Initial Find. Photo: Michael Baker International
multiple phases could be advanced in parallel to minimize the overall project schedule. Within a week of the fracture being discovered, Gen - eral Contractor Kiewit Infrastructure South Co. had also been brought onboard. All partners on the project proceeded in lockstep as repairs commenced, with daily working meetings to resolve challenges and frequent status meetings held throughout the entirety of the project. The Federal Highway Administration (FHWA) was also a key partner throughout the project, aiding with the repair plans. Three-Phase Design With the team in place, ARDOT and TDOT collaborated on a three- phase repair plan – created and executed in collaboration with Michael Baker and Kiewit – with design and construction overlapping between the phases. The plan included: Phase 1: Stabilization The team took a “do no harm” approach to the initial repairs as there was concern that the bridge was severely compromised. The initial evaluations found the remaining section was dangerously close to yielding. The team found no evidence from the structure that the load had not found an alternate path beyond the opposing web and remaining flange. Stabilizing the member was not a long-term fix, but it was the first step toward the repair, ensuring the safety of subsequent phases of work. The Michael Baker team established safe working load levels for construction crews and equipment staged on the bridge. Within the first week of the closure, a stabilization splice was designed to temporarily restore the capacity of the fractured section of the tie and the fabrica - tion of roughly 30,000 lbs. of structural steel plates began by Stupp Bridge. To install the splice, Kiewit assembled a suspension platform and secured the plating with nearly 450 temporary bolts. The splice provided additional redundancy to the partly severed member without applying any corrective twist or loading to the damaged tie. The sus - pended platform allowed the contractor greater access to establish a more permanent repair in Phase 2. Phase 2: Member Repair Knowing that time was of the essence to get this vital transportation link re-opened, analysis and design of the longer-term fix began im - mediately with Michael Baker engineers evaluating ways to repair this bridge. Faced with a range of potential repairs from reconstruction of the bridge to temporarily supporting the structure for the repair, the
Inspection. Photo: Michael Baker International
project participants found a creative solution to repair the structure in place and collectively cut significant cost and schedule impacts out of the project timeline. Advancements in the understanding and applica - tion of concepts in fracture mechanics and redundancy allowed for the fractured member to be repaired rather than completely replaced. The last solution consisted of using external post-tensioning to reduce tension in the existing tie, lessening its level of stress and possibility for further damage. Eight 3” diameter high-strength steel bars were connected to steel weldments at either end of the fractured tie. High- pressure rams were utilized to deliver the required force to partially de- tension the tie. Extensive real-time monitoring of the post-tensioning operations was implemented to ensure success. While the tie was partly unloaded, the temporary Phase 1 stabilization plates were removed and new strengthening plates were installed that have nearly the capacity of a completely intact tie by themselves. These new plates, acting in tandem with the existing steel, provided a redundant load path in the unlikely event of future fracture. The team worked closely with Kiewit and fabricators W&W/AFCO and G&G Steel to design the repair around readily available HPS70W material. During the completion of the work, an 18” section of the tie containing the fractured web and flange plates was removed for further forensic examination. Once the strengthening plates were installed and fully bolted, the post tensioning was removed, signifying successful repair of the damaged tie girder. Phase 3: Overall Tie Girder Repair While Phase 2 repairs were going on, extensive nondestructive test - ing (NDT) of all similar welds in the tie girders was completed and provided information leading to what became the Phase 3 repairs of the tie. NDT discovered indications ranging from very small to very large. The remedy for many of these smaller indications was to either core or grind them out, thereby removing the potential flaw. Larger indications were plated over to provide a redundant solution. The details used in Phase 2 were readily adaptable for Phase 3 and Kiewit worked with supplier AFCO/W&W Steel to obtain the necessary HPS70W plate.
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csengineermag.com
DECEMBER 2021
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