| Summary: | One of the major concerns in earthquake disaster resilience is understanding the risk posed by existing buildings that are not conformant with modern building codes. A related challenge is how, if necessary, to mitigate the risk through retrofit policies or other measures in a cost effective manner. For some types of buildings, such as unreinforced masonry, the risks are so obviously large, that mandatory laws have been enacted to assess and retrofit the buildings. However, in other cases, such as with non-ductile concrete buildings or older tall steel buildings, the risks and mitigation strategies are not as clear cut. This research addresses the risks posed by older seismically deficient steel buildings, which constitute a significant portion of tall buildings in western US cities with high seismic hazard. These buildings include many steel moment resisting frames (MRF), constructed during the late 1960's through mid-1990's, with the type of welded connections that experienced sudden brittle fractures during the 1994 Northridge earthquake. This work applies performance-based earthquake engineering (PBEE) tools to this potential seismic safety problem. San Francisco is selected as a case study city in order to permit engagement with the city’s ongoing earthquake safety initiatives. The performance of existing 1970s tall steel MRF buildings is evaluated through the development of archetype buildings. A series of studies that progressively explore the performance of individual archetype buildings, within a probabilistic framework, are carried out, including scenario-based, intensity-based and time-based assessments. Additionally, a method is proposed to extend such assessments to evaluate clusters of buildings and how their performance may impact the resilience of the community; going beyond individual building performance, towards more holistic seismic performance evaluations. The results of this body of research are communicated not only in terms of structural response, but also in terms of direct economic losses, downtime and recovery, which are more accessible to decision makers. The scenario-based and intensity-based evaluations are carried out to assess performance under an expected earthquake scenario, and design level shaking, respectively. The results indicate that, while the archetype buildings considered are expected to guarantee the life-safety of occupants, the associated economic losses and downtime entail a costly and slow recovery, which can, additionally, result in considerable indirect losses. The impact of adopting structural retrofit schemes, enhanced non-structural building components, and seismic mitigation measures is explored. The results indicate that, through a combination of these interventions, significant reductions, in both losses and downtime, under the earthquake ground motion shaking intensities considered, can be achieved. In order to benchmark the performance of 1970s steel MRFs versus modern design standards, a comparative time-based evaluation is carried out. The results indicate that the probabilities of collapse of the 1970s archetype buildings considered are well in excess of the 1% in 50 year target implicit in modern design standards. The results also illustrate that while modern designs result in performance that complies with the code intended collapse-safety margin, the level of damage control may be insufficient to enable a swift recovery and ensure the seismic resilience of these buildings. A methodology to assess the earthquake risk of existing tall buildings on the urban community is proposed. This method is implemented in a simple case study of a cluster of tall steel MRF buildings in downtown San Francisco. The results suggest that under a range of realistic earthquake scenarios, a considerable loss of occupancy and functionality is expected in buildings consistent with the 1970s archetype. Furthermore, permanent deformations in these buildings can result in large cordons around the damaged structures, which would prevent access to other buildings within a considerable area. The results of this research serve to inform the debate over the expected seismic performance of existing 1970s tall steel MRF buildings. This work provides an array of results from different types of assessment that can be informative to different parties including design practitioners, building owners, policy makers and the insurance sector.
|
|---|
