Dynamic Response of Highway Bridges Subjected to Heavy Vehicles

Evaluation of existing structures is critical for an efficient management of transportation facilities, especially bridges. Knowledge of actual load effects and structure resistance can be very helpful for the determination of the load carrying capacity and condition of structures. It can help to make management decisions, such as establishing permissible weight limits, and can have important economical and safety implications. In particular, the dynamic nature of live loads and bridge-vehicle interaction is not sufficiently considered in current bridge codes. Impact factors suggested by the codes usually lead to inappropriate solutions for overweight vehicles which are the major concern of the Florida department of transportation which is involved every day in processing overload permit applications from trucking companies. This research consisted of analytical work validated by field tests. Static and dynamic field tests were performed on a selected two-lane highway bridge on US 90 over Mosquito Creek in northern Florida. The three-span bridge was a concrete structure with simply supported, precast girders and continuous, cast-in-place deck. One or two fully loaded truck(s) crossed over the bridge, which was instrumented with strain gauges, accelerometers and displacement transducers. Data collected from the tests were used for comprehensive assessment of the bridge under dynamic loading and validation of analytical procedures. The vehicle-bridge interaction was investigated using finite element models with different levels of representation. In the simple analytical model, the vehicle was modeled as a 3D mass-spring-damper system with eleven degrees of freedom. The bridge was discretized to a combination of plate and beam elements which represented slab and girders, respectively. The equations of motion for the vehicle were formulated with physical components while with modal components for the bridge. The coupled system was solved step by step using central difference method. More sophisticated bridge models with consistent stiffness and mass distribution and truck models with detailed representation of suspension systems and wheels were developed using LS-DYNA, a commercial explicit FE code. The advanced features of multi point constraint (MPC) and contact algorithm made it suitable in analysis of vehicle – bridge interaction. The advanced features of the truck model included the suspension system allowing wheel rotation, as well as application of internal pressure in tires. Good agreement was found between the field measurement and FE simulations in both frequency domain and time domain. Impact factors were calculated for some overweight vehicles using the validated finite element procedures. The effect of some parameters to bridge response was also investigated, including road roughness, bridge length, vehicle weight, vehicle speed and vehicle/bridge frequency ratio.