Seismic Performance of Base Isolated Buildings At Different Stages of Construction

Abstract eng:
Seismic isolation is one of the most effective vibration control techniques in current earthquake engineering practice. Its main function is to mitigate the damage in structures exposed to strong ground motions, through both the increment of the fundamental period of vibration and the adequate addition of damping, which lead to diminish floor accelerations and drift ratios. Currently, this technology is applied in many earthquake-prone zones in the world, including leading countries like United States, Japan and New Zealand, and others in which its use has started more recently, which is the case of Peru. Given the seismic hazard which threatens countries located in regions of high tectonic activity, particularly in developing countries, it is possible that a severe seismic movement may occur when a projected isolated building is under construction. Usually, the analysis and design of base isolated structures do not take into account their dynamic behavior at different stages of construction. Although an isolated structure under construction tends to have a greater isolation ratio, it has lesser mass and consequently higher isolation effective stiffness which conduct to shorter periods and greater associated damping ratios. These may result in reduced efficiency of the isolators. Aware of that fact, the authors herein studied the seismic performance of the Information Center Building of the School of Civil Engineering at the National University of Engineering, in Lima, at several construction stages. The structure was designed to withstand optimally the displacements and forces due to earthquakes when its eight stories are completed. However, the construction will be carried out in two stages, and only the first four stories will be completed in the near future. Several nonlinear time history and response spectrum analyses of the isolated building were carried out, considering the two stages of construction previously mentioned in addition to a stage in which only the first story on the base isolation system exists. These numerous analyses were performed using maximum considered earthquake (MCE R ) ground motions. The results obtained were assessed, contrasted and discussed in this paper. The conclusions were then extrapolated to other base isolated buildings on the basis of the analysis of the uniform linear shear beam on a linear isolator model. Detailed and easy-to-use graphics are here presented to estimate seismic responses quickly, which include the augmentation of the higher mode contributions because of isolation system damping. That increase is considerable when such damping is greater than twenty percent.

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