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Nonlinear Evaluation of the Proposed Seismic Design Procedure for Steel Bridges with Ductile End Cross Frames
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Neither the current AASHTO LRFD Bridge Design Specifications nor the AASHTO Guide Specification for LRFD Seismic Bridge Design provides a design procedure to achieve the desired performance of essentially elastic substructure and ductile superstructure. This design strategy that is termed Type 2 Design Strategy in the Guide Specifications limits the inelastic activity to the superstructure of steel plate girder bridges. Due to the lack of this information, bridge engineers have been reluctant of using this strategy which will limit the damage to the support cross frames in steel plate girder bridge. This will also keep the substructure essentially elastic and thus limit the repair of the substructure after a design level earthquake. This report presents a proposed force-base design procedure that will achieve an ‘essentially' elastic substructure and ductile superstructure. The reinforced concrete (R/C) substructure flexural resistance is designed for the combined effect of seismic forces similar to conventional seismic design with a force reduction factor equal to 1.5. Meanwhile, the shear resistance and the confinement requirements are similar to the conventional seismic design. To achieve ductile superstructure, the horizontal resistance of the support cross frames is based on the nominal shear resistance of the substructure divided by a proposed 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. 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. The shear resistance of the substructure is also checked based on the expected lateral resistance of fully yielded and strain hardened pier cross frames. 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 with Critical and other Operational Categories are also shown. Thus, a total of eight 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 seven 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 when compared to Other bridge operation category. However, it also showed a decrease in in the size of the substructure when compared to Critical bridge operation category. The nonlinear evaluation showed the Type 2 design strategy has indeed achieved an essentially elastic substructure and ductile superstructure. In bridge with stiff substructures such as pier walls designed using Type 1 strategy, inelastic activity was observed in the support cross frames. This will subject the superstructure connections and bearings to seismic forces that they are not designed for, which may result in undesirable seismic performance.
Report No. CCEER-14-04