Impact of base conditions on the bracing performance of LTF walls

Abstract

Residential buildings in NZ are often low-rise light timber-framed (LTF) buildings where LTF walls provide the gravity and lateral load resisting systems. LTF building construction largely follows the prescriptive standard NZS 3604. According to the engineering basis of NZS 3604, seismic design action of a building is determined by the force-based approach assuming a ductility of 3.5. For seismic bracing provision, NZS 3604 specifies the P21 test procedure for evaluating seismic bracing capacities of proprietary LTF walls. In the 2010/11 Canterbury earthquakes, thousands of LTF residential buildings were tested under real shaking conditions, providing an unprecedented opportunity to examine our seismic settings for LTF buildings. The great majority of LTF buildings achieved the code-specified objective “life safety”. However, earthquake damage of LTF buildings was often significant and fell short of societal expectations and resulted in billions of dollars to remediate. This provided an important impetus to advance seismic engineering of LTF buildings. A project was initiated to study seismic behaviour of LTF walls typical of NZ practice and a previous research studied the data of plasterboard walls tested using standard P21 procedure, where the walls are mounted on a rigid base simulating a ground floor. This paper presents the experimental study of bracing behaviour of four LTF walls on suspended timber floors. The test results are compared with those of identical walls tested using standard P21 procedures to investigate the impact of base conditions on bracing behaviour of LTF walls and to help inform seismic design moving towards damage mitigation.