I-35W Bridge Construction

The busy Interstate I-35W bridge over the Mississippi River in downtown Minneapolis, Minnesota, collapsed at the peak of rush hour on Wednesday, August 1, 2007. Once the rescue operation was completed and the site was cleaned up, MN-DOT was anxious to get a new bridge built. The Flatiron company from Denver, Colorado, submitted the highest bid, and suggested a project plan that would take the longest. They did, however, score the highest on the technical review of the bid responses, and there was concern that the other bidders would not be able to meet their proposed timelines. As a result, Flatiron was selected to build the new bridge. They accepted what sounds like a nearly impossible project, to build a modern interstate highway bridge in a cramped urban location in the flight path to a major airport over a river that supported barge traffic, and do it within a year.

Note—these materials are covered by copyright and may not be used without written permission.

Flatiron has risen to the challenge. In May of 2008, 6 months into construction, they announced that they were more than 50% completed. In fact, they stated that they intended to be completed 3 months early in order to capture the maximum amount of bonus money. The Republican leadership became especially excited about this prospect since the RNC convention will be held in Saint Paul the first week of September, and a completed I-35W bridge would be a real feather in their cap.

This page is look at the construction project as of May 24, 2008, just after the halfway point. This morning was remarkable in that it was one of the only sunny weekend days since the bridge construction started. It was also the day before the first main span segment would be erected. Note that these photos had to be shot blindly over a very tall chain link fence, and it was extremely windy over the river. That resulted in some shots being a bit tilted, and not being as sharp as they could have been.

The photo above is an overview shot of the river crossing project site. The approach spans are nearly complete, and the piers are complete. The gap over the river is still about 500 feet. The photo below shows the work on the south approach to the new bridge. The old highway was a combination of steel girder bridge and earth embankment. That embankment is being lowered as much as 20 feet, and is being extended to reduce the overall length of the bridge. The new bridge will be about 700 feet shorter.

The photo above shows some of the heavy equipment and gives an overview of the job site on the south end of the bridge. The University math research building is behind the dirt pile. The photo below is an example of one of the signs erected by MN-DOT to explain the construction project. The site is well marked, and guides are available on Saturdays to give tours from the 10th Avenue Bridge. The guides are retired MN-DOT staff. They do an excellent public service, and the Saturday tours are well attended.

The photo above shows a tunnel that was included in the project for a future regional trail to be developed along the river at the top of the bluffs. While there is no budget for such a trail, the opportunity to build the tunnel added little additional cost to the project. Workers are currently putting in utilities and lights in the tunnel. The Red Cross Building is located one block behind the tunnel.

The photo below is a closer shot of the overall project at the river. Both photos show a number of white vehicles purchased by Flatiron for the project. This caused a small media upset when it was learned that Flatiron purchased the vehicles out of state. Some citizens of the state felt that Flatiron should have purchased the vehicles locally where possible.

The photo above is the first span of the main river crossing on the south end of the bridge. This span is being built using the cast-in-place method. This process involves building an extensive network of falsework under the bridge to hold up the pieces while it is being built. Workers then build forms out of plywood. Rebar is assembled inside of the forms. Concrete is then pumped into the forms. Once the concrete has cured, the forms and falsework is removed. What remains is a self-supporting concrete bridge span. The photo below is a close-up view of the falsework and forms above the main bridge piers. Notice how tall the bridge beams will be compared with the size of the forklift on top of the structure. The building in the background is the University of Minnesota steam plant.

The photo above shows the end of the cast-in-place beams and falsework. We can see that the first two beams are finished by the white concrete sticking out where the forms have been removed at the end of the beams. Workers are setting up jacks in front of the beams. Those jacks will support the first two main bridge segments that will be lifted in place on May 25, 2008. The background shows a great view of the Lower Saint Anthony Falls Lock & Dam, the Stone Arch Bridge, Mill Ruins Park, and the buildings that remain from the old milling district.

The photo below shows the Big Ben crane, a ringer style boom crane built by Manitowoc Corporation in Manitowoc, Wisconsin. The ringer attachment allows the crane to handle much heavier loads, up to 600 tons in this case. The ring supports the crane boom in the front, and the large stack of counterweights in the back. A similar blue crane is located in a staging area about a half mile down stream. The blue crane is nicknamed Big Bohemian due to its location on Bohemian Flats along the river.

