August 5, 2007, Carl Bolander & Sons began cleanup in the Mississippi River after the Interstate 35W Bridge in Minneapolis collapsed. After two weeks of down time, the company began removing pavement on the bridge approaches, driving steel bearing piles for the south side abutment, and proceeded with excavations for structures on both sides of the river.

"The main focus of the first grading was structure excavation for the new footing and for the falsework, the steel that the bridge sits on," said Todd Planting, project manager at the bridge site.

Construction began for the bridge as Flatiron-Manson Joint Venture, general contractors, began hiring more Minnesota companies as subcontractors. Carl Bolander & Sons continued with the excavation. "We've done all the excavations for this project, and we continue to grade the site where grading is needed." In December and January, the Bolander crew continued as the challenges of winter work became more demanding. Planting said, "We were shut down for two days during the entire winter. On one of the days, there was a minus 35-degree wind chill."

Flatiron Constructors had expertise in building post-tension box girder bridges, the design specified for the new I-35W bridge, and Manson Construction had expertise in how a structure can be built from the water using floating cranes, barges and other equipment, according to Bob Edwards, assistant project manager for Flatiron-Manson.

"The process actually began when the Minnesota Department of Transportation asked for a Request for Proposal and the design work had to be started ahead of submitting the proposal," said Edwards. "We had to have a concept in mind and a way for Flatiron-Manson to move forward if we were awarded the contract." Figg Engineering, Tallahassee, Florida, worked with the joint venture to create the design-build bridge.

When Flatiron-Manson received the contract on October 8, project managers, supervisors and other employees came to the bridge site to look at the conditions for building a new bridge and began requesting bids from subcontractors to supply materials, to supply labor on site, and for drillers to start drilling for shafts. "We already had an idea of what the shaft sizes would be and then continued from there with the design work," Edwards stated.

A bridge has to be designed from the top down. "You have to know what goes on top before you make sure that the bottom supports it correctly and allows it plenty of capacity before designing the bottom," explained Edwards. "We drilled shafts into the bedrock and were ready to start the first test of the bedrock on Thanksgiving Day." Case Construction, Chicago, performed the drilling and brought in a second driller, Anderson Drilling, to keep on schedule.

The first test shaft went too deep into the ground and hit artesian waters; the second shaft was drilled to the proper depth and was load tested. Contractors installed large hydraulic jacks at the bottom of the second shaft to test for the amount of pressure the center of the shaft can withstand. Then they tested for uplift pressure, which told them how solid the rock is on the sides of the shafts and how much load the rock would withstand.

"All remaining shafts had to follow the same model and were drilled to the same depths into bedrock, about 100 feet. Geotechnical engineers were with us all the time to tell us when to stop drilling based on the depth of the drill in the bedrock and the type of rock," said Edwards. Crews watched for artesian water pressure, because the site is below two dams and the biggest challenge would be water rising into the shafts.

Crews began working in early 2008 to create four major footings and two smaller footings that were poured. Abutment 1, on the south side of the river near downtown, was placed on top of H-piles. Abutment 5, on the north side, was placed on a double row of 4-inch-diameter drilled casings 30 feet into the bedrock.

"The next step was to build columns and stem walls which go into the footings. Abutment 1 was the first concrete pour," said Edwards. Then crews poured the columns on Pier 2 on the south side and Pier 3 on the north side. "Those columns were 75 feet high and it took three lifts to pour them. Pier 4, a much narrower column, required two lifts. It's narrower, because it was a single row of piling and doesn't support the center span with as much weight," explained Edwards. "There's more support under piers 2 and 3 to support the large center span."

Crews wrapped the piers because of Minnesota winter temperatures, which affect curing. "They had to be continuously heated and, because they were mass concrete, contractors used thermo cooling pipes on the inside of the columns to decrease the temperature difference for curing."

Falsework, the columns with the steel I-beams that eventually support the road bed, began first on the south side. On top of the falsework, crews built soffit forms and the bottom of the bridge forms for cast-in-place concrete work for span 1, which is between Abutment 1 and Pier 2. When falsework began on the north side, crews built the forms for concrete work for span 3, between Pier 3 and Pier 4.

Another area of work that went on simultaneously was building the casting yard where pre-cast segments for the main span were made. "We had eight casting beds, each holding 15 segments for both the interior and exterior segments and cantilever segments for Pier 2 and Pier 3. We planned the casting yard in early November and started pouring segments. We finished on June 6 with 120 segments. We rented the exterior forms made by an outside company and applied plywood sheeting,"Edwards said.

Main span segments were hauled to Bohemian Flats, an area on the south side of the river near the bridge, where crews did finishing work to prepare each segment for placement. When segment lifting and placing began, a ringer crane sitting on a barge in the river lifted each piece to the piers nearest the river. Crews waited for the segments, installed each piece on a temporary basis with 10 pre-stressed rods held in place with about 600 tons of force, and then adjusted the post tensioning.

