Passive Seismic Control of Buildings by Using a Large Mass Resting on Rollers at Base Level and Connected to the Roof by a Stiff Structure

Mass dampers have been used in recent years as a device for passive control of structures subjected to lateral load due to wind and earthquake. This device, which consists of a mass connected to a spring and a damper, is usually installed at the upper story of the building, and its mass is relatively small compared to the mass of the building and even building’s story. In this study, the control mass is very large and is resting on rollers at the building’s foundation level, and its inertia force is transferred into the building’s structure by a relatively stiff structure, connected to the upper story of the building, and can decrease the seismic response of the building by its interactive effect. It is assumed that the building has a large void at its middle, which can place the relatively stiff structure connecting the control mass to the building roof. In this research, three buildings, having five, eight, and eleven stories equipped with the proposed control mass have been studied. For this purpose, seven three-component accelerograms with PGA values of 0.3g to 0.8g have been used for a series of time history analysis (THA). The considered buildings have been designed based on Iranian Standard No. 2800 for seismic loading, assuming type II for site soil, and AISC89-ASD for design of steel members, considering moment frames as the load bearing system. Compatible with the site soil, the selected records for THA cover a relatively wide range of input frequencies to make it possible to investigate the performance of the proposed control system with relatively high generality. The used earthquake for this purpose include San Fernando, Friuli (Italy), Tabas, Coyote Lake, Northridge (from two stations), and Kobe. The PGA values of the selected records varies from o.27g to 0.88g for horizontal components and from 0.15g to 0.40g for vertical component, and their duration varies between 23.78 sec and 47.94 sec. Various amounts have been considered for the control mass and its corresponding stiffness and damping to find the optimal values for each building. Maximum roof displacements in two main horizontal directions, and in two states of without and with the control mass have been used as the response value for comparing different cases. Results of THA show that using the proposed control mass with appropriate stiffness and damping corresponding values can decrease the seismic response of buildings around 40% in average.