Seismic Performance and Collapse Fragility of a 765 kV Transmission Tower–Line System

Based on a real-world project in Pakistan, this study investigates the seismic performance and collapse fragility of a 765 kV transmission tower–line system. A refined finite element model, incorporating three towers and four conductor spans, is developed to systematically simulate the system’s dynamic characteristics, seismic response, and nonlinear collapse process. The Incremental Dynamic Analysis (IDA) method is employed for fragility assessments. The results demonstrate that the fundamental frequency of the tower–line system is significantly lower than that of an isolated tower, indicating that the transmission lines substantially reduce the overall structural stiffness. The vulnerable regions in the system are primarily identified at the second and third segments. The mean Peak Ground Acceleration (PGA) triggering collapse is found to be 1.07 g, with the collapse mode characterized by a progressive failure initiated by cumulative damage in the lower members. The derived fragility curves indicate that the probability of system collapse exceeds 55% at a PGA of 1.0 g. These findings can provide a valuable reference for the seismic design and safety evaluation of high-voltage electricity transmission systems.