Seismic evaluation of existing basement walls

Abstract

Current state of practice for seismic design of basement walls is using the Mononobe-Okabe (M-O) method that is based on the Peak Ground Acceleration (PGA). The National Building Code of Canada (NBCC) has considerably changed the seismic hazard level from 10 % in 50 years in NBCC1995 to 2 % in 50 years in NBCC2010, which leads to doubling the PGA in Vancouver from 0.24g to 0.46g. The current design PGA leads to very large seismic forces that make the resulting basement walls very expensive. Because there is a little evidence of any significant damage to basement walls during major earthquakes, the Structural Engineers Association of British Columbia (SEABC) initiated a task force to review current seismic design procedures for deep basement walls. Presented in this paper are some preliminary results of the work conducted by this committee. A series of dynamic numerical analyses have been carried out on a typical basement wall designed using the M-O earth pressures with the NBCC1995 PGA for Vancouver. This wall is then subjected to three ground motions spectrally matched to the Uniform Hazard Spectrum prescribed by the NBCC2010 and the seismic performance of the wall under this level of demand has been presented and discussed. Particular attention has been given to the resulting drift ratio in the walls.