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The main objective of this study is to assess the structural integrity of the double-deck through-truss Brent-Spence bridge when subjected to a seismic event from the New Madrid, Wabash Valley, or Anna seismic zones. To achieve this, the scope of work was divided into several tasks as follows: field testing of the main bridge, analytical modeling, determination of the site-specific ground motion, and seismic response analysis. This paper deals with the first two tasks. The ambient vibration properties of the main bridge were determined through field testing under traffic induced excitation. The purpose of measuring the ambient vibration properties was to determine the mode shapes and the associated natural frequencies. These vibration properties were subsequently used as the basis for calibrating the finite-element model for seismic response analysis. A three-dimensional finite-element model of the main bridge was developed for the free vibration analysis. The seismic response analysis using this calibrated finite-element model is presented in the second part of this two-part paper.

This is the second part of a two-part paper on field testing and analytical studies related to seismic behavior of the Brent-Spence bridge. This part of the paper deals with seismic evaluation of the bridge, while the first part dealt with free and ambient vibration studies of the bridge by field testing and finite-element analysis. Site-specific ground motion scenarios are developed for the bridge to represent probable earthquakes that may occur in these seismic zones. The time histories of these events are then used in the seismic analysis of the main bridge and response spectra are used for analyzing the approach spans. A three-dimensional finite-element model of the main bridge was subjected to the time histories to determine maximum responses (displacements, forces, and stresses). The seismic analysis of approach spans dealt only with the potential for loss of span due to excessive longitudinal displacement along the highway main line. For the maximum credible earthquake of magnitude 8.5 [M.sub.s] (7.3 [m.sub.b,Lg]) on the Richter scale at New Madrid, the seismic evaluation indicates that the main bridge will survive the earthquake in the elastic range without significant damage and no loss of span. The approach spans are found to be vulnerable to loss of span failure in the 8.5 [M.sub.s] event.