Keywords: seismic response, base isolation, friction damper, passive energy dissipation, coupling mechanism.

Under extreme earthquakes or near-fault ground motions base-isolated buildings may experience large deformation in the isolation layer that can over-reach the seismic isolation gap. Consequently, collision between the base-isolated structure and the retaining wall produce high accelerations in the base that increases the inter-story deflections as well as higher modes of vibration can be excited [1]. The friction damper with coupling mechanism for base-isolated structures is presented as a novel safety device to reduce the base displacement and avoid collision during extreme events. This damper is composed of a coupling mechanism combined with friction element connected in a series with elastic springs. Due to the preset gap of the coupling mechanism, the friction damper does not provide energy dissipation during moderate and severe seismic events. However, when base displacement exceeds the preset gap during unexpected strong motions higher than the design level, the coupling mechanism is activated, which subsequently causes the damper to exert frictional force. Frictional force provided by the friction element is independent of the building weight, and can be settled in order to get the best performance depending on the building base-isolation system, weight or limitation gap. Elastic springs attached to the friction element absorb the shock due to coupling and allow the friction element to move smoothly when sliding starts. To determine the damper properties needed for numerical model static and harmonic loading tests were performed on a small-scale friction damper. Experimental results showed that damper elastic stiffness provided by the springs is independent of motion amplitude and frictional force. The numerical model has been verified through shaking table experiment of a single degree-of-freedom base-isolated structure. Experimental results showed that friction dampers with a coupling mechanism can reduce base displacement while acceleration is slightly increased. Also, a numerical study on a real-scale two-storey base-isolated detached house has been used to investigate the effectiveness of friction damper with coupling mechanism against extreme earthquakes. In the analysis, near-fault JMA Kobe NS and Hachinohe EW with velocity level normalized at 1.1 m/s and 0.75 m/s, respectively, were employed. The results showed that base displacement decreased while storey drift angle remained lower than 1/200. Accelerations were increased due to the frictional force of the damper, however they were small compared with the acceleration response due to impact on retaining wall. Finally, a numerical study has been established to analyze the damper performance after the main shock when the damper is coupled from the beginning and aftershock occurs. The high performance during and after the main shock shown in the numerical study results indicated that the friction damper as a safety device can control base-displacement and avoid collision, even for extreme ground motions.

1

V.A. Matsgar, R.S. Jangrid, "Seismic response of base-isolated structures during impact with adjacent structures", Engineering Structures, 25, 1311-1323, 2003. doi:10.1016/S0141-0296(03)00081-6

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