An inertia force in a seismic response of structure excited by an earthquake generates the seismic force acting on the structure. The handling of seismic forces has been being discussed in terms of how the seismic force on a piping controls the deformation of the piping, load-controlled or displacement-controlled. A seismic design code for nuclear facilities applied in Japan qualifies this kind of seismic forces as primary stress components which shall be limited to prevent any plastic collapse, on the assumption that the seismic force mainly consists of load-controlled loads and the deformation due to earthquakes is caused by the loads. The authors studied about a condition of plastic collapse occurrence by the relationship between response acceleration and displacement of SDOF system. And it was represented in a previous paper that plastic collapse hardly occurred to soft structure due to an inertia force generated from response acceleration tended to oppose a response displacement.
Several elastic plastic response analyses of elastic-perfectly-plastic SDOF model are performed with applying dynamic load and both of dynamic load and static load. By the results of the analyses, three forces, which are inertia force, element force and external force, are investigated the relations against the deformation.