Seismic design of piping systems is usually based on elastic vibration theory. However, increase in design earthquake intensity makes it necessary to permit plastic deformation of piping element. Although it is possible to calculate elasto- plastic vibration response of piping systems using the direct integral method, the approach is not suitable for design applications. Static load test and vibration test of a simplified piping model were carried out in order to study the elasto-plastic behavior. The equivalent damping ratio during elasto-plastic vibration is calculated using equivalent plastic strain and the volume in which plastic deformation occurs. Both nonlinear modal analysis and linear analysis using equivalent damping ratio were applied to two piping models and the analytical results were compared with those of the tests. The results of the two anaytical methods coincided witht those of the tests in the case of small plastic deformation.
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