During the earthquakes of Northridge (USA, 1994) and Kobe (Japan, 1995), unexpected brittle fractures were observed in some steel structures. It was pointed out that high strain rate due to heavy magnitude of these earthquakes and temperature elevation during local plastic deformation might have affected the fracture behaviour. This paper addresses strain rate-temperature effects, as dynamic loading effect on materials, to estimate their effects at the fracture origin of steel bridge bents. The local strain rate change, temperature elevation, and their effects were studied with elastic-plastic FEM considering heat transfer. The strain rate and temperature histories were investigated assuming different combinations of ground accelerations and initial deflections of column flange plate. From the results, at the fracture origin of the bent, it was revealed that strain rate-temperature effects increases fracture toughness at the time of fracture. This conclusion indicates that there is a high possibility that brittle fracture in the bent was caused mainly by prestrain and stress multiaxiality.