Seismic Risk Assessment of Reinforced Concrete Frame Structures in Southern California due to a Magnitude 7.8 Earthquake on the San Andreas Fault Authors: Kathryn Lynch, Kristen Rowe, Abbie Liel Due to the seismic nature and economic significance of the Los Angeles region, it is important to evaluate the safety of structures in the area due to the next big earthquake. This paper predicts damage and collapses possible in reinforced concrete (RC) frame structures in Southern California due to the fictitious magnitude 7.8 ShakeOut earthquake on the San Andreas Fault. The ShakeOut project predicted ground motions for seven hundred sites spaced at ten-kilometer intervals throughout the southern California region, which are used to evaluate the impact of the ShakeOut scenario on RC structures. In order to obtain an accurate profile of structural response, both older and modern RC commercial buildings are analyzed. This study assesses 20 RC moment frame concrete office buildings typical of those found in California. These include 8 older RC buildings (constructed pre-1970) and 12 modern code-conforming special moment frame structures. These buildings vary in height from 1 to 20 stories and include both space and perimeter frame systems. The pre-1970 RC designs are designed based on the 1967 Uniform Building Code specifications and are representative of existing nonductile RC frame buildings. The modern structures meet the requirements of the 2003 International Building Code, ASCE 7-05 and ACI318-02. Since all of southern California is considered in zone 3 from the 1967 UBC, these older RC structures’ design remains constant throughout the sites. However, the modern buildings are modified for the modern site-specific design requirements in order to present more realistic results. The models utilize performance-based earthquake engineering methods and nonlinear dynamic analysis to assess engineering demands, i.e. story drifts and floor acceleration at each site. Since the analysis utilizes robust models capable of capturing the strength and stiffness deterioration as a structure collapses under seismic loads, collapse may be predicted at sites where ground motions are very large. Each building model is implemented in the OpenSees analysis platform, with each structure modeled in 2D and including both lateral and gravity systems. With the comparison of damage incurred to structures between existing and modern structures, the increased seismic resistance of contemporary buildings is demonstrated. Older, non-ductile RC frame buildings have been consistently identified as being particularly vulnerable to significant damage and earthquake- induced collapse, and in this study we can see how this would be borne out by a particular southern California earthquake scenario. The outcome of the ShakeOut presents an examination of the future risks and causalities of both life and property in the next big earthquake, which will help with both seismic risk mitigation and emergency response strategies. Results representing seismic risk and damage will be represented using GIS software to graphically show how damage in modern and older RC frames is predicted to vary over the greater Los Angeles region. This can be used to identify particular buildings, types of buildings or regions that may be particularly vulnerable in future earthquakes.
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