1 Implementation of Soil-Structure Interaction Models in Performance Based Design Procedures

Abstract

A soon to be published guidelines document for the design of seismic retrofits for existing buildings is based on performance-based design principles as implemented through so-called nonlinear static procedures (NSPs). In these procedures, the global inelastic deformation demand on the structure is computed from the response of an equivalent nonlinear single-degree-of-freedom (SDOF) system, the response of which is estimated from that of an elastic SDOF system. The guidelines were developed as part of the ATC-55 project, which is summarized by Comartin (this conference). The objective of the present paper is to describe one component of the ATC-55 project related to the implementation of soil-structure interaction (SSI) principles into NSPs. SSI effects are most important at short periods (i.e., T less than approximately 0.5 s). Three SSI phenomena can contribute to NSPs. First, flexibility at the soil-foundation interface can be incorporated into nonlinear pushover curves for the structure. These foundation spring models were incorporated into NSPs that pre-existed the ATC-55 project, and are not emphasized here. Second, SSI affects demand spectra through the effective system damping, which is the damping ratio for which spectral ordinates should be calculated. Third, kinematic SSI reduces ordinates of the demand spectra. This paper describes how damping and kinematic SSI effects have been incorporated into the recommended seismic analysis procedures for existing buildings.