Determination of impact factor for steel railway bridges considering simultaneous effects of vehicle speed and axle distance to span length ratio

In common bridge analysis method, traffic load is considered as a static load increased by an impact factor. Impact factor is just a function of span length or the first vibration frequency of the bridge according to the present codes. In this paper the effects of various parameters including velocity, train axle distance, the number of axles and span lengths on dynamic responses of railway steel bridges and also impact factor values are studied. s regard dynamic responses and impact factors for four bridges with 10, 15, 20 and 25 m span lengths under trains with 100–400 km/h velocity and axle distance between 13 to 24 m have been calculated. Dynamic analysis results show that in most cases the calculated impact factor values are higher than that recommended by the relevant codes and so the offered rations for impact factor are underestimated. also been shown that the train velocity affects the impact factor, so that the value of impact factor rises incredibly with the train velocity. Another effective element for impact factor is the ratio of train axle distance to bridge span length so that the impact factor value varies for the ratio below and above unity. The train number of axles just affects the impact factor under resonance conditions. In this paper some relations are offered for the impact factor considering parameters: velocity, train axle distance and the bridge span length.