As the Minnesota Department of Transportation (MnDOT) quickly moves forward with plans to build a new bridge to replace the structure that suddenly plunged into the Mississippi River in August, it is now painstakingly removing the collapsed bridge piece by piece.
MnDOT hired Carl Bolander & Sons, a general contractor headquartered in St. Paul, to remove the structure. MnDOT officials estimated it will take $15 to $20 million in manpower and equipment to clear the site. The National Transportation Safety Board (NTSB) is leading the investigation into the cause of the bridge failure.
The 1,907-ft. (581 m) steel trussed bridge collapsed shortly after 6 p.m., Aug. 1 as the evening rush hour traffic slowed to less than posted speed. Composed of 14 spans, with a center span length of 456 ft. (139 m) over the river, MnDOT contractors completed construction of the bridge in 1967. At 113 ft. (34 m) wide, the bridge deck carried four lanes of traffic in each direction.
MnDOT engineers rated the bridge to safely carry legal truck loads up to 80,000 lbs. (36,287 kg) and permitted over weight loads up to 189,000 lbs. (85,729 kg).
Within a week of the collapse, Bolander hit the site with crews, cranes and backhoes to begin preliminary removal operations, according to MnDOT documents. Bolander brought in more than two dozen pieces of heavy equipment including Cat dozers, backhoes and loaders, Link-Belt, American and Grove cranes to remove concrete and steel from the site.
Under the guidance of NTSB and MnDOT staff, crane operators began plucking cars from the bridge deck while victim recovery operations were still ongoing, according to MnDOT documents.
Nearly three weeks after the collapse, Navy divers recovered the 13th and final victim from the twisted metal that had sunk below the water surface. At that time, Bolander crews moved into high gear, according to Brad Estochen, senior engineer of MnDOT.
Placing a high priority on opening the river channel to commercial traffic, the MnDOT demolition division and Bolander crews removed parts of the bridge on the south side of the river in the navigation channel just downstream from the lock and dam, according to Donna Lindberg, MnDOT communications specialist.
Bolander crews broke concrete from the collapsed deck sitting over the south side of the river and methodically cut and removed pieces of steel.
By Sept. 7, Bolander crews had removed enough of the bridge deck to clear a narrow swath of the Mississippi River to commercial river traffic, Lindberg said.
“It’s a very interesting process,” remarked Lindberg. “Commercial divers cut steel below the water surface with blow torches. Each piece is lifted over to a holding area on the north bank of the river, near the bridge site.”
Here, Lindberg said, NTSB staff determined which pieces are critical to the investigation and which pieces could be returned to MnDOT for recycling.
“The areas of the bridge the NTSB is interested in are identified by NTSB investigators and then Bolander crews do some precision cutting with torches to get a nice, clean cut,” Estochen added.
The cut steel is floated downriver on barges to an area known as the flats because of its level topography near the University of Minnesota. Here, Bolander crews lay out the steel pieces “in a fashion that represents their location on the bridge” Estochen said.
After NTSB visually inspects the steel on the river flats, pieces found to require more intensive inspection will be shipped to a lab in Washington D.C., according to MnDOT documents.
Before demolition crews began cutting and removing steel from the section of pavement sitting just above the river surface, the concrete bridge deck had to be removed. Hoping to avoid concrete pieces falling in the river below, Bolander crews hammered out 10 by 12 ft. (3 by 4 m) sections, cut the rebar and hoisted them out by crane, Estochen explained.
Though small amounts of concrete broke off and fell to the bottom of the river, “the majority of the deck was removed in larger sections,” noted Estochen.
Removing damage from the north river bank will be more challenging because of the instability of the damaged piers and the steel and concrete decks leaning on them along with the massive amount of broken and twisted steel, Lindberg said.
To make more room for equipment on the north side of the river, crews poured tons of material composed of coarse Class 1 aggregate, Class 4 rip rap and limestone to build a causeway into the river and adjacent to the deck sitting above the river surface, Estochen added.
Eventually, Bolander rolled in a 550-ton (499 t) Grove crane onto the causeway to begin hoisting pieces of steel from the damaged superstructure sitting in the river. This crane, noteworthy because it is unique to this region is owned by Armstrong Crane and Rigging Corporation, located in New Brighton, Minn.
Sitting on a 7-axle truck, the crane is highly mobile, said Dan Gmach, owner of Armstrong.
At 550-ton capacity, it is outfitted with a “MegaWingLift, which enhances its lifting capacity,” Gmach added.
With all the equipment and manpower on-site, up to 100 workers, MnDOT officials are confident most of the damaged will be removed by Oct.15, the target date for construction to start on the new bridge.
“In general, we’re making some pretty good progress. The entire south approach has been removed and on the north end we’ve got the bridge deck substantially removed,” Estochen explained.
“We’re going to start removing stringers here shortly. We’re doing some strategic removals of some of the truss sections on and under the water,” Estochen explained. “With these sections, we’re going to take a little more care, a little more caution to support the investigation.”
Meanwhile, MnDOT made some extensive modifications to portions of the supporting road system to adjust for the expected increased volume of traffic.
First, MnDOT maintenance crews transformed a 2-mi. (3.2 km) section of TH 280, an alternative route for 35W that feeds into I-94, into a temporary freeway by eliminating access from city streets crossing the highway.
MnDOT contracted with Valley Paving located in Shakopee, Minn., for slightly over one million dollars to add a fourth lane in each direction on a 3.1 mi. (5 km) section of I-94 which shares lanes for a short distance with I-35W south of the bridge.
MnDOT crews closed this section of freeway for nearly 70 hours over a weekend construction blitz to give Valley crews the room and time to mill and overlay 1.5 in. (3.8 cm) of the surface to place 11,000 tons (11,177 t) of bituminous.
With a team, Flatiron Constructors of Colorado, Manson Construction of Washington, Johnson Brothers and Figg Engineering Group, both of Florida, recently selected to build a new bridge, MnDOT is shooting for the end of 2008 to have a new bridge in place. CEG