By Kevin Wilcox
A project by the Minnesota Department of Transportation preserves a historical through truss bridge over the Mississippi River and builds a new concrete span beside it.
A two-bridge solution was chosen to preserve the historic span, completed in 1942, and add capacity to meet traffic projections for coming decades. Minnesota Department of Transportation
August 16, 2016—The City of Winona, Minnesota, occupies a picturesque site on the bluffs of the Mississippi River. The city's location was important first to Native American populations and later to early settlers, who valued the abundant natural resources and the commerce connection that the river provides. It is believed that in 1900, the small city had the highest number of millionaires, per capita, in the United States.
In the early 1940s, as the United States moved closer to involvement in, and then entered, World War II, a steel cantilevered Warren through-truss span known as the Main Channel Bridge was built between Winona and Latsch Island. That, coupled with the North Channel Bridge—connecting the island to Wisconsin—forms a vital connection between Minnesota Route 43 and Wisconsin Route 54.
The Main Channel Bridge, which is eligible for listing in the National Register of Historic Places, has been deemed both structurally deficient and fracture critical—if any key element fails, the bridge could collapse. Following the 2007 collapse of the bridge carrying Interstate 35W over the Mississippi River in Minneapolis, repairing main Channel Bridge took on new urgency.
The bridge is a critical infrastructure link in the state's transportation system. That became quite evident when it was closed in 2008 for two weeks to replace aging gusset plates. The resulting 70 mi round-trip detours for area residents caused serious economic impacts on local businesses, according to Terry Ward, P.E., MSISE, PMP, a project manager with the Minnesota Department of Transportation (MnDOT).
A new project now under way by the MnDOT provides a much more extensive rehabilitation, extending the existing bridge's design life by 50 years. The project, which is nearing the halfway point, began with construction of a new concrete box girder span north of the existing bridge. When the new bridge opens later this month, traffic will shift onto it, and workers will rehabilitate the historic steel span. This is known as the two-bridge option and it began to emerge as the best solution in 2008.
The project will preserve the historic cantilevered through-truss bridge and extend its design life by 50 years. Minnesota Department of Transportation
"MnDOT looked at a number of options for rehabbing and keeping the existing bridge in service," Ward says. "Those options included a temporary bridge or a ferry service during construction to maintain the crossing." A temporary bridge—essentially a $30-million disposable asset—was ruled out in favor of a strategic investment in a fast-track, permanent bridge. This will leave the state with a four-lane crossing over the river, something traffic projections indicated will be needed by 2038.
The department decided to pursue this project using a construction manager/general contractor (CMGC) delivery method, a first for the MnDOT. The department had become disappointed with how far apart its estimates and contractors' bids were when using a traditional design/bid/build method. That decision was driven by the department's disappointment with the fact that submitted bids often didn't align with projected budgets. "One of the reasons we used CMGC was to better understand the scope of work and the risks on the through-truss rehab," Ward says. "In previous jobs, it wasn't uncommon for bids to come in two to three or even four times higher than our estimate. It's all about a clear understanding of the scope of work and risks, who owns the risks, and how much does it cost?" Ward says.
Ames Construction, headquartered in Burnsville, Minnesota, secured the contract. The decision was driven by the company's experience in working on similar historic steel bridges rehabilitation projects in the past. "In the final design stage, we put our design team and our CMGC contractor together with our bridge inspectors on the through truss, so that we could look at, touch, and [better understand] the deterioration out there as we were putting together our final plans and bidding packages," Ward says. The process also included field tests in which rivets were removed plate fit-ups were installed.
The new bridge will open soon and traffic will shift off of the existing span, which will be extensively rehabilitated. Minnesota Department of Transportation
Using the CMGC delivery method for the first time on a fast-track project created some logistical challenges for the MnDOT. As Ward explains, "We didn't have any CMGC program documents, processes, or procedures. So we not only had to move quickly—because they wanted the new bridge built quickly—we had to develop the program from the ground up at the same time. And that was pretty challenging back in 2013 when we started."
The state considered two other bridge types—tied arch and cable stayed—before selecting the concrete box girder design, in part for aesthetic reasons associated with the historic nature of the existing bridge. "The tied arch and cable-stayed designs were not selected because it was [believed] that they would distract visually from the existing through truss; they would compete with it from a vertical element perspective," Ward says. "So we ended up with a lower-profile, segmented box."
