Effect of Strut Stiffness on Seismic Performance of Fully Integral Steel Bridge with a Strut-Braced Pier

Recently, the fully integral bridge system that integrates the entire superstructures and substructures together to form a monolithic rigid frame has been presented, since it is anticipated that this approach will lead to improvements in aesthetics, economic efficiency, and seismic performance of a bridge system. This study is related to a fully integral steel bridge with struts installed in-between the piers at the middle of the bridge span, which is called a strut-braced pier. Thus, it is expected that the strut-braced pier mainly prevents horizontal loads like earthquake load or vehicle braking load. In this study, the seismic performance of the fully integral steel bridge was evaluated in accordance with Caltrans Seismic Design Criteria which involves displacement criteria, displacement ductility capacity requirement, and member force criteria. The capacities of the member forces and the displacement were determined through nonlinear static pushover analysis using OpenSees. As a result, the fully integral steel bridge met the seismic performance criteria specified in Caltrans with a great margin. A parametric study was conducted to investigate the effect of strut stiffness on the seismic capacities and effects from the horizontal load of the fully integral steel bridge. The results show that the displacement capacity and displacement ductility capacity of the fully integral steel bridge have a slight change when the strut stiffness increases. The member force capacity is primarily affected by the strut-braced pier and increases significantly along with the strut stiffness. The lateral displacement and the sectional member forces are well controlled to a converging value by a proper application of the strut stiffness. Therefore, it was found that the minimum stiffness required for the struts can be defined to sufficiently resist design seismic loads, and thus, the sectional properties of all intermediate piers can be reasonably adjusted by varying only the stiffness of the struts connected to the braced piers. It has a great significance in that such results lead to the feasibility of various economical designs of bridge substructure including piers suitable for each situation.