Traffic induced stress range cycles in bridge main members

The dynamic characteristics of a bridge determine the live load stresses and number of stress cycles when subjected to vehicular loads. Therefore, the dynamic characteristics of a bridge have a strong influence on the fatigue strength of the bridge.;Based on the condition of flexural mode vibration, a beam and sprung rigid body model incorporating a beam on elastic foundation model is developed to simulate the dynamic response in each girder of the simple span bridge to a test truck of known weight and axle spacing. The results of both magnitude of stresses and the pattern of stress fluctuation compare quite well with measured data.;The magnitude of primary stress range and the number of stress range cycles are controlling factors of the fatigue life of a steel bridge component. A linear relationship is obtained between the primary stress range and the gross vehicle weight by the Weigh-In-Motion technology, despite the trucks' different dynamic characteristics. An algorithm and a procedure of predicting live load primary stress cycles for fatigue design are proposed considering the dynamic characteristics of the bridge.;A method is derived for estimating the dynamic characteristics of girder bridges from measured live load stresses in the girders. Stresses due to regular truck traffic can be used. The evaluation includes the determination of the vibrational modes and the corresponding damping ratios. Two bridges, one being a single span span multigirder bridge and the other a three span skewed bridge, have been strain gaged. The measured strains were analyzed using the derived method. The results show that both bridges are dominated by a flexural mode of vibration with a damping ratio of approximately two percent. A third, simply supported span was also studied. No dominant mode could be identified because of the interference of horizontal vibration of the bridge transmitted from another span.