American Society of Civil Engineers

Vertical Distribution of Seismic Forces for Simplified Design of Base-Isolated Buildings

by Keri L. Ryan, (Utah State University, Logan, UT 84322-4110 E-mail: and Kelby York, (Utah State University, Logan, UT 84322-4110)
Section: Simplified Analysis, Design, and Assessment Tools for Base-Isolated Buildings, pp. 1-10, (doi:

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Document type: Conference Proceeding Paper
Part of: New Horizons and Better Practices
Abstract: The lateral design force distribution given by the IBC equivalent lateral force method for base-isolated buildings is often regarded as very conservative. These provisions neglect the inertia at the base mass entirely, and apply the same triangular force distribution as for a short period fixed-base building, even though the first mode shape of an isolated building is closer to uniform than linear. However, the nonlinearity of the isolation system, which can induce a higher mode response, is often overlooked. Using nonlinear regression analysis of median response data from nonlinear response history analysis of representative systems, improved equations are developed to estimate the seismic force at the base level and the distribution of lateral loads in the superstructure. The IBC distribution and the improved distribution developed here are evaluated in comparison to seismic response data. While the IBC distribution is conservative for many systems, the improved equations are accurate over a wide range of system parameters.

ASCE Subject Headings:
Seismic effects
Structural design