Seismic Margin Evaluation Methods Using Equivalent SDOF Model and Elasto-Plastic Response Spectrum

Two simplified methods for evaluating seismic margin due to elasto-plastic response were proposed. Generally, elasto-plastic response is evaluated by nonlinear time-history response analysis using three-dimensional FEM model (3D FEM model). It, however, takes an immense amount of time with commonly used computers. In order to evaluate it in a shorter time, this study developed seismic margin evaluation methods using Equivalent Single Degree Of Freedom (ESDOF) model and elasto-plastic response spectrum. Additionally, the accuracy of the two methods was verified by static loading tests and vibration tests. Simple cantilever test specimens with several natural frequencies were used in the vibration tests, and input waves with several frequency characteristics were applied to each vibration test. Response displacement, response acceleration of the test specimens and input acceleration were measured in each vibration test. Maximum displacement given by ESDOF model of the test specimens was compared with the corresponding measured values of each vibration test in order to verify the accuracy of ESDOF model. Difference between the maximum displacement given by the ESDOF model and the vibration tests was around 5%, and computation time of the ESDOF model was one-tenth of 3D FEM model of the test specimens. In addition, elasto-plastic response spectrum of input waves in the vibration tests were compared with measured yield accelerations of the specimens in order to verify the accuracy of elasto-plastic response spectrum. Difference between the calculated elasto-plastic response spectrum and the measured yield acceleration of the test specimens was around 10%, and computation time of elasto-plastic response spectrum was one-tenth of the 3D FEM model. As a result, it is concluded that ESDOF model and elasto-plastic response spectrum are powerful tool to evaluate seismic margin.