Modal Analysis versus Time History-Analysis, Concepts for the Seismic Design

Th. Baumann & J. Böhler (WALTER/DYWIDAG, Central Technics, Munich, Germany)

The forces induced into a structure by earthquake ground motions are prescribed traditionally by elastic response spectra. They can be reduced by behaviour factors to account for the possibility of energy dissipation due to plastic deformations. Presupposition for the utilization of this favourable effect is a sufficient ductility. The consistent definition and verification of this ductility is an essential part of the structural design for earthquakes. In the present paper, the principal features of this task are discussed. In order not to persist in general terms, the governing effects are quantified within a case study of a 3-storey steel frame with composite beams, considering the following steps: Modal analysis, application of behaviour factors, usual assumptions for the required plastic deformations, push over-analysis with plastic rotation of hinges, connection of modal analysis and push over- analysis, nonlinear time history-analysis. Only the last step creates the basis for a reliable assessment of the relevant deformations of the plastifying frame, depending on the applied accelerogram and its peak value PGA.

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