Seismic Response and Train-Running Safety of Two-Story Reinforced Concrete Viaduct with Seismically Repaired Middle Beams

On March 16, 2022, a magnitude 7.4 earthquake, hereinafter referred to as the 2022 Fukushima EQ, struck off the coast of Fukushima Prefecture. The 2022 Fukushima EQ caused the derailment of a Tohoku Shinkansen bullet train and inflicted damage on seismically repaired middle beams of a two-story reinforced concrete (RC) viaduct. This study evaluated the seismic performance and train-running safety of a two-story RC viaduct, specifically focusing on those with seismically repaired middle beams, using incremental dynamic analysis. To evaluate the impact of seismic repairs on train-running safety, we adopted a method that calculates the spectral intensity (SI) of the acceleration response time history at the top of the viaduct during seismic events. This study defines the SI value derived from the acceleration response at the top of the viaduct as SItop, distinguishing it from the commonly used SI value, which describes the intensity of seismic waves. The numerical model representing the as-built condition of the viaduct was labeled the “As-built Viaduct Model,” while the model featuring a repaired middle beam was termed the “Repaired Viaduct Model.” It should be noted that the aim of this study is not to determine the cause of the bullet train derailment in the 2022 Fukushima EQ, but rather to assess the impact of repairing middle beams on train-running safety on two-story RC viaducts. The incremental dynamic analysis revealed substantial differences in maximum displacement values within a response drift range below 0.01 between the As-built Viaduct Model and the Repaired Viaduct Model. However, the repair of the middle beams did not significantly affect the ultimate behavior of the two-story RC viaducts. Regarding train-running safety, the SItop value of the Repaired Viaduct Model consistently exceeded those from the As-Built Viaduct Model. In some instances, the SItop value increased by approximately 1.5 times due to the repair of the middle beam. Notably, in many cases, the disparity in SItop values between the two models became more pronounced as it approached the design limit value. Therefore, the alterations in dynamic response of two-story RC viaducts due to the seismic repairs on middle beams could significantly influence train-running safety.