Construction began in July 2014 and continued through the harsh Minnesota winter of 2014-2015, the foundation and pier work timed to beat the spring flood season on the Mississippi River. The new bridge is founded on 42 in. diameter steel shell pilings, driven approximately 100 ft to bedrock, excavated to below the river's scour line, and filled with concrete.
A network of vibration and tilt meters, set with automatic alarms and trigger values, was placed on the existing bridge to monitor any effects from driving the piles and vibrating the sheeting for the coffer dams into place during that foundation work. The impact was minimal, but when the alarms were triggered the team slowed down or altered their operations. More often than not, it was traffic that triggered the alarms, Ward says.
It was a challenge for the team to place concrete in the "dead of Minnesota winter," Ward says. The team had to pay very close attention to the concrete mix and monitor temperature gains to avoid thermal cracking. "We had to be very careful when and how we would add heat to the mass of concrete," Ward explains. "You don't want to get it too hot, because you have a maximum temperature of 160° F. And then you have to be careful how you bring that mass back to the temperature of the environment. It's a very delicate process—pouring a mass of concrete, housing it, heating it, but keeping it below 160° F, and then opening up that enclosure system to the elements and cooling it down. It takes careful timing and choreographing."
The sweeping piers of the new bridge are designed to evoke the flowing river and the trees draped over it from the shore. They maintain the approximately 420 ft wide navigational channel created by the existing bridge and offer a vertical clearance of 70 ft above normal pool.
The piers are engineered to withstand the impacts of a barge hit. Because the existing bridge is founded on shallower, timber pilings and would be vulnerable in such a collision, the design team developed a system of struts that will transfer collision forces to the new bridge's foundation. "We had excess 42 in. steel shell, open-ended piling," Ward says. "We're going to take the excess pieces of that piling, fill them with concrete, and use those as the collision struts. We designed in a receiver on the new bridge pier to anchor them. And they will butt up against the existing piers of the historical bridge," Ward says.
Although it would have been a significant engineering challenge to bolster the existing bridge's shallow timber piles to withstand a barge collision, they are in good condition. In the final design phase, the team sent divers into the river to take core samples from the piles and sent them to a laboratory for analysis. They are in remarkably good shape for having been in service 74 years, Ward says.
That is not the case with some of the span's steel, however. "The through truss is the main, predominant visual feature over the river, [and] it is in deteriorated condition, no question about it," Ward says. "In the last few years we found the deterioration has actually accelerated." So the through-truss will be rehabilitated, and the six deck-truss approach spans on one side of the truss and a series of short, concrete beam spans on the other will all be replaced.
"The design of the through-truss rehab really started with a discussion of what type of design life [extension] we were looking for," Ward explains. "As we moved into final design, our team proposed a 50-year design life. The goal is to minimize new material as much as possible." He estimates that between 20 and 30 percent of the bridge will be new material, some of it retrofitted over existing bridge members to provide additional support.
The engineer of record for the new bridge portion of the project is FIGG Bridge Group, headquartered in Tallahassee, Florida. They are teamed with WSB, of Minneapolis, for the civil designs. The engineer of record for the rehabilitation of the existing bridge is Michael Baker International, headquartered in Pittsburgh, together with Kimley-Horn, headquartered in Raleigh, North Carolina.
The Baker team developed an innovative approach to the historic span that introduces posttensioned rods inside some of the steel structural members of the truss to take the load if a member fails. Although the bridge will still be classified as fracture critical, the rods will provide a welcomed element of redundancy and additional safety.
Another advantage of the CMGC delivery method is that the rehabilitation engineering team was able to work with the contractor early in the project to analyze the equipment, materials, and sequencing to identify any potential for destabilization of the fracture-critical span. The sequencing was designed to eliminate the need for temporary stabilization.
But before the rehabilitation project begins in earnest, there will be a celebration.
"In May 2013, in working with the Winona community, we committed to open a new bridge by the end of 2016. There were a number of people who thought that would not be possible, but here we are," Ward says. "We're going to open the new bridge on August 27. That's really a testament to a lot of people from around the country doing some great work—going way above and beyond the call of duty."