Abstract:Near-fault ground motions can cause severe damage to structures because of its high peak accelerations and macrocyclic velocity pulses.In the last two decades,the seismic isolation technique,which was proved to be very effective for the seismic protection of new constructions and the seismic retrofitting of existing ones,had been applied for buildings,bridges,industrial facilities,and structures of historical value as well.As a consequence,design guidelines for seismically isolated structures have been developed in many countries with a high seismic hazard.The insertion of an isolation system at the base of a building structure allows the horizontal seismic loads to be reduced through a decoupling of the structure motion from that of the soil;moreover,the superstructure behaves like a fixed or base isolated structure along the vertical direction,depending on the value,respectively very high or very low,of the ratio between the vertical stiffness of the isolator and the horizontal one.Base isolation technique,taken as an effective method to control structure＇s vibration that is induced by earthquake ground motions,has been widely accepted,however,only such an approach can not ensure structure＇s safety under near-fault pulse-like ground motions with high energy in low frequency band,because strong near-fault ground motions are characterized by long-duration horizontal pulses and very large displacements,which can lead to an oversizing of the isolation system and an amplification of the response of a base isolated structure.Specifically,the frequency content of the motion transmitted by the isolators to the superstructure can become critical when the pulse intensity is so strong that the superstructure undergoes plastic deformations.Moreover,the structural response can be amplified due to the long duration of the pulse.To overcome these problems,many authors have proposed a lot of solutions based on different kinds of isolators and dampers.Nevertheless,some problems remain unsettled if additional damping is adopted,because the displacements at the top of the isolators can be substantially reduced,but the contribution of the higher vibration modes of the superstructure increases,if this contribution becomes too large,it can give rise to significant increase in inter-story drifts and floor accelerations,so it is necessary to find more effective vibration control strategies.A pulse model and a stochastic ground motion model are combined to simulate the near-fault ground motions.The simulation of directivity pulse type ground motions through superimposition of modeled directivity pulse on a non-pulse type motion is a possible approach to meet the scarcity of such motions in studying the structural response.Typically,pulse duration or period,pulse amplitude,and the number and phase of half cycles constituting the pulse have been considered as the parameters which are necessary for such a characterization. The structure dynamic responses are solved by using MATLAB under these artificial ground motions.The effectiveness of TMD-Base isolation hybrid control method is studied through the comparisons of response time histories of a base isolated structure with and without TMD.The results show that this hybrid control method is capable of controlling isolation layer＇s displacement effectively.
