Accelerated Bridge Replacement Reduces Time and Cost by Reusing Structures

Replacing a swing span in 48 hours in a difficult-to-access, environmentally challenging area requires a custom-fit solution and precise planning. And with less than 15 hours, even more so. 

Originally installed in 1928, the single-track, 163-foot-long Bayou Sara Swing Span was scheduled for replacement as part of CSX’s program to upgrade and remotely operate many of their 47 movable bridges. While its approach spans had been recently replaced, the swing span was approaching 100 years in age and exhibited extensive deterioration. To replace this critical link on its Montgomery to Mobile mainline, CSX turned to HDR to design a durable replacement, including remote operability, minimized maintenance and minimal rail service interruption for installation.

As the lead design firm and construction manager, we recommended an in-kind swing span replacement to minimize environmental concerns, and reduce time and cost. We were able to reuse the existing approach spans with minimal modification, as well as the majority of the swing span substructure, eliminating significant in-water construction. 

An in-depth review of the existing substructure analysis was performed to verify that it would be adequate to support the replacement span. The team drilled several geotechnical borings around the bridge to develop a subsurface profile, which determined that the existing piers were sufficient to support the new structure. The decision to reuse the existing substructures for the Bayou Sara project reduced overall project costs and schedule as well as avoided substantial waterway work and resultant environmental impacts.

A Fast Bridge Replacement Gets Faster 

The top portion of the concrete pivot pier that the bridge rests upon was deteriorated and required replacement. Our original design included a precast concrete disk that would achieve the accelerated replacement within the original 48-hour window in a two-step process. However, as the planned installation date drew closer, CSX requested that the maximum rail outage window be reduced to nearly a quarter of that. 

We worked with the contractor and communicated extensively with the fabricator to find a solution by completing the replacement in a single major step. This was achieved by using a circular steel “grillage” suspended from the new span — with center wedges, pivot bearing, and rack gear premounted — to bear the self-weight and rail traffic loads immediately upon installation. Once the concrete was cast around the embedded grillage, the swing span could be operated. This innovation made it possible to meet CSX’s reduced outage window and minimized interruption of rail traffic.

Within an overnight, 14-hour rail outage, the existing span and 200,000-pound swing span cap were removed, and the new superstructure, grillage and elevated hydraulic operating system were floated in and installed. Shortly after float-in, the first train successfully crossed the new span.

Designed for the Future

Located a few feet above the water, and just 10 miles upstream from Mobile Bay, the structure is susceptible to near-annual brackish flooding due to storm surges. To introduce resilience, the new swing span structure is metalized, hydraulically driven with enclosed motors, and features a center-pivot service tower to keep most equipment elevated above any flood levels.

Using modern communications, the bridge can be controlled from an operations center or another nearby movable bridge, eliminating the need for an on-site bridge tender. As part of a larger movable bridge remote control retrofit program, CSX and HDR have developed a preferred menu of standard equipment for remote control operation use on CSX movable bridges. Under that standard, CCTV cameras, a weather station, motion sensors, limit switches, hydraulic system sensors, radio, and navigational channel sensors were installed with the new Bayou Sara span. 

Many of the innovations applied on the Bayou Sara swing span replacement will be employed in future CSX movable bridge repairs, rehabilitations and replacements.