"That means they're making sure the transverse strands are in place and the segments are joined together with post tension. The longitudinal tension takes place from the force of the segment back to the heart of the cast-in-place bridge to hold each segment tightly and give it strength," explained Edwards. "The rods are connected when the segments are brought up from the river. At that time, the segment is two feet apart from the installed segment, and workers use a coupling nut to install the post tensioning rods.

"Then they apply epoxy to the joint as a seal, not as an adhesive, and bring the segment into place. Once the epoxy is applied, crews have one hour before they have to tighten the segment. They pull the new segment into the installed segment and, because all the segments were match cast, they fit perfectly. A tensioning jack and nut on the end of these rods helps workers jack each rod to 60 tons so the 10 rods in each segment total 600 tons. This method temporarily holds each segment in place until post tensioning work is complete."

Edwards said the epoxy is used to seal the joints from water entering the bridge where it can corrode rebar, cables and post tensioning strands to weaken the bridge over time.

Some concrete has been completely poured and set. Cemstone, the largest supplier of concrete in Minnesota, is supplying all concrete through a subcontract with Nordic Contracting, Clear Lake, Minnesota.

John Thompson, project manager for Cemstone, was at the first test pour on the north side on December 3. "The test was very involved, but our mixture was fine. These mixes are not difficult but very different because they're performance mixes that will exceed the expectations for compressive strength and density in the mix. [The concrete] will last 100 years if it was cared for under normal conditions," he said.

Cemstone's Minneapolis plant produced the mix and was available 24 hours a day, if needed. To date, the plant provided 50,000 cubic yards that have been poured. "The original bid was for 55,000 yards; we're almost done," said Thompson, who is at most sites when concrete is poured and is responsible for all concrete placed on the bridge. "In the amount of time this year, we produced and placed a lot of concrete."

Yes, 90,000 tons, 4,500 truck loads of four different mixes to meet the demands of the new bridge. One grade of mix was for the superstructure, a second mix was optimized for below-grade foundation, a third mix addressed the substructure, and Cemstone developed a fourth, very low heat-of-hydration mix with 15-percent cement in the mix.

The blue Cemstone truck, along with its pumps and Nordic Contracting pumps, is recognizable any place around the bridge site. In the casting yard, Nordic Contracting pumped most of the main span segments. Concrete work has shifted to the Second Street Bridge with the first pour of 300 yards on its deck on June 12. Two additional pours are scheduled to complete that bridge, Thompson stated.

MnDOT also wanted to build a tunnel connecting the University area with a road under the bridge. Flatiron-Manson provided a contract to Hancock Concrete Products, Hancock, Minnesota, to produce the concrete tunnel.

Gary Schmidgall, vice president, said Flatiron-Manson met with Hancock's design engineers, TKDA engineering firm and MnDOT to develop specifications for a tunnel that MnDOT and the city of Minneapolis would approve. "The committee settled on a 20-feet-wide by 11-feet-high tunnel that stretched 252 feet. We had to think about how to move concrete pieces in winter, and the only practical way was to make shorter pieces. We divided the length by 4-foot sections with each section weighing 49,600 pounds that could be loaded one per flatbed truck," said Schmidgall.

The concrete pieces were produced in Hancock's Cannon Falls plant, 45 miles southeast of the bridge. "We produced four pieces per day using 13 yards of concrete each and took about four weeks to pre-cast all the pieces," said Schmidgall. Each piece has a tongue and groove all around on the ends to connect with the next piece. "We stored the pieces in our yard and used our cranes and forklifts to load the flatbeds inside our facility." To transport the pieces in March on a two-lane highway near the plant, Hancock laid the pieces flat while MnDOT provided a route through the city that did not disrupt traffic. The now 71-mile trip to the job site required 10 trucks, some leased, making 65 trips.

Once Hancock arrived at the bridge site, Flatiron-Manson inspectors looked over the products and wrote their initials with spray paint for approval before unloading. Meyer Contracting, a Minneapolis general contractor that is Native American and female-owned, was Hancock Concrete's subcontractor.

Jason Rykal, project manager at Meyer Contracting, said his crews unloaded each piece with cranes, installed each section by connecting pieces, then used Bobcat skid steers and large loaders to cover the tunnel. Meyer Contracting had been on the job site with various grading projects, including the entrance and exit ramps at University Avenue, since January.

Edwards emphasized that Flatiron-Manson is always mindful of safety on the site. "We have spent over 500,000 worker hours on this bridge and have no lost time injuries; that's very important to us. Another crucial aspect is the quality of the bridge. We're designing this bridge to last 100 years, and we work hard on the quality side to make sure it has been designed and constructed with the highest quality."

Work continues to complete the bridge by December. Flatiron-Manson has said the work is ahead of schedule and expects to complete the job in September. Stay tuned.

To see pictures of this project, please go to our photo gallery.