"Seismic Base Isolation Using GT STRUDL Time History Analysis, Nonlinear Modeling, and Signal Processing"

Paul Bradford, Ph.D., P.E.

PB Engineering

East Amherst, NY

Abstract

The last decade has seen dramatic increases in the sophistication of structural analysis software (e.g. GT STRUDL, STAAD, SAP2000, ADINA), and along with that an increasing trend to utilize time history, analyses in industry (recent examples include the US 40/1_64¹, St. Louis and the US 1_40², Memphis). The recognition of the importance of nonlinear events in complex structural subsystems (e.g. cap/beam hinging, soil structure interaction, hysteretic isolators, etc.) has led to a marked increase in the research and development of nonlinear models. With the continuing development of advanced modeling techniques and analytical software, the increased usage of time history analyses for bridge structures promises to continue.

Base isolation, the seismic protection technique of decoupling a structure from the ground using low shear resistance bearings, has evolved into a mainstream design option over the past few decades. Among the simplest and most effective type of base isolator are a class known as "bilinear" or "hysteretic" type isolators. The nonlinear elasto-plastic characteristics of bilinear hearings present significant analysis challenges when attempting to use linear finite element programs. An iterative bearing linearization technique is currently used to circumvent the inherent nonlinearities in the analysis. However, this approach is time consuming and inefficient, requiring the analyst to spend significant amounts of time in bookkeeping operations. To this end, a front end program called GTTRANS was written to assist the analyst in seismic analyses when using GT STRUDL.

Utilities within GTTRANS include a custom .dxf file converter, iterative batch file GT STRUDL solver calls, GT STRUDL input file manipulations, integration of a material property library. as well as other seismic analysis related functions.

Crucial to the accuracy and applicability of a time history analysis is the time history record itself. Design response spectra may have been derived from a general source such as AASHTO or may be as specific as site profile generated. AASHTO DIV IA 4_6 only mentions that excitations must be compatible with the design spectrum. The AASHTO Guide Specifications for Seismic Isolation Design, Section 7.4 delineates some compatibility requirements. However, further guidance is needed related to how design spectrum compatible excitations are obtained. Outlined is a method to construct excitations that are AASHTO compatible with a given design spectrum. Included are the basic approach of frequency and time domain manipulations, as well as some discussion related to computer code and numerical methods. Single degree of freedom models and GT STRUDL bridge models are used to demonstrate characteristics of frequency and phase scaling.

¹ Capron, M., Modeling and Analysis for Seismic Retrofit of the US 40/I64 _ Double Deck Complex in Si. Louis, Missouri; Mid-America Highway Seismic Conference, March 1999

²Imbsen, R., Pecchia, D., Davis, G., Chang, G., The 1_40 Mississippi River Bridge Seismic Retrofit

Biographical Data

Paul has 14 years experience in the research and development, analysis, and design of structural bearings and joints used in the bridge construction industry. Paul has developed various custom software programs for seismic and structural applications. Co-inventor of the most widely implemented sliding isolation system for bridges, activities include base isolation committee work with ASCE, NIST, and AASHTO. Paul was a member of the T-3 technical subcommittee charged with authoring AASHTO's 1999 Guide Specifications for Seismic Isolation Design.

Education

Mr. Paul Bradford is a graduate of the State University of New York at Buffalo, Paul holds a Ph.D. degree in Mechanical Engineering.