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 performance predictions. The effect of modeling uncertainties was investigated by conducting sensitivity analyses to probe the relationship between model random variables and structural response, and then fitting a response surface to sensitivity analysis results. The response surface is a functional relationship between the input random variables and the limit state criterion for structural response, such as collapse capacity of the structure. Monte Carlo simulation, together with the response surface prediction of structural response, is used to propagate modeling uncertainties through the structural performance assessment. These uncertainties may have a significant impact on the structural performance predictions, particularly in cases where the underlying model random variables are characterized by large dispersion (high coefficient of variation) or where structural response is very nonlinear.

Even when they do not collapse, non-ductile reinforced concrete frame structures may also incur significant damage in future earthquakes, forcing building owners to invest in costly repairs. The cost of repairing earthquake damage is assessed, using data from previous researchers relating structural response to damage in non-structural and structural components and the cost of related repairs. These losses are shown to be more significant for owners of non-ductile reinforced concrete frame structures than those of newer code-conforming structures. The collapse assessments are also extended to provide estimations of earthquake-related fatalities in existing and modern reinforced concrete frame structures.

These metrics of earthquake-induced collapse, losses and fatalities are used to evaluate the effectiveness of replacement and retrofit strategies for mitigating hazards posed by nonductile reinforced concrete frame structures. The effectiveness of policies for seismic strengthening is measured in terms of costs and benefits, where the benefits include reduced economic losses and fatalities. The cost-benefit assessment is used to develop recommendations to enhance the efficiency and equity of policies for seismic safety in California.