The plan for building the center span is simple. Bridge segments are being cast in a casting yard next to the Metrodome. Segments are loaded onto a truck that has a zillion wheels, and they are transported down to Bohemian Flats. The big blue crane lifts them onto a barge. The barge is pushed to the construction site, where the big red crane lifts each segment into position. Cables are attached to the segments to pull them tight against the segments that are already in place. They pull the cables so tight that the segments essentially act like one huge beam. When all the segments are put in place, they will hold themselves up mechanically.

The photo above is a closer photo of the north end of the main span. Like the south end, it is being cast in place on a huge falsework platform. While the south end uses a large yellow tower crane typical of skyscraper projects, the north end has two large Manitowoc crawler cranes. Each span also has a small tower crane segment topped with a concrete pumper. Note that the bottom of the piers are sitting on a concrete platform at the edge of the river. There is a storm water outlet below one of the piers. This is yet another obstacle that had to be worked around when building in a crowded urban setting.

The photo below is a close up of the falsework and forms. The photo was shot through the chain link fence, resulting in the white shadows. Notice how large the structure is in comparison to the size of the workers. Also notice the large white barrels sitting on the scaffolding. These hold cooling water for the concrete. Concrete undergoes a chemical reaction when it cures, which gives off heat. In order to maintain the desired strength levels, the concrete cannot be above a certain temperature. Crews maintain the correct temperatures by running cold water in pipes through and along the surface of the concrete structure. On this particular day, the temperatures were running on the high side of normal, so workers put dry ice into the water tanks to make the water colder. You can see the dry ice vapors coming out of the tank on the right.

The photo above is the north end of the bridge showing the concrete finishing machine that will be used to pour concrete on the deck. Even though the beams are not poured yet, work is in high gear building the rebar for the deck. This is an example of the parallelism being used to speed up construction. In most cases, a contractor would conserve materials and build repeatable processes by doing one cast-in-place span at a time, repeating the 4 builds in sequence. On this project, time is the key driver, so all of these items are done at the same time.

The photo below shows the north end of the bridge with downtown Minneapolis in the background. The row of green material just above the plastic is the rebar for the highway deck. Notice the large number of workers on the structure. In all, there are 600 workers on the project, running in two 12-hour shifts each day, 7 days a week.

The final span on the north end has yet to be started. The area is being used for staging of the construction of the main spans. This location used to be a major rail yard. That yard was cut back in size in the 1990s, and part of it was developed for housing. The last major rail shippers in the area were Pillsbury and the University of Minnesota steam plant. The long term plan for this area is to remove the rail and establish a new parkway along the river and past the north side of the U of M campus.

The photo below is the bridge span over SE 2nd Ave. This area was crossed by the old bridge, but since the new bridge is much shorter, a stand-alone bridge was needed.

The photo above is the last photo of our tour crossing the 10th Avenue Bridge from south to north. This photo is looking at the area between SE 2nd Ave and University Ave (the overpass bridge). This area is being lowered up to 28 feet. That requires excavation of the old highway approach as well as building retaining walls. A small daycare center had to be torn down near where the yellow Grove crane is parked.

The photo below is a view of the casting yard that was set up on the old highway between the old bridge and Washington Avenue. In this casting yard, they set up forms to build the individual segments that make up the main channel span of the new bridge. The segments are cast under controlled conditions, and then trucked to the job site and erected using large cranes. There is a nice view of downtown and the Metrodome in the background. Also notice that a concrete pumper is at work near the right side of the photo. Finally, notice the series of white objects under and to the left of the concrete pumper. Those are completed bridge segments that are ready to be transported to the job site.

The photo above is the north section of the casting yard. A bridge builder would normally use one set of casting forms for a project, and use them a number of times. In this case, time is more important than money, so the goal is to cast the segments quickly. Notice the white H-frame machine behind the forms just above the dust cloud. That is a Shuttlelift model SL300 gantry crane. It is used to move the segments out of the forms and to load the segments onto the transporter truck. Shuttlelift machines are built in Sturgeon Bay, Wisconsin.

The photo below is an overview of the area north of Washington Avenue and south of the river crossing. The cranes supporting the bridge project can be seen in the distance. The activity taking place is that the roadway is being broken up and the pavement is being removed for recycling. This roadway will be lowered up to 20 feet, so a new grade will be required, along with new pavement.