Displacement/velocity-based control of a liquid spring—MR damper for vertical isolation

Seismic isolation is an effective technique used to mitigate effects of shaking and helps to achieve higher seismic performance. Recent researches have suggested that vertical excitation has significant effects on structures behavior during earthquake. In this study, three-dimensional (3D) isolation is proposed that combines an elastomeric bearing to resist horizontal ground shaking in series with a bilinear liquid spring (BLS)–controllable magnetorheological fluid damper (CMRD) to resist vertical shaking. A numerical model of a simplified rigid 2D block was developed to predict BLS-CMRD response under earthquake loading. The response of the BLS-CMRDs was simulated through a combination of nonlinear stiffness and viscous and hysteretic (semi-active) damping. A Disp/Vel-based control strategy was proposed that adjusts the input current according to the instantaneous vector combination of feedback displacement and velocity of the damper. Two variations of the control strategy were explored. First, with linear current variation, the current is activated when a threshold lower bound vector magnitude is reached, and maximum current is applied when threshold upper bound magnitude is exceeded. Second, the simplified ON–OFF strategy uses single threshold vector magnitude that triggers the maximum current to turn on when the instantaneous vector magnitude exceeds the threshold, and turn off otherwise. Results show that for ground motions that exceed the design level, Disp/Vel-based Control is effective to moderate the level of energy dissipation, keep the device vertical displacement within the design stroke limit, and attenuate vertical acceleration below PGA. In addition, Disp/Vel-based control reduces all responses relative to the well-known clipped optimal strategy used for structural control.