Tom DeHaven still is not sure of the exact contents of the most identifiable structure lying near what was once the site of the age-beaten I-35W bridge crossing. But in a midnight’s moment, it was gone.
“They told us they had something that made them concerned of the vibration [caused by demolition work],” the FIGG bridge design coordinator for the new I-35W bridge told me as we drove past the University of Minnesota power plant—which showcases a series of giant smokestacks—en route to the busy cleanup work alongside the Mississippi River. “We asked them what it was and they never did tell us. Then one day they told us it wasn’t a problem anymore. It’s like they secretly moved it in the middle of the night.”
Though probably a little foggy on the details themselves, those at the Minnesota DOT (Mn/DOT) can certainly appreciate the approach: the center of controversy one day, out of mind the next.
However, a downed bridge and its tragic images cannot be moved overnight. By industry standards, they have certainly come close. Walking the work area on a crisp and gray October morning, I realized how hard those involved have worked to make what happened on Aug. 1 not a problem anymore.
With a couple of Caterpillar excavators breaking up the last remains of two bridge piers, the site was being pruned for a reawakening. Back to calling the interstate crossing the St. Anthony Falls Bridge, the team of Flatiron-Manson, FIGG and Mn/DOT are out to regenerate the pure beauty of the region.
Public address
Dropped in the center of an urban area, a 200-yd footprint around the St. Anthony Falls Bridge site is a picture frame of history and natural aesthetics. Nearby is a stone arch bridge now serving bikers, walkers and runners. Just up the river sits a waterfall, and over on the east end of the bridge site there is a railroad abutment wall constructed in the late 1800s. Designing off the theme “Arches, Water, Reflections,” FIGG wanted to bring the people back to the shoreline.
“One of the things we are really trying to achieve is something that is context sensitive,” Alan Phipps, design manager for FIGG, told Roads & Bridges. “Something that really fits with the site and something the community really gets behind. We want it to become part of their community and be something they can get excited about.”
After the I-35W collapse, residents were fired up over a number of issues, most notably the replacement bridge. Mn/DOT held over 15 public forums in neighborhoods of all types, and conversation seemed to gravitate to safety, quality and aesthetics.
“There was a combination of themes that came out of those meetings,” Jon Chiglo, Mn/DOT project manager, told Roads & Bridges. “Some people wanted to be involved in the development of the design, and a lot of them wanted to know how we were going to ensure quality. They also wanted to make sure it was safe, but they also were wondering about the schedule.”
The schedule for this project could fit in the palm of your hand. Work was supposed to begin this month, with a completion date set for December 2008. Prime contractor Flatiron-Manson has the approach for an aggressive design-build programmed into the brain. The joint venture has executed a number of them, but even Flatiron Project Manager Peter Sanderson seemed preoccupied with concern during the first week of operations at the temporary construction headquarters in Minneapolis. Before answering a question, he made a decisive turn to his computer and showed the hundreds and hundreds of unread e-mails regarding the project. In just a few short days, the job was beginning to feel like a clumsy elephant sitting on his shoulders.
“I think we have a very aggressive schedule,” Sanderson told Roads & Bridges. “Normally you would want somewhere between two to three years. That would be most economical. One year isn’t. This is nothing new to us, but this is particularly rushed.”
On the design end, FIGG made sure the public set the pace. The design firm held its trademark charette to cut the final look of the bridge. It came down to two options. Both featured eight sleek hourglass-shaped piers, which will stand about 70 ft tall. In option A the face of the piers was looking out over the Mississippi. The other option had thinner piers with a blue stripe down the middle turned 180°. Other shared elements were a waterfront observation area, the preservation of the historic railroad abutment wall, a unique lighting system and the option of constructing a pedestrian bridge below. There will be a northbound and southbound span, each carrying five lanes of traffic with 13-ft shoulders on the outside and 14-ft shoulders on the inside. The bridge also will accommodate bus rapid transit in the median and can carry light rail in the future. Both designs called for post-tensioned concrete box girder bridges. Despite the problems with the old I-35W bridge, Mn/DOT was not partial to any one type of building material.
