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Base isolation technique is a new approach in seismic design of structures in which an isolation layer at the bottom of structure helps to reduce earthquake effects on the system. However this powerful technique is less effective in the near-fault regions due to long period pulses of earthquake records. In this work two base isolated structures subjected to pulse type earthquakes were studied. The fixed base natural periods in these structures are chosen far apart (0.4 and 0.75 second), but their isolated periods are chosen equal to each other (2.5 seconds). The main objective of this study is to investigate on the role of linear and nonlinear viscous dashpots on responses of base isolated buildings subjected to near-fault earthquakes. So that, a number of near-fault records along with some far-fault ones are used in a series of time integration analyses on both structures. The model for base isolation system in this work is a simplified model in which the base isolated building is assumed as a system with only two degrees of freedom. The main structural system and the isolation components are all assumed to behave linear in the analysis procedure. Analyses have been carried out for both structures, equipped with linear and nonlinear viscous dashpots. In case of nonlinear viscosity, velocity exponent is assumed equivalent to 0.5. Average values of maximum responses, including damper force, super structure drift and base displacement are calculated for both types of earthquakes (near-fault and far-fault records). The results show considerable gains in reducing base displacement and superstructure drift in both structures while using nonlinear viscous dashpots. However, the role of nonlinearity in damping devices on acceleration reduction in super-structure is considered marginal. The results also indicate the existence of an optimum damping value for the system in both cases of using linear and nonlinear damping devices. Providing this amount of damping for the system, minimum value for the selected responses in the structure will follow. In practice, defining an optimum damping value for the system needs a cost benefit study based on the desired structural responses in different dashpot technologies

Keywords: Base isolated building, Nonlinear viscous dampers, Near fault earthquakes

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