A Case Study on the Initial Costs and Life Cycle Benefits of Base Isolation for a Low Rise Office Building

Base isolation provides a proven and effective means of protecting structures from the damaging horizontal components of earthquake ground motion. However, first cost increases and uncertain future benefits act as strong disincentives for building developers and have slowed wider adoption. Implementations of base isolation in the United States have been predominantly limited to critical facilities such as hospitals, for which post-earthquake function is essential. To encourage the implementation of base isolation to wider classes of structures, quantitative evidence on the financial benefits of base isolation across different building typologies over a building's design life is needed.

This paper presents a case study, comparing the life cycle cost of a fixed base and a base isolated three-story office building using the ATC-58 methodology. The buildings have been designed as steel braced frames situated over a soil class D site about 10 miles north of Los Angeles.

Unique to this investigation, a moat wall was included in the dynamic model of base isolated building. Two different ground motion suites were used in the loss analyses for the fixed base and base isolated buildings. These were scaled to match a conditional mean spectra at 0.5 s and 3.0 s for the fixed base and base isolated buildings respectively. A damageable component inventory based on the normative quantity recommendations in the ATC-58 manual for office occupancy was included in the loss estimation analyses. The Performance Assessment Calculation Tool (PACT) was adopted as the tool for this analysis.

This case study illustrates that base isolation significantly reduces the expected financial loss under design basis event, but less so for the maximum credible event due the impact of pounding against the moat wall. Considering all earthquake intensities, base isolation is shown to reduce the life cycle costs from earthquake hazard substantially, and the life cycle benefits of adopting base isolation exceed the initial cost difference when both direct and indirect losses are considered.