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Seismic Design and Nonlinear Evaluation of Steel I-Girder Bridges with Ductile End Cross-Frames
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The AASHTO Guide Specifications for the LRFD Seismic Bridge Design define three Global Seismic Design Strategies based on the expected behavior characteristics of bridge systems. Type 2 design strategy is dedicated to essentially elastic substructure with a ductile superstructure. This category applies to steel superstructure where the nonlinear response is achieved by ductile elements in the pier cross-frames. The current guide specification does not provide bridge engineers a complete design procedure on achieving the desired performance of Type 2 Design Strategy. This report presents a proposed design procedure that will achieve an ‘essentially' elastic substructure and ductile superstructure. The reinforced concrete (R/C) substructure flexural resistance is designed for the longitudinal seismic forces that are based on the design spectrum using a force reduction factor equal to 1.5. Meanwhile, the shear resistance of the substructure is conservatively designed based on the plastic hinging of the substructure which is not expected to occur in Type 2 design strategy. In addition, the lateral steel of the substructure is designed and detailed to achieve the required confinement, in case the substructure will undergo nonlinear response. The pier cross frames are designed and detailed to achieved a ductile response. The horizontal resistance of these cross frames is based on the nominal shear resistance of the substructure divided by a response modification factor equal to 4. This will ensure that the superstructure will act as a ‘fuse' and will not subject the substructure to forces that may cause nonlinear response in that direction. The shear resistance of the substructure is also checked based on the expected lateral resistance of fully yielded and strain hardened pier cross frames. In order to achieve a ductile response of pier cross frames, the diagonal members, which are expected to undergo inelastic response, are detailed to have limits on width-to-thickness and slenderness ratios. The diagonal member connections and other cross frame members are designed for fully yielded and strain hardened diagonal members. Three bridges were selected to illustrate the proposed design procedure for Type 2 design strategy. The substructure of these bridges included single-column pier, two-column pier, and wall piers. Examples showing the design of these bridges using Type 1 design strategy are also shown. Thus, a total of six bridge design examples are shown in this report. The design and performance of these bridges were then compared. The performance was evaluated through nonlinear response history analysis using ground motions representing the design and maximum considered earthquakes. The proposed design strategy for Type 2 design showed an increase in the size of the substructure but with smaller bridge displacement. The Type 2 design strategy achieved an essentially elastic substructure and the inelasticity was concentrated in the support cross-frames. In Type 1 design, the inelasticity was concentrated in the substructure but required larger support cross-frames and bearings. In bridge with stiff substructures designed using Type 1 strategy, inelastic activity was observed in the support cross-frames. This may subject the cross-frame connections and bearings to seismic forces that they are not designed for, which may result in undesirable performance.
Report No. CCEER-13-16