Masonry arch bridges are vital infrastructures in the system of railway transportation. In Iran's railway network, there is a large number of masonry arch bridges built 80 years before. These structures have been designed just for gravity loads, not for earthquake excitations. Thus, their seismic assessment is critical. Probabilistic performance-based earthquake engineering (PBEE) is preferred to other seismic response assessment methods, since the randomness and uncertainty in structural resistance, as well as seismic excitation, is involved in this method. The PBEE procedure assesses the seismic performance of structures concerning decision variables of downtime, fatalities, and monetary losses. The decision variables involve the evaluation of the limit state risk. For this purpose, 22 far-field earthquake records are employed for incremental dynamic analysis (IDA) of two railway arch bridges located at km 23 and 24 of the old railway of Tehran-Qom, respectively. Using the results of IDA, the capacities of performance levels, the mean annual frequency of exceeding the performance limits and levels of confidence of meeting seismic performance objectives, fragility curves, damage state probabilities, mean rate of a certain limit state per year, deaggregation of annual rate of limit states, and probability of reaching limit states over different life spans are obtained.
