Seismic Assessment, Retrofit Strategies and Policy Implications for Vulnerable Existing Steel Buildings

Francisco Galvis, Greg Deierlein, Anne Hulsey, Wen-Yi Yen, Carlos Molina Hutt

One of the major concerns in earthquake disaster resilience is understanding the risk posed by existing buildings, which are not in conformance with modern building codes.  This project addresses the risks posed by the older seismically deficient steel buildings, which constitute a significant portion of tall buildings in San Francisco, Los Angeles and west coast cities with high seismic hazard. These include many steel moment frame buildings, 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. Many buildings of that era were designed and constructed without capacity design principles to protect against story mechanisms, without seismic drift limits, and with lower base shear strengths that those specified in current codes. Compounding these deficiencies are the new seismological data and models indicating that earthquake ground motion hazard at long periods, which can affect tall buildings, may be larger than previously thought. Furthermore, there is a growing realization that even in the best circumstances, minimum building code requirements may not ensure a level of damage control to ensure the seismic resilience of our communities.
The objectives of this project are to (1) develop and apply a comprehensive methodology to assess the safety risk of seismically vulnerable existing tall steel buildings, (2) evaluate the implications of these risks on the resilience of cities, and (3) develop quantitative strategies to promote the development of risk mitigation policies for building owners and communities. The risk assessments of this project consider implications of the risk of significant damage, downtime, and collapse of tall buildings on nearby structures and the surrounding community. As part of this project, we are developing a new material capable of simulating fracture in fiber-based structural models. This model is capable of replicating reasonably well the distribution of fractures observed during the 1994 Northridge Earthquake as shown in the Figure below.