The effect of Stiffness on Behavior of Isolated Tall Buildings

The basis of seismic separators in reducing the acceleration applied to structures is based on an increase in the natural period of vibration of the structure. In high-rise structures, the natural period itself is high. In this case the flexibility of superstructure may be in conflict with its elastic behavior. In the present study, 240 structures were modeled and analyzed in the first step to the question of whether the addition or reduction of the stiffness of the substructure and superstructure (1-100 times and 0.001 to 1 times respectively) affected the acceleration distribution or not? The results of the analysis of structures that were of five types of plans and at elevations 10, 15, 20, 24 and 28 floors showed that adding the stiffness of the superstructure can lead to a decrease of about 30 percent of the maximum roof acceleration (relative to the isolated structure without increasing the stiffness of the superstructure); However, the use of base isolation in the structures decreased about 50% of roof acceleration. The significant decrease in roof acceleration was related to structures with a 10 times increase in the stiffness of the superstructure compared to normal structure. In the next step, with the push-over analysis of 15 structures of this set, it was determined that in all isolated structures (with varying stiffness), the superstructure will remain in elastic state and none of the hinges formed in the members, exceeded from IO region.