There are numerous old arch bridges in Iran that have been used as railway bridges for more than 70 years. Field load testing of old railway bridges in km-23 and km-24 of Tehran–Qom railway have revealed important characteristics of the bridges and have proven that there is still a large capacity under service load. Since most of these bridges are not designed for earthquake excitation, seismic vulnerability of these structures is uncertain. This fact necessitates the investigation of the earthquake resistance of these kinds of bridges. In this paper, seismic performance of bridges is assessed by the pushover analysis. These bridges are plain concrete arch structures with different span length and mechanical properties which have been built 70 years ago. The results of dynamic load tests are used to calibrate a finite element model of the bridges in which a plain strain analysis is carried out. Using non-linear static analysis method, suggested by several modern standards, the capacity curves of the structures and non-linear demand spectrum are obtained. The choice of control nodes in the pushover analysis and their influences on seismic performance of plain concrete arch bridges are investigated. The crown, centre of mass and a virtual point are selected as control nodes and it is demonstrated that the capacity curves are almost identical in all control node cases. Finally, the demand levels of the bridges are determined and compared with capacity curves. The results show that the earthquake resistance of these kinds of bridges totally depends on material properties and geometry of the structures.
