effects of torsion, bidirectional excitations, the variability in ground motion, structural modeling, collapse capacity, and vibration period were investigated. It was found that uncertainties due to ground motion and structural modeling are the major sources for increase in estimated structural demand. Coupling of torsion and bidirectional excitation also causes significant response magnification. Overall, the adverse effects considered could cause more than 150% increase in demand or, in probabilistic terms, a 1 order of magnitude increase in exceedance probability for a given demand. These findings could be helpful in putting into context the findings and conclusions of analytical studies of woodframe building performance under earthquakes that ignore these uncertainties and effects. Torsion should be minimized in design to alleviate coupling magnification with bidirectional excitations.