Quantifying unreinforced masonry seismic risk and mitigation options in Victoria, BC

Despite their well-known seismic vulnerability, unreinforced masonry (URM) buildings continue to be a leading source of loss of life and property damage in earthquakes around the world. Victoria, British Columbia is a community with substantial seismic hazard that has yet to experience a damaging earthquake and, thus, has not seen widespread interest in mitigating seismic risks. It is also a city with a substantial stock of URM buildings, constructed around the turn of the 20th century, reasonably similar in form to those devastated by the 2010/2011 Canterbury earthquake sequence in New Zealand. To promote seismic upgrading of URM buildings in the region, cost-benefit analyses were undertaken specifically for Victoria, considering the seismic hazard, typical pedestrian and occupant exposure, building vulnerability, and local retrofit costs. The loss estimates are underpinned by motion-damage relationships derived in this thesis from observed damage in past earthquakes in California and New Zealand. Upgrading measures considered range from parapet bracing to full seismic upgrades consistent with local practices. Parapet bracing is shown to have favorable cost-benefit ratios – up to 4:1 for buildings on soft soils, indicating that the expected benefits outweigh the costs by a factor of four. Partial retrofits (eg. tying walls back to all floors/roofs, plus parapet bracing) are shown to have favourable cost/benefit ratios for buildings on soft soils (up to 1.7:1). Full upgrades are shown to have unfavorable cost-benefit ratios (maximum 0.7:1). In all cases, public benefits (i.e. reduced casualties) represent a significant portion of the total benefits, which provides evidence for cost sharing among building owners and the public. The motion versus structural damage relationships derived in this study are also used in developing proposed refinements to the FEMA 154 screening methodology. New, additional score modifiers (specific to URM) are presented, including score modifiers for various levels of earthquake strengthening. A trial application on buildings in Victoria shows that the revised scoring system is able to more effectively differentiate amongst URM buildings, making it more suitable for URM-only surveys, such as are often implemented by communities interested in quantifying and mitigating URM seismic risk.Applied Science, Faculty ofCivil Engineering, Department ofGraduat