Research sheds new light on integral abutment bridges behavior

Integral abutment bridges are constructed without expansion joints. Due to the high maintenance costs associated with expansion joints—as a result of water seeping through the joints and causing damage to the beams and bearings at joint locations—integral abutment bridges (IAB) have become increasingly popular in the past 30 years, not only because of the economic advantages of these structures, but also because of their proven functionality and durability.Despite the numerous benefits of using IABs, however, there have been concerns that this type of bridges may have significant superstructure stresses that are not accounted for during design. This led the Illinois Department of Transportation, in cooperation with the Illinois Center for Transportation, to initiate Project R27-115 under the title: Analysis of Superstructures of Integral Abutment Bridges.

The primary objective of this research is to better understand IAB structural performance from the perspective of superstructure behavior. The study was conducted in two parts. In Part I, Integral Abutment Bridges under thermal Loading: Numerical Simulations and Parametric Study, which was completed in June 2016, numerical simulations were developed to evaluate the behavior of IABs with composite steel I-girders subjected to temperature changes consistent with seasonal fluctuations in the State of Illinois. The study recommended that the superstructure geometry of IABs—including bridge skew—be considered in the substructure design and that thermally induced stresses and strains be considered in the superstructure and substructure design.

In its second part, Integral Abutment Bridges under Thermal Loading: Field Monitoring and Analysis, completed in August 2017, the study focused on field monitoring of two Illinois IABs: Kishwaukee River Bridge and the Union Pacific Railroad Bridge. Data related to the global bridge movements; pile, deck, girder, and approach-slab strains; and rotations at different abutment interfaces was collected, and field results were compared with the finite-element models of each bridge.

“Owing to the geometry of an IAB, there might exist locked-in stresses, thermal stresses, and load distributions that may be different than those assumed in the design of the bridge,” reports Mark D. Shaffer, Policy, Standards, and Final Plan Control Unit Chief at IDOT’s Bureau of Bridges and Structures. Shaffer, who served as Chair of the Technical Review Panel overseeing the project continued, “These stresses and load distributions may result in beams being underdesigned in some areas of the bridge and overdesigned in other areas of the bridge.”

Professor James LaFave and associate professor Larry Fahnestock of the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign served as Principal and Co-Principal Investigators for the study, respectively.

According to Fahnestock, this project provided a unique opportunity for collaborating with IDOT’s engineers to develop more efficient bridge designs with improved long-term performance. The combination of numerical simulations and field monitoring employed in this research has provided an important new perspective on IABs and the results will be contributing to enhancements in IAB design. “The field monitoring campaign was particularly insightful as we confirmed some fundamental aspects of IAB behavior and gathered some structural response data that we did not anticipate.”

In light of the findings of this research study, the bridge planning charts for integral abutments are being re-evaluated, in order to better determine when integral abutments are appropriate for use in design. Shaffer expects that a more comprehensive design of integral abutment bridges will be achieved, which should result in long-lasting bridges. He also anticipates the number of integral abutment bridges to increase in the coming years, so there will be less maintenance costs due to the elimination of bridge joints in some cases.