Theoretical and experimental investigation on the seismic performance of a novel variable-damping viscous fluid damper

This paper presents a novel three-stage variable-damping-coefficient viscous fluid damper (VCVFD) and investigates its effectiveness in mitigating the seismic structural response. The concept and design of the displacement-dependent VCVFD were based on the equations for one-dimensional viscous flow in the damper orifice. A series of performance tests on VCVFDs were conducted to investigate the mechanical behaviour of the dampers with different design parameters under various loading conditions. The damping force increases more significantly than in the conventional viscous fluid damper (VFD) when the operating displacement is larger, and the expected effect of variable damping is achieved. A smaller gap height is more beneficial to energy dissipation in the VCVFD. Moreover, the hysteresis loop of the VCVFD was consistent with the theoretical results. To validate the effectiveness of the VCVFD in structural control, shaking table tests of two-storey steel frames equipped with VCVFDs under different seismic intensities were conducted. VCVFD can slightly increase the natural frequencies of the steel frame and significantly increase the structural damping ratio. The effectiveness of the VCVFD in mitigating the structural acceleration under low earthquake intensities was not prominent, but as the earthquake intensity increased, the acceleration amplification factor decreased. The added damping contributed by VCVFDs can significantly reduce the displacement response. It is concluded that with reasonable damping structural design, the VCVFD can achieve an almost similar mitigation effect as VFD in low-intensity earthquakes, whereas the VCVFD can provide higher damping under high-intensity earthquakes.