This study describes inelastic seismic design of piping systems considering the effect of plastic deformation of a pipe support structure. The damping coefficient of a piping system is focused on, and the relation between seismic response of the piping system and elastic-plastic behavior of the support structure was studied using nonlinear time history analysis and complex eigenvalue analysis. The analysis results showed that the maximum seismic response acceleration of the piping system decreased largely in the area surrounded by pipe elbows including the support structure which allowed plastic deformation. Furthermore, modal damping coefficient increased a maximum of about seven-fold. The increase ratio of the modal damping coefficient was proportional to the size of the effective mass ratio, when a relatively large increase was seen in the increase ratio of the modal damping coefficient. On the other hand, the amount of the initial stiffness of the support structure made a difference in the increasing tendency of the modal damping ratio. In the case of relatively small initial stiffness, the modal damping ratio of only one vibration mode increased. The increment of the modal damping ratio was proportional to the effective mass ratio in the case of large initial stiffness. In the viewpoint of the inelastic seismic design, the seismic response of the piping system was little affected by the plastic deformation of the support structure with 10% variation of the secondary stiffness to the initial stiffness. The result suggested that the seismic response of the piping system with the support structure can be estimated by using only the support model which has the elastic perfectly plastic property even if there are various shapes of steel type of support structures.

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