A high-traffic, two-lane crossing providing an east-west connection from the southwest section of Davidson and Williamson counties into Brentwood and Interstate 65, the existing bridge over Otter Creek on state Route 254 in Davidson County had become structurally deficient and functionally obsolete after more than 50 years of service. Too narrow for the traffic it was carrying, the deteriorating overpass was on the Tennessee Department of Transportation’s (TDOT) list to be upgraded as a repair project.
Having worked on multiple projects together, TDOT solicited GS&P’s guidance in determining the most feasible approach for repairing the aging structure. After close collaboration with the Department regarding the bridge’s current condition and the possibilities for rehabilitation, GS&P advised that the timeworn crossing was beyond repair, and that replacing the portion of the bridge that supported the deck was the only viable option. This would involve removing the existing deck, beams and piers, and replacing them with a new single-span superstructure.
“Like many bridges built around the same era, the existing structure was past the point of repair due to the amount of deterioration in the reinforcing steel,” explains senior structural engineer Ted Kniazewycz. “However, the substructures were in good enough condition to be preserved as part of the replacement. Reusing those in the new bridge configuration and replacing the existing superstructure presented the most cost-effective solution.”
Planning the Replacement
Because of the high volume of traffic along this stretch of Old Hickory Boulevard (SR 254), GS&P worked with the client to determine the most effective and expeditious means of carrying out the necessary and complex bridge repairs. It was ultimately decided that Accelerated Bridge Construction (ABC)—a process that dramatically reduces construction time as well as long-term inconvenience to motorists—provided the best solution. A first-of-its-type ABC method for TDOT, this short-term total closure of a 1-mile section of Old Hickory Boulevard would allow crews to carry out their work around the clock without being encumbered by road traffic.
To be completed within a single weekend, the fast-tracked project included minimal roadway widening, substructure modifications, and full superstructure replacement and widening for the existing two-span bridge. Along with significantly reducing the project’s schedule, the ABC process would also result in significant sustainable and environmental benefits, including less disruption to Otter Creek and reduced carbon emissions, as extended traffic gridlock would be reduced.
Senior transportation engineer Mark Holloran discusses the key benefit of applying ABC methodology to the project versus a traditional phased construction approach:
“This section of Old Hickory Boulevard often carries up to 30,000 vehicles a day. Given the heavy traffic volume on-site, executing the project in a more traditional way would have been untenable, and most likely resulted in a three- to six-month project schedule. By employing this particular ABC technique, the entire superstructure could be replaced in just one weekend.”
Building a New Bridge
Starting Friday, September 12, 2014 at 8 p.m., TDOT closed Old Hickory Boulevard from Chickering Road to Hillsboro Road to allow crews to replace the existing bridge over Otter Creek. As part of the comprehensive project, GS&P was tasked with developing a plan to utilize prefabricated superstructure units that could be constructed off-site, installed, and then joined together during a single weekend road closure. The team also created plans for precast approach slabs and sub-footings to transition the bridge to the approach roadway.
Once the existing bridge and section of roadway was closed and traffic detoured, demolition commenced at 9 p.m. The only portion of the original structure to remain were the central portions of the original abutments. Prior to the closure, these surviving bridge supports were widened allowing for the addition of 6-foot shoulders on each side of the overpass to improve sight distance for residents of an adjacent subdivision. After demolition was complete, the site was cleared. Concrete from the former bridge was repurposed as fill material, while rebar was collected and recycled.
Just after 6 a.m. Saturday, the first superstructure unit—one of two that weighed approximately 225,000 pounds each—was lifted off the ground by two cranes and then transferred to a third crane that moved it across the creek. Once both precast units were set in place, the concrete bridge rail was poured. By 10:30 a.m. the units were joined together using high-strength grout.
Weathering steel beams, which don’t require painting, as well as epoxy reinforcing steel that offers enhanced protection in freeze-thaw conditions were used, providing a durable, low-maintenance and sustainable structure. A deck seal was also utilized to further protect the bridge.
Constructed with minimal disturbance to Otter Creek, the new superstructure with a projected life span of 75 years was opened to traffic by 8 p.m. Sunday, coming in 10 hours ahead of schedule.
“Our design solution successfully integrated the bridge’s end-walls into the prefabricated units,” explains Holloran. “This allowed the entire superstructure to be cast in only two pieces, and also provided an economical and structurally sound detail for attaching the units to the existing substructures.”
“The single-biggest challenge of this project was getting the superstructure from Point A to Point B without imposing any unfavorable loads to the structure itself,” adds Kniazewycz. “So we designed it to be self-supporting once the concrete was cured onto the steel beams. This let it function as a separate unit and handle the load capacity as it was being moved. It was a great moment to see those two pieces finally come together. Everyone involved was delighted that by Sunday evening there was a nice, smooth road to drive across.”
Setting the Path Forward
Executed without major long-term impact to motorists, the Accelerated Bridge Construction process used in the new bridge over Otter Creek eliminated months of inconvenient traffic backups, resulting in a reduced carbon footprint, and saving end users in both fuel and maintenance costs. The innovative pilot project was awarded the Grand Iris Award (Best in Show) in the ACEC of Tennessee’s 2015 Engineering Excellence Awards program.
“This project signifies a bold step forward in embracing construction methodologies that could forever change how complex transportation projects are delivered in Tennessee,” says Kniazewycz. “The amount of planning in design and construction that was required to ensure the project’s success was staggering.”
In addition to working closely with the client, the project also represented a coordinated effort between GS&P and general contractor Bell & Associates.
“Once we started demolition of the bridge, there was no turning back,” notes Kniazewycz. “I was amazed at the high degree of redundancy the contractor built into this project to allow for anything that could potentially go wrong on-site. If something broke down, he provided backup—extra equipment, extra personnel, and anything else you could think of to make sure the project went as smoothly as possible. This effort truly emphasizes how a team approach, from design through construction, sets the path forward for greater projects to come.”
Terry D. Mackie, project manager of the Bridge Inspection and Repair Office at TDOT, confirms the project’s success:
“The Otter Creek pilot project was a tremendous success due in part to the development of detailed construction plans and the close coordination between the engineer, owner and contractor. The positive results taken from the project will be applied to future efforts, further enhancing the transportation network and the environment. The main goal of this Accelerated Bridge Construction project was to significantly reduce the impact to the motoring public. By replacing this bridge in a single weekend closure, we accomplished that key objective.”