Impact factors for a composite steel bridge using non-linear dynamic simulation

Traditionally, bridges are designed using static loads that are increased by the dynamic load allowance (DLA) factor (or dynamic amplification factor). The DLA factor is a function of span or first flexural natural frequency of the bridge, and indirectly incorporates the dynamic effects of moving vehicles in the design. This article firstly reviews the literature on impact loading of bridge decks. Analytical methods published previously are evaluated and the bridge-vehicle interaction is found to be the most reliable method among them. The article then presents a 3D finite element model to study the bridge-vehicle interaction. Finite elements are developed to simulate the trucks, the road surface and the composite girder bridge itself. Truck parameters include the body, suspension and tires, with variables being the total weight and the speed. The bridge superstructure is treated as a 3D composite steel girder bridge incorporating special end springs that simulate the elastomeric bearings. A parametric study is performed to identify the effect of various parameters on DLA, such as vehicle speed, aspect ratio of steel girders, stiffness of neoprene, type of vehicle, vehicle lane eccentricity and initial bounce of the vehicle due to road surface roughness. The results indicate that the DLA is correlated well with the velocity of the truck, especially at high speed. DLA is vehicle dependent and the dynamic and static live loads can be considered uncorrelated, except when the truck weight is less than 10 percent of the total deck weight, for which a low degree of correlation is observed. The DLA is decreased as the vehicle lane eccentricity (with respect to the deck centerline) is increased, and the same relationship exists with the bridge span length. No distinctive correlation is observed between the DLA and the initial bounce of vehicle at the time of entrance to span.

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