Preliminary Experimental Study on the Effect of Live Load on the Seismic Response of Highway Bridges
AuthorSanford, Danielle Margaret
Civil and Environmental Engineering
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The objective of this research was to conduct a preliminary study of the effect of live load on the seismic response of highway bridges using a large-scale model of a 3-span bridge supported on multiple shake tables. This objective was achieved in two stages. The first included selecting and characterizing a test vehicle, as well as developing a numerical model of the test vehicle to be used in future bridge modeling. The second stage involved the development of a test plan for placement and instrumentation of a set of trucks on a large-scale bridge model, conducting shake table experiments of the vehicle-bridge system, and making preliminary observations of the effects of live load on the behavior of the model bridge.A Ford F250 truck was selected to be the test vehicle for these experiments due to its availability and high weight-to-length ratio. A set of system identification experiments was performed on a single test vehicle using the 6-degree-of-freedom shake table in the Large Scale Structures Lab at the University of Nevada, Reno. The system identification included a series of snap test motions in the vertical and lateral directions of the truck, as well as earthquake motions using the Sylmar record from the 1994 Northridge Earthquake. The truck was tested both with and without wheels, as well as empty and loaded (with sand in the truck bed).Both a single and two-axle model were developed in order to characterize the suspension system of the truck using MATLAB and SAP2000, respectively. The single-axle model was used to determine the vertical properties (stiffness and damping) of the suspension system and tires while the double-axle model was used to characterize the lateral properties of the suspension system and tires of the vehicle. The final suspension system and tire properties were verified against the experimental earthquake acceleration and displacement data using the two-axle model. Since the three main modes of vibration of the vehicle model mimicked realistic movements of the truck, and the model demonstrated displacements and accelerations of the same order of magnitude as the experimental data, the model was determined to be suitable for use in future bridge-vehicle interaction modeling. For the bridge experiment, six of the test trucks were placed on a 2/5th scale model bridge that was part of a larger FHWA study on the resilience of curved highway bridges. The data from the experiment with the vehicle loading was compared with previous data from experiments without trucks on the bridge. Results from the bridge with the vehicle loading showed a decrease in lateral displacements, accelerations, and visible column damage (column spalling and exposed reinforcement) when compared against the bridge without live loading. Adverse effects found in this study were generally within 10% of the no-live load case. Until further studies are complete, it appears generally conservative to ignore the dynamic effect of live load on the seismic response of ordinary bridges.