Seismic assessment of base isolated structures using n2 method

Abstract eng:
In the paper a simplified nonlinear method (N2 method) is applied for analysis of base isolated structures. In the first part of the paper a new three-linear idealization of the capacity curve for a base isolated structure is proposed. The initial stiffness of the capacity curve is defined based on the first yielding point in the superstructure. In this way the N2 method is able to detect the first yielding of the superstructure (which might be a crucial parameter for a base isolated structure) as well as to estimate the behavior of the superstructure further in the nonlinear range. A consequence of such idealization is much steeper second slope (hardening) which does not correspond to presumptions used in the original N2 method. For this reason, a parametric study of idealized SDOF systems with different hardening slopes has been performed first. Different parameters were examined: initial stiffness/period, post yield hardening ratio, reduction factor and damping, while the hysteretic model remained the same (Q-model) in all cases. The inelastic spectra were derived for the systems in the long periods (T>Tc) that are characteristic for base isolated structures. Top displacements were obtained as average of nonlinear dynamic analysis results for the ensemble of seven EC8 spectrum-compatible artificial accelerograms. It was concluded that the equal displacement rule could be assumed also for the systems with proposed non-zero post yield stiffness. In the second part of the paper the proposed three-linear idealization is used for the seismic analysis of base isolated symmetric four-storey RC frame building designed according to EC8 and isolated with lead rubber bearings (LRBs). The building response is analyzed for three different ground motion intensities. Nonlinear dynamic and pushover analyses were performed with the computer program SAP2000 and the results of pushover analyses are presented for four different lateral load distributions. The target top displacement of base isolated structure was determined as an intersection of the proposed idealized capacity curve and inelastic demand spectrum curve. The results are presented in terms of top and base displacements, as well as damage patterns of the superstructure. Comparisons of the results of nonlinear static with the results of nonlinear dynamic analyses have shown a very good agreement. It has been shown that presented simplified approach might be a valuable tool for design, analysis and verification of behavior of base isolated structures.

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