Hastings, Minnesota, will be home to the longest freestanding tied-arch bridge in North America when the project is completed in 2014. Courtesy of Touchstone Architecture
The City of Hastings, Minnesota, once a tourist attraction thanks to a unique spiral bridge, will once again be home to an innovative bridge when the longest freestanding tied-arch bridge in North America is completed in 2014.
January 17, 2012—In Hastings, Minnesota, at a sharp bend of the Mississippi River that challenges barges traveling to and from Lock and Dam No. 3, an innovative new bridge is taking shape. When completed in 2014, it will be the longest freestanding tied-arch structure in North America—with a main span of 545 ft. The bridge employs a massive pair of trapezoidal boxed steel ribs—more than 9 ft wide and nearly 8.0 ft tall at the base—and posttensioned concrete ties.
“Technically speaking, that was a challenge in itself, since there isn’t much defined literature in the design of that rib,” says Vincent Gastoni, P.E., the project design manager and a senior project manager for Parsons, in Pasadena, California. “A lot of it is good old-fashioned engineering and trying to apply code accordingly as you work through that.”
The existing bridge, completed in 1951, was deemed to be structurally deficient by the Minnesota Department of Transportation (MnDOT) and placed on a list for accelerated replacement after an inspection in the days following the deadly I-35 bridge collapse, August 1, 2007. [Click here to see the article on the collapse in Civil Engineering magazine.] MnDOT estimates the new bridge, part of Highway 61, will carry 45,000 vehicles a day by 2030, the highest volume of any two-lane trunk highway in the state.
The new bridge, which will have four lanes, full shoulders, and a bike path, has big shoes to fill for this city of more than 22,000. The welcome sign of Hastings bears the image of the first bridge to cross the Mississippi there, a unique “spiral bridge” built in 1895 that bore a passing resemblance to a roller coaster. The bridge was the result of competing requirements that the structure be high enough to provide clearance to barges yet still lead directly into the city’s downtown, which is near the riverbank. The $39,050 bridge, which became a tourist attraction, incorporated a spiral approach from Second Street onto the bridge, leading horse-drawn wagons and later automobiles in a full circle. (Click here to see.)
An artist’s rendering shows the south river span touchdown.
Courtesy of Touchstone Architecture
MnDOT didn’t start out to build the longest freestanding, tied-arch bridge when it requested proposals. Instead, it specified either a traditional basket-handle, tied-arch steel structure, or a single-tower cable-stay structure, Gastoni says. After researching those alternatives, Parsons, working with a joint venture of Lunda-Ames Construction, began investigating options that met the state’s goals of a highly redundant, long life-span bridge that minimizes construction time in the shipping channel and provides the highest value at the lowest cost.
“As we went through the process and we weighed alternatives, we weighed them against these criteria, and that really drove our selection on a lot of the elements,” Gastoni says. “For instance, the traditional tied-arch bridge is a steel rib and a steel tie as well. The steel tie is made up of multiple plates and they are all bolted together. That’s a mess to inspect and maintain and clean and paint.” It can be difficult to achieve redundancy by using such a design, “so if one element fails there are enough remaining elements that it all stays intact,” he adds.
“By going with a concrete tie, it was a lot of birds with one stone,” Gastoni says. “It’s solid concrete, so it’s more durable. We use multiple, posttensioned ducts, and a posttensioned duct is made up of multiple, posttensioned strands, which is made up of multiple wires, so there are numerous redundancies through that system.”
The floor beam/lower hanger bracket connects to the concrete
tie girder. Courtesy of Parsons Core Services Graphics
Gastoni says that by framing the bridge into the piers, Parsons was able to eliminate some of the joints and bearings that typically create long-term durability issues for bridges, helping the new bridge achieve a projected lifespan of 100 years instead of the typical 75.
The foundations for the new bridge are complete, Gastoni reports. Contractors are finishing the south pier and are at the midpoint of the north pier. The two piers are in vastly different soils: the south pier is founded on bedrock, while the north pier must extend through 200 ft of mud. To anchor the north pier, engineers specified 200 ft, 42 in. diameter steel pipe piles, with 1 in. thick sidewalls, driven to bedrock.
The north pier is designed to withstand a multibarge impact of 2.2 million lb of force at normal water level, Gastoni says. The pier can also withstand the forces of a runaway barge at flood levels. The bridge offers 67 ft of clearance for barges when the river is at “standard pool.”
The south approach to the bridge is composed of two posttensioned slab structures, each with a maximum span of 137 ft. That length, coupled with a state requirement for solid slabs, meant the slabs needed to be 5 ft thick, Gastoni said. “[They] are just monsters.” The state required solid slab structure to minimize long-term maintenance requirements. Gastoni says the design team considered a typical hollow concrete box structure as an alternative, but the contractor determined the solid slab was more cost effective because of the reduced labor costs and schedule impacts.
The south approach columns will provide support for massive
5 ft thick, 80 ft long slabs. Courtesy of Parsons
The north approach is a more conventional, with precast I girders. Gastoni says Parsons has designed 96 in. deep precast concrete I girder that can extend as far as 200 ft in length, which the state then used as a new standard for prestressed concrete beams. The beams utilize a 4 ft wide top flange, 6.5 in. web and can accommodate up to 70-0.6 in. diameter high strength prestressing strands.
Weather has presented an enormous challenge on the project, Gastoni says. Heavy snow and rain put the Mississippi River at record levels in the fall of 2010 and spring of 2011, which delayed the start of pier construction. The project was again delayed when the Minnesota state government shut down for 20 days in July over a political impasse in the state legislature.
“The Minnesota government staff could not be on the project any further, [and] we could not progress with the design or construction without them on board, through the quality processes we have,” Gastoni says. “So we essentially had to halt construction at that point, as well as fabrication of the arch rib and steel floor system.” That halt has ripple effects that extended far beyond the 20 days of the actual shutdown.
“All of the fabricators, from our bearings to our main steel fabricators, as soon as the state shut down, they had to take any Minnesota work off their shop floors—because they didn’t know how long it would last,” Gastoni says. Having lost their place in fabricators’ schedules has added months to the project.
“We were supposed to have steel delivered this fall and actually [be] erecting the main span this month into next month,” Gastoni explains. “As it turns out, because of the delay, we are getting steel in the spring—which puts us in the shipping channel navigation season—so we have to wait until next fall before we can get steel up in the air.”
The bridge’s large ribs, steel grid floor system, and temporary steel ties will be erected on the shore using a falsework, about 0.5 mi upstream from the site. Engineers will then move the structure onto a barge using self propelled motorized transports (SPMT’s), float it into position between the piers, and then raise it 70 ft into the air and lock it off onto the piers using strand jacks to handle the 800 tons per corner. The team will have 48 hours in which to accomplish this feat.