Initial stiffness degradation and its effect on seismic capacity of shear wall with high reinforcement volume: An experimental study

•Results of change in the natural period and stiffness degradation based on long-term earthquake observation on Onagawa NPP.•Clarify the influence of drying shrinkage on the initial stiffness and seismic performance of the RC wall.•Clarify the influence of prior damage on the seismic performance of the RC wall. Nuclear simulation analyses and field investigations were performed to study the seismic response of the reinforced concrete (RC) walls of the Onagawa NPP Unit 2 reactor building to the Tohoku Region Pacific Coast Earthquake on March 11, 2011 (hereafter referred to as the “March-11 Earthquake”). They indicated that the degree of deformation of the walls remained within the elastic range and did not reach the plastic range. Small residual cracks appeared in the shear walls on some floors, and the observation records of the natural period of the building exhibited stiffness degradation. In addition to the effect of large seismic loads, which are among the factors responsible for such reduced initial stiffness, a correlation between reduced stiffness and deterioration due to aging was recognized in the results of the analyses based on long-term earthquake observation records. The likely factors responsible for deterioration from aging include internal stress and cracks resulting from the drying shrinkage characteristics of concrete. Therefore, we conducted seismic resistance experiment #1, a static load test, to understand the effect of drying shrinkage on the initial stiffness and maximum strength. The results for the specimens subjected to drying showed better agreement with the results of the observation record analysis of the reactor building than those that were not subjected to drying. In addition, because small residual cracks occurred in the reactor building, we conducted seismic resistance experiment #2, a static load test, to understand the effect of such cracks on the ultimate strength and deformation capacity. The test confirmed that the seismic performance of the building after the earthquake was ensured. From the results of these investigations, it is considered that both the drying shrinkage of concrete and large seismic forces from the March-11 Earthquake were factors in the initial stiffness reduction of the Onagawa Unit 2 reactor building. However, there was no significant impact on the seismic performance of the building.