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Abstract
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Research on the dynamic response of bridges due to moving loads has received considerable attention in recent years. Around the world, passenger and freight traffic have increased significantly in size and number in recent times. Many older bridges are now subjected to heavier traffic flows than designed for, and new bridges are to be designed for higher load levels without excessive deflections or vibration. This has made it increasingly important for engineers to estimate the dynamic effects of vehicle passage on the serviceability of existing bridges accurately and to consider it in the design of new bridges.
In order to check the capacity of existing bridges to handle heavier traffic and for the proper design of new bridges, a bridge engineer requires improved techniques that simulate the bridge–vehicle interaction. This is even more important these days due to the current trend towards longer spans and lighter deck systems, combined with the natural limited damping of these systems. Because of the structural form of composite girder highway bridges, being wide but shallow in depth, this type of bridge is highly susceptible to vertical vibration, which can cause excessive dynamic deflection.
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. |