Seismic Fragility Analysis of Steel-Reinforced Concrete (SRC) Frame-Bent Hybrid Structure of Main Turbine Building in Conventional Island of Nuclear Power Plant

Steel-reinforced concrete (SRC) frame-bent hybrid structures are widely used in conventional island buildings of nuclear power plants because of their favorable seismic performance and economic efficiency. However, the seismic fragility of these structures has not been adequately investigated. In this study, the main turbine building of the CAP1400 nuclear power plant in Rongcheng, Shandong Province, China, was selected as the prototype. A three-bay frame-bent substructure was extracted, and a 1/7-scale model was designed for pseudo-dynamic testing to investigate the evolution of seismic damage and the failure mechanisms of the structure. Based on the experimental results, a refined numerical model was developed in OpenSees. Incremental dynamic analysis (IDA) was subsequently conducted to evaluate the seismic fragility of the SRC frame-bent main turbine building under far-field and near-fault ground motions. The results indicate that the structural stiffness progressively decreases with increasing seismic demand because of concrete cracking and cumulative damage. Damage is primarily concentrated in the short columns, beam-column joints, and column bases. The fragility response also exhibits pronounced directional dependence. Because of the lower lateral stiffness in the X direction, the structure develops larger interstory drift demands and higher probabilities of exceeding the prescribed damage states in the X direction than in the Y direction. For the selected ground-motion suites, near-fault records generally produce slightly higher exceedance probabilities than far-field records at the same peak ground acceleration (PGA), with a maximum difference of 3.68%. However, the magnitude of this difference varies with the damage state, excitation direction, and ground-motion intensity. These findings indicate that pulse-like near-fault ground motions may have a measurable but moderate effect on the seismic fragility of SRC frame-bent main turbine buildings. This study establishes an experimentally validated framework for assessing the seismic fragility of SRC frame-bent structures in nuclear power plants and identifies their vulnerable components, dominant damage mechanisms, and fragility characteristics under different types of ground-motion input.