Ahead of its time

Aug. 14, 2008

A few miles to the north of Minneapolis, tornado sirens were blaring. At the site of the I-35W St. Anthony Falls Bridge project, the hail was warm and embracing. It had to feel good.

On May 25 crews from Flatiron-Manson placed the first segment of the job in front of hundreds of spectators on the 10th Street Bridge. Despite the threat of severe weather, the support of the community could not hold itself back from one of the premier signature construction jobs in the state’s history. The clouds were ominous, but the salute—the hail—was obvious.

A few miles to the north of Minneapolis, tornado sirens were blaring. At the site of the I-35W St. Anthony Falls Bridge project, the hail was warm and embracing. It had to feel good.

On May 25 crews from Flatiron-Manson placed the first segment of the job in front of hundreds of spectators on the 10th Street Bridge. Despite the threat of severe weather, the support of the community could not hold itself back from one of the premier signature construction jobs in the state’s history. The clouds were ominous, but the salute—the hail—was obvious.

“They had to put up that first one and the severe weather alert came through,” Alan Phipps, I-35W project manager for FIGG, the lead designer, told Roads & Bridges. “The storm was actually a little north of the city, but at the project site they got some high winds very suddenly, and it actually took one of the boom cranes and swung it around. At that point in time they decided to not lift the second segment they had planned for that day.”

Gusts of all kinds have been peppering the jobsite since officials first broke ground back in October. Whether it was a sudden drop in temperature or a quick flip in a change order, the sails were straightened almost instantaneously—keeping the winds of momentum on a forward push.

It was that salvo that had officials, creators and shakers fast-forwarding to July 10, when the last segment of the new I-35W bridge was put into place. In a striking case of coincidence, severe weather came back for the milestone, as did the crowds of hailers.

“It was a milestone of the project,” said Phipps, who was glued to project webcams from the FIGG offices in Tallahassee, Fla., during the launch and tie. “But it was a short-lived feel-good being there were many more things that needed attention and many more details to be complete to finalize the project.”

Professional movers

Project details dominated the run of the operation from the onset. The geometry of the collapsed bridge enveloped a series of design exceptions.

“The most notable one was when you came from the north to the south [on I-35W] you go underneath Fourth Street and University Avenue, across Second Street and then you are in a sag vertical curve, then you go up to go over the bridge,” said Phipps.

The sag was too sharp for Federal Highway Administration design requirements and often was the culprit of accidents due to lack of proper sight distance.

“People would come up over the hill and see all of the traffic stopped,” remarked Phipps.

The new proposal eliminated all of the design exceptions of the original project geometry. A thin superstructure was drawn up for the overpass on Second Street, and crews also excavated to lower the profile.

“What that allowed us to do was give a sag curve underneath University Avenue that meets the design criteria and eliminates the substandard geometry associated with that and the on-ramps,” said Phipps.

Reducing the profile also allowed crews to establish the correct 16-ft- 6-in. vertical clearance on the University Avenue bridge and will allow workers in the future to come in and provide superstructure depths of 3 ft greater than the existing bridge when the new bridges are built at the University Avenue and Fourth Street sites.

Surveying at the start of the project also was suspect. With the crumpled remains of the old bridge still on the ground well into October, designers had to go off past reads of the site, which contained some unknowns. The most notable was the location of a historic wall that was going to be preserved. When pier 4 of the new structure was originally staked it landed right in front of the wall, which would require demolition and the excavation of the cliff. Neither was going to happen.

“There also were steam tunnels with a conveyor belt for coal loading buried in that cliff, and the railroad for the coal train on top of it all,” observed Phipps.

So pier 4 was moved about 25 ft closer to the water, which opened up more challenges. Another project criterion included providing an 80-ft corridor behind the historic retaining wall for future transportation to accommodate rail lines, a two-lane road, a bike trail, a sidewalk and greenspace. Moving pier 4 increased the span length from 121 ft to 147 ft, which required increasing the structure depth and maintaining the vertical clearance under University Avenue as well as following depth limitations over the envelope that included the railroad tracks.

Spans 2 and 3 of the new bridge also were forced into the equation. Both are on fixed locations due to the river, utilities and the old foundation, and they tie into pier 4. With the member now moved, the balance of the dead load between the two spans needed to be adjusted. Phipps and his group decided to use thicker elements in span 3 to retain that structural harmony.

“We used additional concrete over and above of what we needed stress-wise to counterbalance that,” said Phipps.

The location of an old storm drain also was a little fuzzy. The structure, which comes out right underneath the pier 3 northbound footing, wanders a little bit to the west as it heads north, right into the pier 4 footing. The design called for straddling the tunnel, but previous surveys did not pinpoint the path. Further excavation needed to be done to determine the location of the edges of the drain.

Running test drilled shafts proved to be another touch-and-go situation. In the attempt to find solid foundation, the first test shaft ran into artesian conditions after crews drilled down over 100 ft.

“You never know when you drill these shafts near a river like [the Mississippi] where you are going to run into artesian conditions, because with the falls nearby the fractured rock has water running through it. If you hit a seam that is very hard to predict,” said Phipps.

