AN EVALUATION OF BI-DIRECTIONAL EARTHQUAKE SHAKING ON THE PROVISIONS OF THE AASHTO GUIDE SPECIFICATIONS FOR SEISMIC ISOLATION DESIGN

摘要

The AASHTO Guide Specifications for Seismic Isolation Design (a.k.a., Guide Spec) contain simplified procedures for estimating the displacement and force demand imposed on a bridge designed to be seismically isolated. The Uniform Load Method in this document is a linearized method for estimating uni-directional displacement response for purposes of design. It is understood that real ground motion time histories represent bi-directional shaking along two orthogonal axes at a given site. The Guide Spec stipulates that peak bi-directional force demand may be estimated by combining uni-directional maxima in a combination of 100 to 30% (or visa-versa). These procedures were established from the evaluation of elastic systems, and therefore may not adequately capture the nonlinear complexities of the bi-directional response of isolated bridges. Due to these underlying difficulties with current code procedures, a systematic evaluation of the effects of directivity and bi-directional input on the response of seismically isolated bridge systems was undertaken. It is evident from these results that the effect of bi-directional input on seismically isolated bridge systems is significant. It was found for all cases considered that on average peak displacement response due to bi-directional input is considerably larger than peak response due to uni-directional input applied separately. Further, current Guide Spec procedures due not adequately account for this coupling effect. This disparity is most significant for more rigid structures, employing stronger isolation systems, subjected to larger magnitude earthquakes, nearer the active fault. For these cases, softer site specific soils tend to increase the bi-directional effect. Coefficients to account for the effect of bi-directional input on systems designed by the Guide Spec procedures are suggested.