Karaj bridge, located on Km 38+000 of Iranian railway network, is a 70 years old unreinforced concrete bridge. The bridge consists of seven spans of 21m, with a total length of 190 meters. The bridge has a width of 4.5 meters and accomodates a single track, with an allowable axle load of 20 tons, and a permissable speed of 80 km/h. Superstructure consists of UIC60 rails, wooden sleepers, and screw fastners. All rail joints on the bridge are welded.

Structural integrity of the bridge is assessed in SLS and ULS. It is concluded that the bridge is capable of withstanding an axle load of 25 tons. Moreover, seismic assessment of bridge suggests that the bridge could withstand longitudinal earthquakes, but requires retrofitting for lateral earthquakes.