Benefit-cost evaluation of seismic risk mitigation in existing non-ductile concrete buildings

 

Benefit-cost evaluation of seismic risk mitigation in existing non-ductile concrete buildings

Gregory Deierlein Abbie Liel

Stanford University University of Colorado

Abstract

The risks of damage and collapse to older (non-ductile) reinforced concrete buildings and the cost-effectiveness of seismic retrofit are investigated through analyses of archetypical designs representative of construction in California prior to the introduction of more rigor- ous seismic design requirements in the mid-1970’s. These risks for older buildings are compared to those in buildings that are designed to modern building code provisions that employ capacity design and ductile detailing requirements. The comparisons indicated that older non-ductile buildings have expected annual economic losses that are about twice those of the ductile buildings and risks of collapse and fatalities that are about 35 times higher. The cost effectiveness of seismic retrofit is examined to reduce damage and life safety risks. Considering the monetary benefits of both reduced damage and lives saved, these cost-benefit comparisons justify retrofit costs of up to about 20% to 40% of the building replacement value, implying that in most cases the retrofit of non-ductile concrete buildings would be cost-effective.

Introduction

Emerging performance-based earthquake engineering methods can offer significant new insights to the systematic evaluation of design criteria and policy-related questions for new and existing buildings. In this study, recently developed performance-based technologies are applied to assess the comparative performance of older “non- ductile” reinforced concrete (RC) buildings versus modern “ductile” buildings that employ capacity design approaches and ductile rein- forcing bar detailing. Concerns with older non-ductile designs stem from buildings constructed in the high seismic regions of the west- ern United States, prior to the mid-1970’s when major changes were instituted for seismic design of RC structures as a result of damage observed during the 1971 San Fernando earthquake. Prior to these changes, seismic design requirements for concrete frames did not require capacity design provisions to inhibit the formation of story mechanisms or column shear failures. While it is generally recog- nized that such buildings do not provide the same level of safety and damage control as modern buildings, there are many debates as to the safety of existing buildings and whether policies should be adopted to require detailed risk assessment and mitigation.

This study aims to improve understanding of earthquake risks in non-ductile RC buildings and the cost-effectiveness of mitigating these risks through building replacement or retrofit. Performance- based methods are applied to assess the risk of damage and collapse to a set of archetypical RC-framed office buildings that are repre- sentative of those in the high-seismic regions of California. A re- lated objective is to illustrate the application of the performance- based cost-benefit analyses, which can be generally applied to other building types in other seismic regions.

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