UBC Theses and Dissertations
Seismic assessment of basement walls in British Columbia Amirzehni, Elnaz
The current state of practice for seismic design of basement walls in Vancouver is based on the Mononobe-Okabe (M-O) method using a Peak Ground Acceleration (PGA) mandated by the National Building Code of Canada (NBCC, 2010). Because there is a little evidence of any significant damage to basement walls during major earthquakes, the Structural Engineers Association of British Columbia (SEABC) became concerned about designing the walls under the code-mandated PGA and set up a task force to review the current procedure for seismic design of basement walls in British Columbia (BC). The University of British Columbia (UBC) was asked to carry out this investigation. This thesis aims to provide solid base for designing the basement walls using an appropriate fraction of the code-mandated PGA in the M-O analyses. To this end, a series of dynamic nonlinear soil--structure interaction analyses are conducted to examine the seismic resistance of typical basement walls designed according to current practice in BC, for different fractions of the code mandated PGA (100% to 50%). The seismic responses of the walls are evaluated by subjecting them to ensembles of ground motions comprised of shallow crustal, deep subcrustal, interface earthquakes from a Cascadia subduction events and near-fault earthquake motions. Input motions are matched to the intensity of the seismic hazard using both spectral and linear scaling techniques. Representative 4-level and 6-level basement walls are analyzed. The nonlinear hysteretic response of the foundation soil is characterized in order to obtain realistic estimates of an interaction between the basement wall and the surrounding soil. In addition, the effects of the local site conditions in terms of geometrical and geological structure of soil deposits underlying the basement structure on the seismic performance of the basement walls are evaluated. The analyses show that current engineering practice for designing basement walls based on the M-O method and using 100% PGA is too conservative. The analyses suggest that a wall designed using 50% to 60% PGA results in an acceptable performance in terms of drift ratio.
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