Design and analysis of LRB base-isolated building structure for multilevel performance targets

According to China's current code for the seismic design of buildings (GB50011-2010), in the course of designing base-isolated building structures, the isolation system and superstructure are designed separately. Hence, the horizontal seismic isolation coefficient in terms of the design-based earthquake (DBE) and horizontal deformation of the isolation system under the maximum considerable earthquake (MCE) cannot be considered simultaneously. Consequently, the optimum mechanical parameters for isolation bearing and the corresponding horizontal seismic isolation coefficient used in the seismic design of the superstructure cannot be obtained. To address these shortcomings, we propose an optimum seismic design procedure for LRB base-isolated building structures to meet multilevel performance fortification criteria, where explicit mathematical formulations for horizontal seismic isolation coefficients and the horizontal deformation of isolation systems in terms of the mechanical parameters of the LRB isolation system are established by combining equivalent linearization and modal decomposition response spectrum methods. Through a parametric study, we find the critical yield strength ratio, i.e., the ratio of the yielding shear of lead core to the total weight of the LRB base-isolated structure, which leads to the minimal value of the horizontal seismic isolation coefficient. Beyond this critical yield strength ratio, an optimum solution is unavailable, as the horizontal seismic isolation coefficient and horizontal displacement of the isolation system are of a conflicting nature. By converting the multi-criterion formulation into an auxiliary problem with a scalar objective, the optimum design method of an LRB base-isolated building structure oriented to multilevel performance fortification criteria is proposed, so as to obtain the optimum yield strength ratio and the optimum second shape coefficient for the seismic design of the superstructure. A numerical example demonstrates that the seismic horizontal seismic isolation coefficient and maximum horizontal displacement of the isolation system calculated from the optimum yield strength ratio and the optimum second shape coefficient agree well with the results from the time-history method. In a comparative study, LRB base-isolated building structures designed by the critical yield strength ratio, the optimum yield strength ratio and the optimum second shape coefficient, the optimum yield strength ratio and the corresponding fixed-base superstructure are subjected to MCE and mega earthquakes, where the LRB base-isolated structure shows good seismic performance in terms of the optimum yield strength ratio, even when subjected to mega earthquakes.