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Guidelines for the Seismic Design of Ductile End Cross Frames in Steel Girder Bridge Superstructures
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Current practice in the seismic design of bridges assumes that their superstructures do not need to be explicitly designed for earthquake loads. They are assumed to remain elastic by virtue of their inherent strength and in-plane stiffness which is required for service loads. As a consequence few codes require detailed design of these members. Whereas this assumption appears valid for concrete box girder superstructures, the performance of steel bridges with concrete decks in recent earthquakes has cast doubt on the validity of this approach for this class of bridges. In particular, damage has occurred within the end cross frames of steel superstructures which are known to be the primary element in the lateral load path of straight bridges. It is also known that designing these end frames with special ductile details and allowing the braces to buckle and yield can significantly reduce the lateral loads transferred to the substructures. But little is known about how to maximize this effect while at the same time minimizing any associated damage. In this report, finite element analyses are conducted on multi-girder, multi-span, steel plate girder superstructures to identify load paths, factors influencing cross frame stiffness, tolerance for drift, and robustness of studded steel-to-concrete connections. Moments and shears transmitted through these connections rotate the girders about their longitudinal axes, and since this rotation is not uniform along the girder, the torsional stiffness of the girder-deck system plays an important role in the behavior of the cross frame. Furthermore, these moments are transmitted through the connections by pairs of tensile and compressive forces which, as the transverse loads increase, may cause yielding in the studs and breakout of the concrete. This report also discussed the experimental investigations that were conducted on a set of five subassembly specimens to establish their lateral cyclic response including the initial stiffness, ultimate strength and failure modes of subassembly models with various shear connector configurations. The specimens were one-half scale models of a steel girder bridge superstructure prototype. Two of the specimens represented typical end cross frames details without diagonal bracings. The results of the experimental investigations showed that the shear connectors near the end cross frames will be subjected to combined tension and shear forces. Any premature failure of these shear connectors will interrupt the load path and may not transfer the forces to the cross frame and the bearing. Simplified analysis and design method are also developed as part of this study to determine the seismic response parameters of single and multi-span steel girder bridges with ductile special end cross frames. The proposed methods are based on an iterative solution and show good agreement with results from nonlinear time history analyses.
Report No. CCEER-09-04