The assessment and mitigation of seismic risk associated with existing buildings in Canada present technical and economic challenges to building owners, especially in medium and high seismic zones. This study presents a multi-criteria and multi-level decision support framework for seismic risk management of existing buildings in Canada. The framework consists of the following three levels: (i) Level 1 - Preliminary Seismic Risk Screening Tool, which identifies the buildings that are exempt from further seismic risk assessment; (ii) Level 2 - Semi-Quantitative Seismic Risk Screening Tool, which identifies potentially hazardous buildings for detailed seismic evaluations; and (iii) Level 3 - Detailed Seismic Evaluation, which can be used to quantify the seismic risk of existing buildings and to identify any weak links in the building that could lead to component failure or building collapse.
The Level 1 screening is based on three key criteria to assist in the decision to take no action or to proceed with Level 2 screening, namely (i) seismicity; (ii) building condition; and (iii) remaining occupancy time. The Level 1 also identifies the criteria that immediately trigger the Level 3 Detailed Seismic Evaluation, in presence of any of the following hazards: (i) geologic hazards; (ii) site class F (such as liquefiable soils and quick and highly sensitive clays); (iii) deterioration or damage; and (iv) designated heritage buildings.
The Level 2 screening consists of both structural and non-structural scoring systems. The structural scoring system quantitatively evaluates the seismic risk of existing buildings largely based on FEMA P-154 methodology but incorporating additional building attributes including building importance, deterioration and ageing, and remaining occupancy time. In addition, seismicity is adapted to reflect Canadian earthquake characteristics; consequence classes are defined to describe different levels of consequence of failure of buildings to life safety rather than probability of collapse. The non-structural scoring system is developed based on a simple but comprehensive approach that qualitatively assesses the seismic adequacy of non-structural components. The final structural and non-structural scores are compared to certain structural and non-structural score thresholds, respectively, which enable to determine whether Level 3 Detailed Seismic Evaluation is triggered or not.
The Level 3 Detailed Seismic Evaluation can be performed using simple analysis methods or more robust analysis methods depending on the seismic risk level of a building. In this evaluation, several criteria are considered in determining the structural adequacy and needs for retrofit of an existing building, including the acceptable probability of failure or minimum reliability index, which in turn depends on life safety, consequences of failure, existence of complete load paths, redundancy, quality and reliability of visual inspection and seismic safety margin.
The proposed framework ensures an acceptable and consistent risk for existing buildings with a focus on minimizing the threats to life safety, and will help building owners in making risk-informed decisions in order to: (i) optimize the prioritization of buildings for detailed seismic evaluation; (ii) minimize the cost for such evaluations; and (iii) minimize the seismic risk for a portfolio of buildings.