Assessing the collapse risk of California's existing reinforced concrete frame structures: Metrics for seismic safety decisions

The emerging field of performance-based earthquake engineering enables the evaluation of seismic performance of building structures. In this study, performance-based earthquake engineering tools are applied to a potential seismic safety problem: older non-ductile reinforced concrete frame structures. Because these buildings were constructed before significant advancements in building code provisions for reinforced concrete were instituted in the mid-1970s, they may be vulnerable to earthquake-induced collapse, posing a threat to public safety in future earthquakes. Assessment of collapse risk for non-ductile reinforced frame structures is examined here to quantify differences in safety between existing and modern structures and to investigate the effectiveness of mitigation strategies, providing much-needed data for the ongoing discussion of seismic safety in California. A central component of this work involves assessing collapse risk of non-ductile reinforced concrete frame structures through dynamic analysis of nonlinear simulation models. Collapse performance assessments are conducted for a group of structures, varying in height, framing system and other design and detailing characteristics, which represents older reinforced concrete frame structures of the type constructed in California between 1950 and 1975. Nonlinear analysis models are constructed that are capable of capturing the effects of critical design and detailing features on structural behavior. Important aspects of the assessment procedure – such as propagation of sources of uncertainty, incorporation of nonsimulated failure modes, and adjustments for appropriate spectral shape of input ground motions – are treated systematically such that collapse assessment results for different structures and structural systems can be compared. These evaluations are used to discover trends in performance associated with design variability in non-ductile reinforced concrete frame structures and to show how much more likely these structures are to collapse than their modern, code-conforming counterparts. Assessments of collapse performance provide one possible metric of life safety of building structures. Structural modeling uncertainties, including those associated with component strength, stiffness, deformation capacity and cyclic deterioration, are incorporated in structural