The second test drilled shaft was moved over about 20 ft and inserted 93 ft into the river bank.

“We stopped at that higher level and got amazing results,” Tom DeHaven, design coordinator at the site for FIGG, told Roads & Bridges. “We found a foundation 20% stronger than what we were supposed to get.”

The test shafts were outfitted with two sets of Osterberg load cells. One set was at the tip of the shaft, with the other set positioned 10 ft above. The two levels allowed crews to test the tip capacity separate from the side capacity. The cells at the tip were activated first until there was sufficient displacement from the rock underneath. The cells 10 ft above were then switched on to measure displacement versus friction.

Two types of rock rest at the bottom of the river: a St. Peter Sandstone and a shale. Sandstone can create cleanliness issues with the tips of the shaft. A submersible shaft inspection device was used to check for debris prior to concreting, and cross hole sonic logging was used to verify soundness of the completed shaft.

“Once we got out of the ground, we controlled our own destiny,” remarked DeHaven.

“This isn’t cold”

Everybody talked about how the Minnesota weather would bring the operation to its knees, but no one could predict the spring storms that at times threatened to grab the project by the throat. Both, however, failed to stop the circulation.

“There was a day when the temperatures dipped well below zero and somebody commented how cold it was. One of the workers said, ‘This isn’t cold. Cold is doing 70 mph on a snowmobile across a frozen lake,’” said DeHaven.

Sense of humor and sense of urgency kept everything ahead of schedule. The main span of the new bridge contains 120 precast segments, and all were cast by the first week of June. Flatiron used four temporary shelters that could be wheeled around the casting yard to provide protection from the elements. Thermal blankets also were used during cast-in-place operations. Flatiron also elected to use eight forms for the piers all at once, instead of building two or three and rotating them around. Working on all of the cast-in-place elements at the same time helped speed up productivity.

A thermal control plan made sure the concrete, temperature-sensitive mass concrete in some areas, cured in the right conditions throughout the winter. Probes embedded in the concrete allowed the crews to constantly track the temperature. If the temperature was rising too fast, blankets would be removed. If it was dropping, more insulation could be added.

With the mass concrete items, particularly in the footings of the bridge, cooling pipes ran up and down to keep the environment under control. Those pipes have since been filled with grout.

“We had continuous thermal monitoring 24 hours a day from automatic readers that would show you where you were going,” said Phipps.

“Flatiron did such a good job with quality control,” added DeHaven. “The local work force was efficient. They were not just working during the eight hours of sunshine, but at midnight, too. That is why the project got off to a great start.”

Substituting cement with fly ash and ground-granulated blast furnace slag also proved to be successful during cold pours.

The extra work produced extra strength. The high-performance concrete used for the superstructure was hitting the 8,000-psi level. Design requirements called for 6,500 psi.

The natural sequence of events also helped during the winter. While the precast segments were being shaped in the sheltered casting beds during the bitter months of December through March, crews were working on the footings of the bridge. Construction of the piers began in early March, when temperatures were a little more tolerable.

Mastering pieces

When six segments were erected on the main span in one day in June, people wondered if eight in a day was even possible.

“The goal is to get as many up as we can,” Peter Sanderson, project manager for Flatiron-Manson, told Roads & Bridges during a site visit on June 17. “The absolute physical limit is eight. We can’t get more than eight.”

Anywhere up to six were erected on a daily basis, but the controlling factor in the erection sequence was not the physical limitations of the crew, rather it was what took place as soon as the piece was placed. Each cantilever comprises two parallel box girders, and there was a 4-ft-wide closure pour connecting the cantilever wings of the box girders at deck level.

“That had to be poured in sequence a couple of segments behind erection, so as we stress our post-tensioning the deck got longitudinal compression in it.

“So the sequence included pouring that closure and stressing transverse post-tensioning through it and stressing the longitudinal post-tensioning and doing some grouting and anchorage pour-backs before they could erect the next segment,” explained Phipps.

Despite the multiple-step proc­ess, all 120 segments were installed in a blur by construction standards—it took just 46 days.

At press time several elements of the bridge were still awaiting completion before the grand opening in mid-September. Span 4 needed to be cast and post-tensioned, and the approaches needed to be finished, which included grading and paving. Lighting (LED lighting will be installed, the first on an interstate in the country) along with the anti-icing system and the intelligent transportation system needed to be installed. Connecting the structure with smart bridge sensor wires, signing, striping and final landscaping also lay ahead. The bridge will be coated a “snowbound white” color upon completion.

Balancing budgets

As for aggressive schedules becoming the norm with bridge projects across the U.S., the people behind the I-35W St. Anthony Falls project advised to weigh each situation carefully. The greater Minneapolis area was losing $400,000 a day with the downed bridge, an estimate many said was light. The contractor also was encouraged behind a $200,000-a-day early-completion incentive.

“The contractor can weigh how much the time is worth in relationship to initial costs to accelerate the project,” said Phipps.

“The months of December and November, they really are not much use to you, so we really needed to be finished by September,” said Sanderson. “It is not necessarily economical, but in a case like this the value makes it economical.”

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