Seismic response of base-isolated high-rise buildings under long-period ground motions

Base isolation systems may be considered as one of the most powerful tools of earthquake engineering pertaining to the passive structural vibration control technologies. It may enable a building or non-building structure to survive a potentially devastating seismic impact. Generally, it is thought that application of seismic isolation is limited to low- and medium-rise structures, and the use of isolation for high-rise buildings considered as impractical or unfeasible. However, existing examples of isolated high-rise buildings in Japan, also in Iran, suggest that these viewpoints clearly disagrees with the real state-of-practice that exists there. Since the 1995 Kobe earthquake, just fewer than 200 isolated high-rise buildings, ranging from 60 to 180 meters height, have been constructed in Japan. However, this strategy is still uncommon in most countries of the world. Implementation of base isolation can greatly decrease inter-story drifts and floor accelerations, which results in protection of building’s contents. As a result, high-rise buildings can be kept fully operational during the earthquake and also immediately occupiable just after the event. In other words, isolation can be adopted for the improved performance of high-rise buildings. To maintain the efficiency, the period of isolation system has to be considered between 4 and 7 seconds. Clearly, structures like this will be vulnerable to long period ground motions. Therefore, it is necessary to study the behavior of these structures under such earthquakes. Long-period ground motions can be divided into far-source and near-fault classes. Most far-source long-period ground motions were generated by large earthquakes and effective propagation paths. Therefore, far-source long-period ground motions are generally associated with offshore earthquakes in subduction zones. Near-fault long-period ground motions are generated mainly by rupture directivity effects in the vicinity of earthquake source faults,. They consist primarily of rupture directivity pulses, which can be damaging, especially when combined with site effects and basin edge effects. In this paper, three base isolated models of 8-, 14-, and 20-story shear buildings using isolator type of lead-rubber bearing (LRB) and friction pendulum system (FPS), under long-period ground motions are studied. A set of 14 long-period ground motions – 5 far-source long-period motions and 9 near-fault long-period motions – as well as 14 short-period ground motions were selected. Total earthquake input energy per unit mas was used as a measure to distinguish long-period motions so that those which had a significant input energy over the periods of 2 seconds were considered as long-period motions. For each model two isolators – LRB and FPS – were designed so that the design displacement and the period of systems were exactly the same. The isolators were designed carefully and all dimensions and parameters were checked to insure practicality of the design. Then nonlinear dynamic analysis was implemented to evaluate the response of the structures. Results show that in the cases that input motions are short-period, increasing the height of the structure doesn’t significantly affect the structure response and the isolation displacement are nearly the same. On the other hand, as the height of the structure is increased, its response due to the long-period ground motions becomes more significant, and these motions impose a great displacement demand in the isolation system.