To Build a Bridge
Several years ago, I wrote a four part series about the story of the Stillwater Bridge across the St. Croix River. Part 1 covered the project from 1950s promises to build a high bridge and get traffic well away from downtown up to the point the original iteration of the 1980s proposal was officially sunk in 1998. Part 2 covered the endless bickering and litigation from then up to the time the roadblocks were removed and the new bridge officially was authorized. Part 3 looked at final design revisions, and Part 4 speculated on the future of downtown Stillwater. Now that the bridge is open it’s time for a part 5 to look at the actual construction.
There’s really two methods for building a concrete bridge. You can pour all the concrete at the site called cast-in place. This requires bringing in trucks onto the site, pouring in less controlled conditions, and building elaborate falsework. Or you can cast segments in advance in a convenient location, which may even be some distance away (precast), which requires moving very heavy, bulky concrete segments. Poured in place (or alternatively using steel) is generally good for custom, curved pieces while cast-in-place is ideal for identical straight segments. On the Stillwater Bridge both were used; the main span and straight approach sections are precast and the curved approach sections are cast-in-place.
Structurally, the segments of a precast bridge are held together with internal steel cables that are tightened after being placed (post-tensioning) and the force of friction against each other keeps them from falling. As a practical example, take a stack of books and press them hard against a wall without supporting the bottom. Notice how they stay put. The external cables on an extradosed bridge are more to provide additional tension rather than to hold the bridge up against gravity.
Originally I hated the aesthetics of the bridge; looking at renderings the scale seemed off. But now that I’ve seen it in real life I’ve grown to like it. I’m not sure it’s superior to the Braun proposal, which would have looked like a slightly wider high bridge, nor superior to a cable stayed bridge. Keeping the towers low enough so they could not be seen outside of the valley was important to the stakeholders, but really, everywhere you could see the towers there’s also other buildings around. It’s definately unique, only the second of it’s type in the US. And there’s at least some engineering justification for the design (it’s particularly hard to build piers into the St. Croix River here) as opposed to just building extremely expensive fluff like the arches on the Lowry Avenue Bridge or the towers on the Hennepin Avenue Bridge.
To limit the scope of the article I’ve focused the pictures and text on the bridge itself. Throughout I’ve used official pier/tower numbers. The large piers in the river are numbered 8-12 from east to west. Piers 1-7 are smaller ones on the Minnesota shore and pier 13 is a smaller one on the Wisconsin Shore
The original plan was to begin construction on the bridge in 2014, but engineering on the foundation had advanced to the point that it was split off and bid separately to get a head start. Meanwhile, work on the Minnesota approach began.
The foundations having been completed, the primary work on the bridge structure in 2014 was completing the piers and towers. On the Minnesota side the second of two years of the approach work happened, and was completed except for the short stretch that was used to build an on-site casting yard.
Pier 8 rising from the waters, March 2014
Once the piers were built to their full height, a temporary steel crossbeam was placed between them to allow the pier caps to be poured. Three beams were fabricated, They were placed on piers 8, 9, and 10 in that order, then once the cement pours were done the beams on piers 8 and 9 were reused for 11 and 12. These were made of standard lengths of steel, and once done the pieces were broken up and reused for other projects. Meanwhile, work on the towers continued.
Public Boat tours were offered. I went on two of them. Here’s a few photos from a September 6, 2014 trip.
During the fall segments for the approach span started being cast on the on-site Minnesota casting yard, and bridge spans were cast indoors at a facility on Grey Cloud Island, to be barged in.
Placement of the horizontal segments could not be done over the winter as the epoxy needed to be maintained above freezing while it cured. The hope was to get a start at placing the segments in late fall, but an early freeze meant this did not take place.
The major highlights of 2015 were the completing of vertical construction of piers 11 and 12, and the beginining of horizontal construction on piers 8, 9, and 12. Three blue-painted segment lifters were fabricated.
A milestone was reached on May 27th, 2015 with the first river segment lifted into place on Pier 8.
Once again in 2015, an early freeze delayed construction. The last segments that were waiting on barges to be placed had to be returned to Grey Cloud Island for the winter.
To nobody’s surprise, it was officially announced on January 7, 2016 that the bridge would open a year behind schedule, in fall 2017. Delays were blamed on weather, labor and material shortages, the overall project complexity, as well as not being able to get steel forms in a timely fashion.
