Dynamic Identification of Seismic-Isolated Bridges Subjected to Near–Fault Ground Motion with Soil-Structure Interaction

Bridges are very important and vital structures in any country and have main roles in transportation. Collapse of bridges due to earthquakes in San Fernando (1971), Northridge (1994), Hanshin (1995) and Chi-Chi (1999) showed that there are still many unknown and unsolved problems in seismic design of bridges. In this research, an existing overpass bridge in Urmia was analyzed by nonlinear dynamics, using SAP2000 software, and its dynamic response has been investigated with different kinds of seismic isolation systems (elastic neoprene and LBR isolators), considering the effects of soil structure interaction. One of the important tasks in this project is modeling the abutment backfill of the bridge which can affect its response depending on the type of bridge isolation. To perform a time-history dynamic analysis, this research uses near-field and far-field earthquakes records to compare the bridge response affected by these earthquakes. Results of the analysis showed that LRB isolators would increase deck displacement and decrease pier and abutment displacement, as compared to elastomeric isolators, and also decreases the soil-abutment interaction. On the other hand, bridge isolation causes a decrease in forces and better distribution of shear force between abutment and piers. Also, the abutment backfill would dissipate some parts of the earthquake energy. The decrease of the earthquake shear force in non-isolated models is more than the isolated ones. On the other hand, by comparing the periods of absence of proper distance between superstructure and infrastructure in the expansion joint position, it follows that this parameter is 0.52 in model E and about 0.36 in E-S. If there is sufficient distance between superstructure and infrastructure in the expansion joint position, these values increase to 0.89 and 0.87, respectively.