Effects of Secondary Elements and Joints on Strain Distribution in Composite Steel Girder Bridges

When considering the design of bridge girders, the American Association of State Highway and Transportation Officials (AASHTO) determine how the loads will be transferred to each girder. The equations used in AASHTO Load and Resistance Factor Design (LRFD) neglect the inertia added from secondary elements such as barriers and curbs. By neglecting these added effects, many bridges that are already built could have more strength than initially designed for. If the effects of these secondary elements were considered, it would be possible to reduce the distribution factors that are given for interior and exterior girders. The bridge of concern for this project has four spans and was built in the early 1950s. Some repair work and modifications were conducted on the bridge and a load test was performed a week afterwards. The tests were done to find out if the repairs were adequate based on full composite action between the girders and the slab. During this initial test, some problems were discovered in one of the interior spans. This interior span is unique because it contains an expansion joint and a physical barrier and curb joint at the mid-span of the bridge. After problems were discovered, the physical joints were recommended to be grouted and a second load test was conducted afterwards. The second load test yielded much lower strains in the exterior girders due to the decrease in localized stress at the physical joint. In order to prove that filling the joint could improve the bridges strength, a finite element model was constructed to simulate this activity. Two models were made, one prior to filling the physical barrier and curb joints and one after. The test data was compared to the data from the finite element model to ensure accuracy. After the model was calibrated, the secondary members of the bridge were modified to study their effects. The primary goal of this research is to prove that a physical joint in a continuous exterior secondary element will cause the same amount of strain at its location as if they weren't there to begin with. By analyzing the finite element model data, it was found that when the joint is filled the behavior of the bridge changes and the exterior girder has up to 50% reduction in strain. The effect of concrete cracking and stress distribution that is associated with it is a secondary topic that was discussed because it was a driving factor in the model calibration.