Comparison Between the Coupled - Bidirectional and Uncoupled Unidirectional Excitation Response of Lead - Rubber Isolators in Isolated Bridges

Highway bridges are the most important members of transportation network, which play the key role of lifelines at the time of natural disasters like earthquakes. Several solutions for rehabilitation of highway bridges have been proposed by engineers among which seismic isolation solution seems to be a good solution from the practical point of view.Replacement of existing elastomeric bearings with lead-rubber seismic isolators is a solution that exempts the substructures from expensive retrofit work. Since the use of seismic isolators increases the displacements of the deck, more exact estimation of the displacements is essential to provide enough gap distance at the end of girders. In case of underestimation of the gap, there is a high probability of collision between two adjacent decks or deck and abutment. Such collision causes not only seismic isolation to be non-effective but also harmful for bridge health and serviceability. Deck fall and abutment rotation are some of the results.In this paper, for more exact estimation of the deck displacements, a coupled plasticity model has been used for nonlinear modelling of the seismic isolators. Earthquake excitation also has been applied bidirectional in two orthogonal (longitudinal and transverse) directions simultaneously. A comparison between the results of coupled plasticity-bidirectional excitation and uncoupled plasticity-unidirectional excitation has been carried out. The results show that the deck displacements are estimated unrealistically in the uncoupled plasticity-unidirectional excitation modelling (common engineering practice) relative to the coupled plasticity-bidirectional excitation modelling.