Here’s some photos from a bridge viewing cruise in 2016. (Apologies – there was water in my lens.)
Later in the summer “ringer cranes” were brought in to lift segments for piers 9 and 10. Ringer cranes swivel on a ring, rather than move on a track. The original plan was to finish installing segments on piers 9 and 10, then move those lifters to piers 12 and 13. However, at this point it would likely have delayed the bridge another year, as segment placing would not have been finished before it had to cease for the winter, meaning the types of construction that could be done during the winter wouldn’t be possible since the segments had not been placed. No doubt the cost was less than keeping the site mobilized for another year as well as the costs of continued motorist congestion. The cranes typically rent for $80,000 a month, a drop in the bucket for a project this size.
The final segment was cast at Grey Cloud Island on August 3rd, and lifted into place October 4th. Although 2-1/2 foot gaps remained to be filled in closure pours, you could now walk from Minnesota to Wisconsin.
The segment lifters were constructed specifically for this project; once they were no longer needed the contractor elected to buy them back and they were removed in the winter of 2016.
With the bridge segments done, work focused on the “closure pours”, the small sections left in between the towers. With the completion of the closure pours, the bridge was essentially complete with an unbroken slab from one shore to another. Work also started on the drainage system and railings.
With the concrete work of the bridge proper completed in the early spring, attention turned to the finishing touches and the final few hundred feet of the approach roads. Chip-sealing the driving surface, railings, lights and signs, drainage pipes, and painting. The field office on the Wisconsin side was dismantled and the final sections of approach roadway were paved on both sides.
Finally the day Stillwater has waited 66 years for arrived; MnDOT delivered on their promised to open a “High Bridge” away from downtown following the flood of ’51. It was a gorgeous Midwestern summer day. The ribbon cutting was attended by hundreds of people. In attendance were two Stillwater residents that had been at the ribbon cutting of the Lift Bridge in 1931. The speeches by governors Dayton and Walker and other dignitaries were the usual back-patting. Dayton made a joke that “tomorrow we start construction on the Minnesota toll booth and will charge double for people with Packers paraphernalia”. And the overall theme was of bipartisanship: that it’s possible for Democrats and Republicans to put aside their bickering and work together and make government work and get something done. Following the meeting, those in attendance boarded the shuttle buses back to their cars and as a surprise the buses drove over and back across the new bridge.
I returned to Stillwater in the evening for the closing of the Lift Bridge. The area was a mob scene with people wanting to take part; it seemed that it took almost as long to get from Oak Park Heights to Lowell Park as from Bloomington to Oak Park Heights. If the ribbon cutting was like the dry pomp of an Olympic opening ceremony, this was the party atmosphere of the closing ceremony. Lowell park was filled with old cars for the “Cruisin on the Croix” classic car and hot rod show. Thousands of people were in attendance. As the bridge rose one final time, people wandered into the roadway on both sides to look and take selfies.
As the time approached more and more people gathered on the sides of Chestnut Street street. At 8:00 PM sharp the flashing red lights came on and people swarmed into the street, engulfing the waiting cars. It only took a matter of seconds for the people of Stillwater to reclaim their downtown from the cars. I could see it would be some time before they could clear the area to allow the final parade of classic cars through, so I went to drive the new bridge. And once again there was a party atmosphere, people in cars driving slowly back and forth across, horns honking, waving from the overpass as well as people on foot and bicycle exploring the trail. The bridge’s first “traffic jam” was less than 30 minutes after opening.
As they said about the Big Dig, sometimes doing something wonderful is worth the cost. Getting rid of an elevated freeway, the “Green Monster” made a vast difference in downtown Boston just like the new bridge allows the people of Stillwater to reclaim their downtown while allowing people to continue to pursue the American Dream. It’s forecast to see 99,820 cars a day by 2065.
And the old bridge will remain, as something for Stillwater to treasure. We’ve done something wonderful for ourselves and left a legacy for our kids, our grandkids, and our great grandkids.
Besides the obvious places, the St. Croix Crossing Facebook page and MnDOT site, on my personal blog North Star Highways there is The Stillwater Bridge Construction Galleries Part 1 and Part 2. More pictures including the trail and approach roads, less commentary. On my personal Flickr site, The New Stillwater Bridge Album, there are close to 400 full sized photos.