Keywords: bridge isolation, seismic dampers, near-fault excitation, structural robustness, model uncertainty, stochastic simulation.

Applications of seismic isolation techniques to bridges have gained significant attention over the last decades. This configuration provides enhanced capabilities for energy dissipation during earthquake events while also accommodating thermal movements during the life-cycle of operation of the bridge. It is associated though with large displacement, especially under near fault earthquake ground motions, which may lead to inelastic deformations and to seismic pounding of the deck between adjacent spans or to the abutments. For controlling such vibrations, application of seismic dampers has been proposed [1]. One of the main challenges in the design of such dampers has been the explicit consideration of the nonlinear behaviour of the isolators and the dampers in the design process as well as proper modelling of soil-structure interaction. Another challenge has been the efficient consideration of the variability of future ground motions.

An approach that addresses these challenges in the design of supplemental dampers for seismically isolated bridges is discussed in this work. A probabilistic framework is used to address the various sources of excitation and structural uncertainties and characterize the overall bridge performance. Stochastic simulation is used to evaluate this stochastic performance and an efficient computational framework [2] is adopted for performing the associated design optimization and concurrently establishing an efficient probabilistic sensitivity analysis for the uncertain model parameters. In this efficient, simulation-based framework, consideration of complex nonlinear models for the bridge system and its excitation is feasible. The bridge model adopted in this study explicitly addressed all nonlinear characteristics of the isolators and the dampers, the dynamic behaviour of the abutments and the effect of seismic pounding. A realistic probabilistic model for describing future near-fault excitations is also considered.

An illustrative example is presented that considers the design of nonlinear viscous dampers for protection of a two-span bridge. The fragility of the bridge system due to seismic pounding, but also against failure modes related to pier shear and abutment deformations, is considered as the performance measure quantifying seismic risk. The addition of the dampers is shown to provide considerable risk reduction, especially with respect to the fragility against seismic pounding. Results from the probabilistic sensitivity analysis show that the excitation properties have the highest importance in affecting seismic risk and that the inclusion of the dampers does alter the sensitivity characteristics.

1

N. Makris, J. Zhang, "Seismic response analysis of a highway overcrossing equipped with elastomeric bearings and fluid dampers", Journal of Structural Engineering, 130(6), 830-845, 2004. doi:10.1061/(ASCE)0733-9445(2004)130:6(830)

2

A.A. Taflanidis, J.L. Beck, "An efficient framework for optimal robust stochastic system design using stochastic simulation", Computer Methods in Applied Mechanics and Engineering, 198(1), 88-101, 2008. doi:10.1016/j.cma.2008.03.029

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