“We chose Flatiron and FIGG, and their design happened to be concrete,” said Chiglo. “We did not have any preference for concrete or steel. We would have been comfortable with steel, and we are obviously comfortable with concrete.”
The public favored option A, shifting the focus of the project to the actual jobsite—which was not doing any favors for anyone.
Challenge binge
Snugged in with this tight assembly of history and beauty is a fat layer of challenges. Contamination covers the south end of the site where there used to be a coal gasification plant. Mn/DOT was conducting its own geotechnical investigation, in the form of soil boring, in October.
A gas line and some fiber-optic lines also have to be relocated, but that should cause minimal disruption.
Some of the bigger obstacles are a guide wall to the river locks, a large drain tunnel and the preservation of the railroad abutment wall. Right behind the guide wall, located on the south bank, is the existing foundation of what was pier 6 of the I-35W bridge.
“Those are steel-cased concrete piles,” said Phipps. “We wanted to find a way to not have to pull those out and work around them.”
The contractor also will have to squeeze piers between the guide wall and drainage tunnel without removing the old foundations. The design solution calls for the new footing essentially straddling the old footing. The new footing comes with a clipped corner and a rotated shaft pattern to accommodate the drain tunnel. That is on the northbound bridge. On the southbound bridge, crews will install two 7-ft-diam. shafts down through the pile cap and thread them through the pile arrangement from the existing footings.
As for the old abutment wall, FIGG had to find a way over existing railroad tracks that run coal to the University of Minnesota power plant. Underneath the tracks is a tunnel that carries steam back to the plant and heats the campus.
“Being able to leave that wall and train alone was a key component to laying this whole thing out,” said Phipps.
There is a 20-ft clearance envelope, which Phipps said will be a challenge to meet because one of the other key elements of the project was correcting the geometry of the highway.
Then there is the suffocating 13-month schedule, which will run through what could be a frigid Minnesota winter. Missing a series of days could turn disastrous. The concrete mix was still being developed at press time, but it will be a high-performance mix that will probably be of low permeability and will contain some type of corrosion inhibitor. Flatiron also might use self-consolidating concrete. The minimum targeted strength is 6,000 psi. The approach spans and piers will be cast in place, so whether or not weather will play a factor will depend on when the pours are executed. Curing blankets, heaters and heated aggregate in water are all solutions the contractor could turn to. FIGG, however, comes with local experience. On the Wabasha Street Bridge in St. Paul, crews poured concrete at 19 degrees below zero, and the material was curing at 36 below.
The main span will consist of precast segments weighing about 200 tons each. Lengths will range from 13.5 to 16.5 ft, and a total of 120 segments will be put into place. The total length of the bridge will be shortened from 1,900 ft to 1,350 ft.
At first it was thought that the contractor would have to mold the segments off-site, but in October Flatiron thought there was enough real estate at the bridge site to accommodate precasting.
The erection scheme will take on a different look. Normally, segmental bridge construction is executed span by span or by balanced cantilever. Here they will carry out one-directional cantilever construction. The back spans of the bridge will be cast in place on falsework including the pier diaphragms. A small cast-in-place closure joint will allow crews to attach the precast segments going out in cantilever from the main piers across the river. Post-tensioning tendons will have a unique arrangement because they will go from the open segment joint back toward the back end of the bridge.
“It is different because by erecting this way you are introducing negative moments in the back span all the way to the abutment, which means your post-tensioning tendons, which normally are shorter, end up being much longer,” said Phipps. “We have a lot more tendon anchors back by the abutments. It is a little bit more of a challenge to get them to fit in the box section.”
Flatiron also was not sure of how equipment would be used during the erection phase. The choice comes down to a crane on water or on land. However, Phipps said whatever the method, the segments would have some type of water delivery.
Flatiron is currently installing 7- and 8-ft-diam. drilled shafts about 80 ft into bedrock. Foundation work will continue through March 2008, with pier construction beginning in February. Superstructure construction is scheduled to begin the following month, and segment assemblage should start by midsummer.
