Vertical Seismic Isolation by Viscous and Visco-Elastic Dampers Based on Different States Performance for Response Reduction

In this paper, vertical seismic isolation (VSI) of a building for seismic response reduction by partitioning its structure into two different dynamically behaviorsubstructures and linking them together by viscous or visco-elastic links was investigated. To have a better understanding of the VSI features, study of a sample one-story single frame was supposed to be helpful. The Kelvin model was employed to represent the visco-elastic damper for which the stiffness and the damping coefficients are 𝑘𝑙and 𝑐𝑙respectively. In the vertically isolated structures (VIS) by using viscous or visco-elastic dampers, the influence of system dynamic characteristics, including natural frequencies and modal damping ratios in response reduction was of interest. In a comprehensive study, eigenvalue analyses of non-classically damped system were performed. These analyses were also carried out for different parameters of the connecting link. In this study, color contour graphs were employed for presenting the results. The more important advantage of this representation is creating the possibility to observe results of both stiff and flexible isolated structures in one individual graph. This graphical representation was called Vertical Isolation Contour Graph (VICG). Various ratios were considered for mass and stiffness of either of the two substructures to the mass and stiffness of the original structure to find out which ratios or range of ratios result in maximum seismic response reduction. Depending on these mass and stiffness ratios, three states of Mass Isolation, Interactional State, and Control Mass were differentiated in isolation behavior. To study the seismic performance of the one-story VIS, its response histories subjected to different earthquake excitations were obtained by a series of time history analysis (THA) cases. In these analyses, the aim was examining the effects of different parameters on the efficiency of the VSI. In the VSI, one of the main goals was the determination of an appropriate range of mass and stiffness ratios and the associated interconnecting link parameters to achieve the maximum possible seismic response reduction. To solve the governing differential equations of motion numerically, a program, developed by the authors in MATLAB environment based on Runge-Kutta method, was employed. Response ratios that Compare maximum displacements of each substructure of the isolated structure with that of the original structure were taken into account as the VSI performance assessment.The results of numerous analyses on different earthquakes excitation, performed in this study, implied that employing appropriate link parameters, proportional to mass and stiffness ratios, can lead to satisfactory levels of seismic response reduction. To investigate the advantage of the VSI in low-rise multi-story buildings, the seismic performance of a five-story building with the VIS was also studied. Based on the VICGs, it was observed that response ratios are satisfactoryin the short multi-story buildings for both short-period and long-period earthquakes.
The following remarks can be stated as the conclusions of this study:By applying the VSI technique to low-rise multi-story buildings in Interactional State, up to 40% decrease in the seismic response of flexible substructure and even more in the stiff substructure is achievable.In the Mass Isolation state of the VSI, it is inevitable to use relatively large values of practical link damping as 15 to 20% to achieve the reliable response reduction.Link stiffness in the practical range less than 10% of the original structure does not affect response reduction of the flexible substructure.In the Interactional State of the VSI, it is practical to select a wide variety of mass and stiffness ratios with different link damping values.The values of inter-story drifts in low-rise multi-story VSI buildings is less sensitive to input earthquakes characteristics.