By Aaron Stover, P.E., S.E., Ted Kniazewycz, P.E., and Rick Ellis, P.E.,
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 responsibility of Arkansas Department of Transportation (ARDOT) inspectors 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 inspectors 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 Transportation (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 inspectors 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.
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 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 from ARDOT, 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. Engineers 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 information 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 inspector 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 understanding 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 selected 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 engineers from 20 of the firm’s offices around the country to contribute to the project in design and review/oversight roles to ensure that timely and prudent decisions were made at all phases of the work and that multiple phases could be advanced in parallel to minimize the overall project schedule. Within a week of the fracture being discovered, General 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.
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 fabrication 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 suspended 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 immediately 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 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 application 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 testing (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.
Tests were conducted on a portion of the damaged member that was removed as part of the Phase 2 repairs. The removed portion is currently undergoing forensic examination at the labs of Wiss Janney Elstner Associates Inc. (WJE) in Northbrook, Illinois. The WJE team is conducting various tests on the material to document its properties, as well as microscopically examining the weld and the fractured surfaces to determine where and how the fracture began. By having the fractured component in their possession, the engineers at WJE were able to provide guidance for field-testing of other welds that were completed as part of the inspection in Phase 3.
All Eyes on Memphis
The emergency closure quickly gained attention across the country from the public, media outlets, and politicians alike. Tennessee Governor Bill Lee, Arkansas Governor Asa Hutchinson, and U.S. Secretary of Transportation Pete Buttigieg all visited the bridge in the days following the closure, with Secretary Buttigieg noting “We want to make sure that national attention and resources are available to help the state and local authorities who are resolving this and working toward a safe reopening of the bridge…Even for people outside this region, it is important that we restore this connection quickly because like so much about the Memphis region, it is an area of national logistical importance.”
The impact of the shutdown of the Hernando de Soto Bridge on the economy was felt almost immediately. River traffic below the bridge had resumed by May 14, 2021, but all automobile traffic had to be diverted to the nearest crossing on Interstate 55 (I-55). The added traffic resulted in bottlenecks and delays, with the Arkansas Trucking Association estimating that the additional travel time attributed to the closure was costing the trucking industry more than $2.4 million each day that the bridge was not in operation. This further reinforced that the timely, proper, and safe completion of the repairs was critically important.
As the team moved through the initial find and into inspection, design and repair, it was of the utmost importance to maintain transparency and communication with the public impacted by the closure. A public awareness campaign was carried out in conjunction with the bridge’s repair activities. Project information and updates were shared via media efforts (press releases and press conferences), as well as daily across the TDOT and ARDOT websites and social media channels. To support this, Michael Baker developed a 3D visualization of the repairs as they were being developed.
Reopening the Hernando de Soto Bridge
For several weeks, activities progressed 24-hours a day, supported by extended shifts. Initially, the eastbound and westbound lanes of I-40 crossing the bridge were initially scheduled to open on August 2 and August 6, 2021 respectively, but the eastbound lanes were opened ahead of this date on July 31, 2021, while the westbound lanes opened on August 2, 2021 – traffic was again flowing 83 days after the fracture was discovered.
The project highlighted the importance of transparency, accountability and collaboration and exemplifies how two DOTs can work together to accomplish a common goal. All agencies and firms involved in the project brought a deep understanding of structural engineering to the work, and the commitment and partnership amongst them facilitated the analysis, design and construction needed to safely, effectively, and efficiently repair the fracture.
Aaron Stover, P.E., S.E. is Vice President and Great Lakes Regional Practice Lead – Bridge at Michael Baker International.
Ted Kniazewycz, P.E., S.E., is Structures Division Director at Tennessee Department of Transportation (TDOT).
Rick Ellis, P.E. is Division Head-Bridge Division at Arkansas Department of Transportation (ARDOT).