To Rehabilitate or Replace: Merchants Bridge

By Luke Carothers

Merchants Bridge, also known as Merchants Memorial Mississippi Rail Bridge, is a steel truss bridge that spans the Mississippi.  Built in 1889, Merchants Bridge has long served as a vital artery for rail traffic moving through the heart of the United States.  In 2018, plans were made to renovate the bridge so that it could maintain its service to five Class I railroads.

Part of the team working to bring Merchants Bridge into the modern age is engineering firm Burns & McDonnell who is one of the major subconsultants to TranSystems who is the main consultant on the bridge’s rehabilitation.  The Burns & McDonnell team are responsible for a few aspects of the bridge’s rehabilitation such as designing new trusses, analyzing foundations, and seismic analysis.

Because of the structure’s age and constant use, the rehabilitation project is extensive.  The bridge’s three main truss spans, each 520 feet long, have to be replaced.  Additionally, when the bridge was built, the masonry piers that held the bridge in place were not reinforced which has led to significant scouring.  Because of this, these piers are not able to provide sufficient seismic resistance.

One of the biggest challenges in working on such an old structure is a lack of original plans.  To overcome this, the teams at Burns & McDonnell had to conduct surveys and inspections to determine the original structure and its condition.  In addition, various patchwork fixes have been applied to the bridge over the course of its history.  To correctly plan their rehabilitation of the bridge, Burns & Mcdonnell’s team had to take all of these changes and original features into account.  When the project was initially conceived, it was unclear whether or not it was economically feasible to rehabilitate the bridge rather than simply replacing it.

The other big challenge during the project was widening the two railroad tracks that cross the bridge’s span.  When the bridge was built in the late 19th century, it was much more common to build a track at 12 feet.  To update the bridge for modern rail travel, the track is being extended to 15 feet.  However, this is not as simple as removing and replacing the track.  To compensate for this new load, the foundational piers also had to be widened.

The first step in rehabilitating the Merchants Bridge was to conduct a number of constructability studies on the structure.  Part of this process involved examining the structure and its foundation and correctly determining all its current parts, while the other part of the process involved coming up with various solutions that would fit together.  One of the earliest conclusions the Burns & McDonnell team came to was that the three truss spans on the bridge needed to be replaced, but they had to determine the best course of doing so.

There were several solutions that the Burns & McDonnell team explored regarding the layout of the rehabilitated truss spans.  One of the solutions was to add a pier to the water, reducing the length of the span.   However, hydraulic studies conducted by TranSystems showed that the water beneath the bridge was much too channelized and would present additional problems for the structure.  As such, the team decided that the best course of action was to replace the three truss spans as they were without altering the layout of the structure.

With the decision to replace the three truss spans made, the team had to decide the process by which it would be done.  Because the bridge is still a vital artery in the North American Railroad system, the rehabilitation teams had to work while minimizing disruptions to the movement of trains across the bridge.  In this pursuit, the new bridge spans have to be completed during specified ten day closures to the railroad.  Included in this is two 24-hour shutdowns of the navigational channel.  During the first 24-hour shutdown, the old bridge span is removed, and, during the second, the new span is floated in by barge to be installed during the ten day rail shutdown. Using this method, the bridge’s rehabilitation is progressing smoothly, with the first section of the three being completed in Fall of 2021.

Luke Carothers is the Editor for Civil + Structural Engineer Media. If you want us to cover your project or want to feature your own article, he can be reached at lcarothers@zweiggroup.com.