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Influence of new Bridge Configurations on Seismic Performance
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After the dramatic failure of a connector bridge at the lnterstate-5/State Route-14 interchange during the Northridge earthquake of 1994 in Southern California, designers re-evaluated the overall configuration of the bridge and incorporated new concepts in the replacement structure. The new concepts departed from the norm in three fronts: (1) it used equal column heights within each frame regardless of the terrains, (2) it reduced the number of in-span hinges, and (3) it eliminated intermediate hinge seats and instead placed columns adjacent to the hinge on either sides. While these steps appeared to be logical, the replacement bridge had to be constructed rapidly and no detailed evaluation of the concepts was carried out before the design. This report presents an evaluation of the effects of these changes on the nonlinear seismic response of the bridge based on an extensive three-dimensional analysis of a straight version of the actual structure which is curved. Computer program Drain-3DX was used along with five earthquake records with different spectral characteristics. Both the new bridge and a conventional bridge were analyzed. The conventional bridge had variable columns heights that followed the terrains, had more in-span hinges than the new bridge, and had hinge seats. Displacements, column shears, and column ductilities were used to evaluate the effectiveness of the changes in improving the seismic performance. The study revealed that the changes can improve the earthquake response only if abutment shear keys are designed to yield at relatively low forces so that the superstructure can move in the transverse direction of the bridge. Otherwise excessive transverse displacements could occur due to the pivoting action of the abutments on the superstructure. It was also found that the eccentricity between the superstructure mass center and substructure stiffness center should be considered and minimized at the design stage to obtain a more uniform distribution of displacements and column shears (Abstract by authors).
Report No. CCEER-97-5