Usage of Simplified N2 Method for Analysis of Base Isolated Structures

Abstract eng:
The paper examines the usage of a simplified nonlinear method for seismic analysis and performance evaluation (N2 method) for analysis of base isolated structures. In the paper the N2 method is applied for analysis of a fixed base and base isolated simple four-storey frame building designed according to EC8. Two different sets of base isolation devices were investigated: a simple rubber (RB) and a similar lead rubber bearing (LRB) base isolation system. For each system a Soft, Normal and Hard rubber stiffness and three different damping values were used. The paper shows how we can obtain base displacement and top (relative) displacement for different bearing stiffness and selected damping. The target base displacement was determined as an intersection of the capacity curve of single degree of freedom system with rigid behavior of the superstructure and demand spectrum curve for selected damping of isolators. In the following step the pushover analysis of the whole isolated structure was performed up to the target base displacement using constant load distribution. The results are presented in terms of base and top displacements and ductility factors for those base isolation systems which were not able to protect the superstructure. It has been shown in the paper that N2 method might be a valuable tool for design, analysis and verification of behavior of base isolated structures with different linear or nonlinear seismic devices. Nonlinear pushover analyses were performed with the computer program SAP2000.

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