This study discussed seismic response of a piping system when the pipe support structure was deformed plastically on being subjected to a much larger seismic load than the designed one. This case is expected as one of the conditions beyond seismic design that should be analyzed. To examine the effect of elastic-plastic deformation of the pipe support structure on seismic response, the effect of various shapes and failure mode of support structures was investigated. Difference of failure modes of the pipe support structure was modeled by change of initial stiffness and secondary stiffness of the load-displacement curve in this study. Seismic response analyses were conducted by using a typical small bore piping system shaped in a three-dimensional arrangement. The investigated change of seismic response behavior generated by plastic deformation of the pipe support structure was clarified based on comparison of seismic responses by using the elastic-fully plastic model and bi-linear model with various initial stiffness and secondary stiffness values. The results showed that the secondary stiffness of the load-displacement curve of the support structure did not affect the decreasing area of the response acceleration and the amount of the decrement significantly. It was concluded that the plastic deformation behavior of the pipe support structure could be modeled by using the elastic-fully plastic model when modeling plastic deformation characteristics of the pipe support structure.
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