C+S October 2021 Vol. 7 Issue 10 (web)

Spanning the Mississippi River with a distinctive profile, the Hernando de Soto bridge not only carries Interstate 40 (I-40) across the Mississippi River between Memphis, Tennessee and West Memphis, Arkansas, but also serves as an iconic landmark for the region. As one of only two crossings of the Mississippi River in the Memphis area, the steel-tied arch bridge is a vital transportation, commerce, and defense link. On May 11, 2021, inspectors from Michael Baker International were conducting a routine inspection of the upper portions of the Hernando de Soto Bridge (areas of the bridge below the deck are the respon - sibility of Arkansas Department of Transportation (ARDOT) inspec - tors and outside of Michael Baker’s scope of work) when a fracture was observed in the tie girder of the arch over the primary navigation channel. Initial shock and disbelief quickly turned into swift action to ensure the safety of the traveling public and the 18 rope access inspec- tors climbing the upper portions of the bridge. The team confirmed the critical finding and from there, moved quickly, focusing on doing the next right thing at each step. ARDOT, Tennessee Department of Trans- portation (TDOT), and 9-1-1 were immediately contacted by Michael Baker staff to alert them of the situation and request support to close the bridge to both automobile traffic moving across the bridge and river traffic on the Mississippi River below. The next few minutes were critical. Michael Baker’s rope access in - spectors were called down and as the team awaited support from local authorities to close the bridge, they moved off the structure down each of the westbound and eastbound lanes dressed in neon colors, waving their hands and stopping traffic. With the assistance of the Memphis police, the bridge was quickly evacuated. As I-40 stood empty, much was unknown, but one thing was certain: the team’s #1 objective at all times was safety. Initial Assessment The fracture critical tie girder, the main tension element in the tied- arch bridge, left the structure in a precarious state. Both vehicular and barge traffic were immediately halted. With nautical traffic paused for three days, initial physical and analytical assessments were completed. Once the structure was deemed stable, the U.S. Coast Guard decided to reopen the river for navigation. Vehicular traffic across the structure remained halted for the duration of the repairs. The Michael Baker team first leveraged their considerable experience with unmanned aircraft systems (UAS) – or drones – to fly the fracture location to inform ARDOT and TDOT of the apparent extent of the damage. The initial UAS video confirmed that the fracture included the Hernando de Soto Bridge Emergency Repairs: Critical, Complex, Collaborative By Aaron Stover, P.E., S.E., Ted Kniazewycz, P.E., and Rick Ellis, P.E.,

Hernando de Soto Bridge. Photo: Michael Baker International

complete loss of one of the two web plates, one of the two flanges, and partial fracture of the second flange. More than 50 percent of the member cross-section was lost in the fracture. Within hours, engineers fromAR- DOT, TDOT, and Michael Baker were working toward the ultimate goal of safely repairing the fractured tie girder. At this time, Michael Baker was contracted for design of an emergency repair by TDOT. Thorough analysis and evaluation of the bridge began immediately. Within a single day, Michael Baker assembled teams across numerous offices to gather data, perform calculations, and increase the team’s understanding in order to better evaluate the bridge’s condition. En- gineers generated detailed finite element models of the bridge and the local fracture to begin to shed light on the criticality of the bridge’s condition. Field inspection teams assisted with obtaining critical infor- mation to support early investigative efforts obtained by UAS. Michael Baker, alongside ARDOT’s UAS pilots, monitored the fracture to track any changes during those first critical hours. To support the initial temporary repairs, additional measurements of the crack, tie distortion, and other critical field measurements were needed. This information was gathered by a Michael Baker rope access inspec - tor on the bridge. During the inspection, Michael Baker established a live feed via UAS video linked to a web meeting. This allowed design engineers in multiple locations to communicate in real-time with the inspector while he took measurements and allowed them to request additional information and clarifications as needed. This creative use of technology gave designers real time results and a first person un - derstanding of the implications of the distortion that would need to be considered in the repair design. A Collaborative Process The team recognized that collaboration and efficiency in design and schedule would be important to repairing the fracture and reopening the bridge as quickly as could be properly accomplished. TDOT se- lected the Construction Manager/General Contractor (CM/GC) project delivery method, recognizing the benefit to the project as it allowed owners, engineers, and contractor to collaborate on repairs. As lead designer for all phases of repair, Michael Baker called on more than 60



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