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As the U.S. bridge inventory ages and traffic volumes increase, state highway departments are spending more on maintaining their current structures to extend the service life of the current bridge inventory. This includes two-girder bridges that are currently classified as fracture critical and nonredundant. Because of increased inspection costs associated with these fracture-critical bridges, many state highway departments are electing to evaluate alternate load paths and implement retrofit methods on existing bridge structures to avoid bridge replacement. Significant improvements to member and system redundancy can be achieved by using relatively straightforward retrofit methods and materials. Implementation of web reinforcement plate retrofits is discussed as a possible technique to improve redundancy and reduce the fracture potential of two-girder bridge structures. In each of two presented case studies, the bridge owner selected the design criteria of the redundancy retrofit (such as fracture environment, postfracture capacity, and postfracture performance). The industry lacks a clear, objective, and quantifiable definition of redundancy, and there is no rational minimum benchmark that can be quantified in the design standards.

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The Jefferson Barracks Bridge is a fifteen-span structure that carries I-255 over the Mississippi River, in St. Louis, Missouri. The 910-foot main navigational span consists of a steel tied-arch superstructure, approximately 190 feet tall. In May 2019, during a routine and fracture critical inspection of the main span, inspectors discovered a six-foot crack at the toe of a weld in the steel tie girder. Further investigation revealed that crack extended at least 5/16 inch into the steel web plate. Because the cause of the steel crack was not known and the potential for bridge collapse if the crack were to propagate, the Missouri Department of Transportation (MoDOT) elected to close the three westbound lanes of I-255 on the structure. Several similar cracks were subsequently found at other locations throughout the tie girders. Working together with MoDOT and the Illinois Department of Transportation (IDOT), the inspectors responded quickly to evaluate the extent of the cracking at similar locations, obtain steel core samples, and perform laboratory testing to determine the cause of the steel cracks. Within approximately four days, the team documented the extent of this critical finding, determined the cause, began emergency repairs, and reopened the bridge to limited traffic. This paper/presentation discusses the decisionmaking process for the emergency response and presents the findings from the inspection, the results of the laboratory testing, the cause of the steel cracks, and the approaches used for repair.

Also known as the Great River Bridge, the US-34 Burlington Bridge is a signature stay-cable structure spanning over the Mississippi River between Gulf Port, Illinois, and Burlington, Iowa. Opened to traffic in 1993, the main river crossing span is supported by stay cables connected to two 305-foot tall reinforced concrete pylons. In 1992, prior to the bridge being opened to traffic, vertical hairline cracks were observed on the interior and exterior faces of the concrete pylons. The width of the cracks continued to increase slowly and consistently over time. The size and location of the cracks suggested that they may be structural in nature. Extensive investigations performed between 1999 and 2004 could not completely identify the cause or significance of the cracking. A more recent investigation was performed after the crack widths had grown to 1/4 inch wide. This follow-up study utilized instrumentation, material sampling, laboratory testing, and computer analyses, which determined the cause of the crack growth to be due to an alkalisilica reaction (ASR) in the concrete pylons. This case study will discuss the results of the investigation and why the cause of the crack was so difficult to identify. In addition, potential approaches to mitigate ASR deterioration in existing concrete structures will be presented.Structures Congress 2020 Downloaded from ascelibrary.org by 44.224.250.200 on 07/16/20.