By Dick Rohland
While thousands of daily motorists, truckers and travelers have had their 35W bridge back since last September after a record construction time of less than one year, another busy and popular Mississippi River crossing northwest of the Twin Cities is now closed and under going a complete reconstruction.
Warped gusset plates, found by the NTSB to be the primary cause of the 35W bridge collapse in Minneapolis over a year ago, forced the closure of the Desoto Bridge in St. Cloud, Minn., that carries motorists on Hwy. 23 across the river.
Just 80 mi (130 km) upstream from the bridge disaster that killed 13 people and injured more than 100 others, Minnesota Department of Transportation (MnDOT) closed the TH23/Division St. bridge last March.
Though inspected and found structurally sound on Aug. 3, 2007, as part of a statewide bridge inspection program Gov. Tim Pawlenty ordered in wake of the 35W bridge collapse, MnDOT bridge inspectors found the warped gusset plates during the March 2008 follow-up inspection.
Gusset plates are a key structural piece on steel truss bridges and are critical tie in points for a structure’s load bearing members.
The March 2008 inspection was part of an “extensive, systematic and continuous review” of 25 similar truss bridges across the state, said acting transportation commissioner Bob McFarlin at that time. MnDOT immediately closed the bridge and began looking at alternatives to get it back on line as quickly as possible.
At 890 ft. (270 m) long and 70 ft. (21 m) wide, the four-lane two-way traffic continuous truss steel bridge had been carrying traffic over the Mississippi River for five decades. It is a primary river crossing in this busy, commercial and granite mining city of 80,000 with recent traffic statistics showing a traffic count of 31,000 vpd.
MnDOT researched two scenarios: to rebuild or repair the bridge.
“The bridge was scheduled for replacement in 2015 but even before the closure, we had begun planning for its replacement to some time in the next two years,” McFarlin added.
Entering into the equation was the scheduled funding for a $13 million road project to begin in April 2009 to improve traffic movement and pedestrian safety through the TH 23 corridor leading up to the bridge from the west.
Considering that discussions had already taken place to accelerate the replacement schedule for this bridge, and traffic to the west would be disrupted due to the road improvements, MnDOT decided to make construction for a new bridge a priority, said Dan Dorgan, MnDOT Bridge engineer.
MnDOT let the bids and set a late 2009 completion date.
By mid-summer, 2008, MnDOT had a contract in hand and brought in Lunda Construction headquartered in Black River Falls, Wis. to demolish the old structure and build a new one. A family-run business since 1938, Lunda specializes in highway, bridge and industrial construction. Lunda’s satellite office in Rosemount, Minn., is running the bridge construction.
The new $19.2 million, steel girder bridge will carry four lanes of traffic with enough space to expand to six lanes in the future if necessary. It also will feature 8 ft. (2.4 m) sidewalks and a 6 ft. (1.8 m) shoulder for bicycle traffic.
Though not on the 24 hour, seven day a week fast track as the 35W Bridge, Lunda crews expect to complete all major construction by November 2009.
Since Lunda bridge crews hit the site in August 2008, they are well on the way to opening the bridge for traffic this coming November. Piers are rising from the river surface and the abutments are taking form.
Before any demolition could begin, however, all utilities, including a 16-in. (41 cm) gas main hanging from the bridge which fed most of western St. Cloud, had to be removed and replaced.
The construction of the new gas main was a project in itself. Owned by Xcel Gas and Electric, Michels Construction located in Brownsville, Wis., took on the drill project. Michels Construction specializes in underground utility drilling and digging of all types including cross country oil and natural gas lines, interstate and intrastate pipelines, high voltage underground cable, steam and water lines.
Known as the city of granite because St. Cloud sits above a vast source of the hard stone, original bore tests indicated that there would be a higher percentage of soft soil to bore through.
However, once drilling began, crews discovered that they were boring mostly through hard rock, according to John Restad, project superintendent for the gas main installation.
“The consolidated rock turned out to be the biggest challenge. Original bore tests did not indicate that much rock,” Restad explained. “A great deal of it became a rock bore. You’re under time constraints to get it finished and the biggest stumbling block was drilling through that hard rock.”
Drilling 2,200 ft. (671 m) through mostly solid granite 30 ft. (9 m) below the river bed added several weeks to the project, Restad added.
“The drill shot started with a 6-in. pilot hole and then reamed larger to accommodate the 16-in. pipe,” Restad said. “The drill shot was guided by a computerized wire line system.
Michels drill crews used a bentonite slurry solution to lubricate the hole and rinse out the displace matter.
“It’s captured and sent to a recycling unit, cleaned up, hauled off site and reused,” Restad said.
MnDOT stepped in to speed up the drilling work, Restad said, by closing off the eastbound lanes of Hwy 23 to lay the 16 in. (41 cm) steel pipe out in two strings to save some time.
“This allowed us to stop the pull back only once to make a weld to complete the bore string,” Restad said.
Pipe crews placed an additional 2,200 ft. (670 m) in a combination of 6, 12 and 16 in. (15, 30 and 41 cm) steel and plastic pipe in the surrounding neighborhoods to update the underground gas distribution system in those areas.
Shortly after Michels Construction completed boring the underground gas main in late August, Lunda crews rolled in with two Cat 950 loaders and several Volvo backhoes to start bridge demolition operations.
According to Bruce Reihl project superintendent for Lunda, bridge crews brought the old bridge down in sections, starting with the deck and stringers on top, picking the spans off, lowering them to the river below and blasting the main river piers.
“We took the deck and stringers from the top, took the main span out from the middle and worked our way back to the piers,” Reihl explained. “From the piers to the abutments, we erected some temporary shoring to cut the spans up in small enough pieces to handle.”
To remove the main span crossing the river, Lunda floated barges with two 210 ton (180 t) Terex cranes out to the center of the river and anchored them with spuds. Working from man lifts, bridge crews cut the sections free and lowered them to the barges.
“We also had the option of dropping the end spans but thought using the temporary shoring was a cleaner way to go,” Reihl remarked.
During the blasting of the piers, the footings and the stems sheared off from the old concrete shield and they did not break up as well as we would have liked,” Reihl acknowledged.
It was a difficult challenge resolved Riehl added by bringing in a bigger backhoe to break and pry up foundation concrete still clinging to the old wood piling embedded in the river bottom, Reihl added.
“The biggest challenge is on the operators. They’re working in the blind and can’t see below the river surface. They basically removed the remaining foundation pieces by feel,” Riehl explained.
Lunda crews and equipment removed approximately 1,500 tons (1,370 t) of structural steel and 5,600 cu. yds. (4,100 cu m) of concrete to make way for the new bridge.
In spite of a renegade barge that came loose from its moorings, a long, cold, snowy winter, and rising waters from heavy snow melt, Lunda crews are on schedule. They quickly recovered the barge, anchored it and adjusted the spuds on the other barges as the river rose.
Steel girders from Industrial Steel Construction, Hodgkins, Ill., will be shipped to the site in May to begin the steel erection phase of the project. CEG