Integral abutment (IA) bridges may provide many advantages over conventional bridges during construction and subsequent maintenance. NCDOT has been designing various types of IA bridges. Unlike conventional bridges, IA bridges do not have expansion joints within the bridge deck or between the bridge deck and supporting abutments. Expansion joints and bearings in a conventional bridge are costly, and leaking joints cause deterioration of girders and bearings — leading to potentially unsafe conditions and high maintenance and repair costs. Besides cost savings related to construction and maintenance, IA bridges also provide superior performance during extreme loading events, such as earthquake and blast loading, and are being built at an increasing rate in the United States.
NCDOT began utilizing integral abutments in 2006. Since that time, NCDOT has provided varying guidance and details for integral abutments, but the overall performance of each detail has not been documented. Because IA bridges are built without expansion joints, thermal expansion and contraction must be accommodated by movement of the abutments. Thus, significant forces can develop in the bridge structure, abutments, piles, and soil surrounding the bridge substructure. The magnitude of these forces and response of the IA bridge to them is strongly dependent on the stiffness of the bridge structure, pile foundations, and soil. If the piles and soil are too stiff, large unwanted forces/stresses may develop in the bridge. On the other hand, if the backfill is relatively flexible and the embankment and foundation soil is stiff, unwanted yielding of the piles may occur at the bottom of the abutment. The fact that the soil response is strongly dependent on moisture content, which can vary significantly both seasonally and over the life of the bridge, results in unexpected problems. NCDOT reported that components of their integral abutments have been removed, added, or revised to address construction and maintenance problems without monitoring the influence of the revisions. Evaluation of these revisions is needed and recommendations for updates provided, as required.
The objective of this project is to assess, revise, and make more accurate IA bridge design and construction criteria based on IA bridge structural inspection, behavior, and performance. Existing performance will be evaluated based on field inspection and monitoring of a representative sample of IA bridges in North Carolina. A research program is proposed to evaluate the performance of NCDOT’s IA bridges and to recommend revised or develop, as necessary, analysis and design methodologies. First, a literature review will be completed along with a survey on the use, construction, design, documented problems and NCDOT policy associated with IA bridges. In parallel, a review of best practices for IA details and performance will be completed considering all state DOTs and international applications. This will be followed by field investigations of in-service NCDOT bridges constructed with integral abutments to monitor and document performance. The range of integral abutment types will be investigated and performance documented. Based on the aforementioned tasks, the final task will be focused on existing integral abutment policy to provide guidance for updates. Design methodologies will be updated to consider actual field observations such that the design accurately predicts the structural behavior.
The research project expected outcomes include: state-of-the-practice reference documents; guidelines and specifications covering the analysis, design and construction of IA bridges; report on performance of integral abutment policy based on field evaluations; documented performance with respect to type of integral abutment; recommendations for improved integral abutment performance; and updated Design Manual guidance and plan details corresponding with recommendations. The findings and recommendations of the research can be used to update the NCDOT Structure Management Design Manual guidelines for designing and detailing bridges with integral abutments.