Design of lead-rubber isolators with energy method and evaluating its accuracy for very important steel structures

Base isolation is an effective technology for reducing seismic damage to structural and non-structural components as well as building contents, allowing buildings to maintain their function during and after a rare, high-intensity earthquake. This makes it an ideal seismic response correction system for importance buildings. The main advantage of isolated structures is that seismic responses can be easily and effectively reduced by prolonging the period and increasing damping. Therefore, the natural period of the structure isolated from the base is longer. In this paper, a new energy balance method is used to design a lead rubber bearing (LRB) isolator. Energy balance method is an analysis method to evaluate seismic resistance based on the balance of seismic energy input to buildings due to ground motion and energy absorbed by the building. In other words, the energy balance method is a response prediction method to approximate the seismic response of buildings isolated from the foundation. This method is effective for determining the relationship between ground motion, seismic isolation period and the effect of reducing the reaction of dampers. In this method, the specifications of the isolation system, including stiffness, yield shear force and viscous damping ratio, are adjusted in such a way that the maximum shear and maximum displacement in the isolation system do not exceed a certain value determined by the designer. This allows the designer to limit the maximum displacement at the isolation level to a certain amount when there is a constraint on the supply of separation distance around the building and the isolated level. Also, by limiting the maximum shear of the isolators, it is possible to use the base isolation system for retrofitting the existing structures that have a certain lateral capacity.
This design method was first proposed and used in Japan. This method has been recently proposed in the Iranian regulations (which is being drafted) and has not been used much in this country so far. Its advantages include no need for trial and error in the design process, the possibility of designing a rubber and frictional type of seismic isolator, the possibility of using a viscous or hysteretic damper, or a combination of both at the isolator installation site. To evaluate the accuracy of this method, three 5, 10 and 15-story steel structures with an ordinary concentric braced frames in both directions for clinic usage have been modeled and under eight near and far-field earthquakes in the by the nonlinear time-history analysis method have been analyzed. The results obtained from the time-history analysis are in good agreement with the estimated results of the energy balance method. The error percentage related to the displacement of isolator compared to the value assumed at the beginning of the design for 5, 10 and 15-story structures is 3.4%, 2.57% and 2.12%, respectively. Also, the percentages of error related to the maximum shear of isolator compared to the value obtained from the performance curve for 5, 10 and 15-story structures are 11.08%, 12.61% and 13.98%, respectively.