Quantitative Evaluation of New Strategies To Increase Seismic Resilience of Cities: a Shift of Current Paradigms

Abstract eng:
Lowrise wood-frame structures represent the most frequently used construction type for residences in many earthquakeprone regions such as the United States, Japan, Canada, New Zealand and many other countries. Although in general these structures have shown an acceptable seismic performance in terms of collapse safety, they have resulted in multi-million dollar losses that have large regional economic impact. Furthermore, even if collapse is avoided, damage may result in occupants being displaced from their residences, which for urban areas translates into the need to provide temporary housing to many thousands of people. Investigators at Stanford University have recently developed a new approach, referred to as “unibody design” which allows to significantly increase the lateral strength but particularly the lateral stiffness of lightweight residential construction, with very small increases in cost and changes in construction practice. Contrary to current approaches that allow significant nonlinear behavior and damage to take place, the unibody approach allows lowrise wood structures to essentially remain elastic and nearly damage free even in the event of large earthquakes. This paper proposes a probabilistic performance-based earthquake engineering framework for quantifying the benefits at a regional scale of using an enhanced design, in this case the unibody design, in residential construction. This study demonstrates that benefits of this novel approach are nonlinear, generating an exponential chain of regional risk reduction. Reasons for these large benefits are explained, as four effects can be superimposed: (1) increase the stiffness and strength significantly reduces the displacement demands in short-period structures, (2) reduced displacement demands result in a lower probability of damage, (3) enhanced houses closer to the seismic source present similar damage probabilities as conventional houses much farther away, and (4) the difference in distances to the seismic source are translated into significant differences in affected areas, and therefore, the number of damaged houses. The proposed framework is not exclusive of the unibody approach and can be used for evaluating the benefits of different types of retrofits or new designs which increase the strength and the stiffness of houses in an urban area. This framework provides useful information for decision-makers in seismic-prone regions to evaluate new public policies and the resilience of the residential construction.

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