Propelled in the air

Nov. 13, 2008

We’ve all seen kids do this. A collection of building blocks, scattered all over the floor. The child stands two of them side-by-side and places another across the top, making a bridge. But then the child thinks the red block would look nicer than the blue one, so he lifts off the blue and replaces it with the red.

A simple idea, easily executed. But try doing that same thing with a bridge that weighs 3 million lb and sits on the most extreme angle permitted by engineering standards—and within an unbelievably compressed time frame.

We’ve all seen kids do this. A collection of building blocks, scattered all over the floor. The child stands two of them side-by-side and places another across the top, making a bridge. But then the child thinks the red block would look nicer than the blue one, so he lifts off the blue and replaces it with the red.

A simple idea, easily executed. But try doing that same thing with a bridge that weighs 3 million lb and sits on the most extreme angle permitted by engineering standards—and within an unbelievably compressed time frame.

That describes the challenge met by the Utah Department of Transportation (UDOT) and engineers from Michael Baker Jr. Inc. (Baker), along with Ralph L. Wadsworth Construction and Mammoet USA in Utah last year. They literally built new abutments under a deteriorating bridge, lifted off the existing bridge and replaced the entire bridge with a brand new structure of steel and concrete that had been built along the side of the highway, driven down the road and set in place atop that new set of abutments.

Team effort

The project replaced a bridge along 4500 South, which passes over a section of I-215 near Salt Lake City.

“A simple concept, perhaps, but a huge and complicated effort” was how engineers described the project.

Imagine the calculations and precision engineering required to pull off this job. Figuring out how to cleanly and safely remove an existing 37-year-old deteriorating bridge; building new support structures for its replacement with only the space under the existing bridge to work within and while traffic continued to flow; designing and constructing the new bridge to demanding specifications on an open site alongside the interstate; maneuvering the new structure into position and sliding it in place in a single weekend; and, lastly, finishing up the roadway connections quickly and safely.

And all within full view of the taxpaying public, visiting dignitaries from federal and state transportation agencies and the news media.

“People seemed pretty excited about it,” said Michael Arens, an engineer Baker who was involved with the project. “We’d hear things like, ‘What the heck is that, the ramp to nowhere?’ as the new bridge was being built along the roadside. But there was a real sense of excitement as they showed on the news an animation of how the project would be done. People really ate it up. It turned out to be far less disruptive to the public doing it this way in the end, as far as traffic impacts. More than a thousand people came to watch the bridge move.”

Getting there took an enormous commitment by all involved, Arens said.

“From an engineering perspective, the geometry was extremely complex,” he noted. “Baker’s engineering capabilities became and remained a very important element in this overall project. The bridge was on a 12% grade, the steepest roadway grade allowed by AASHTO standards. So we had an extremely steep bridge to begin with, then the interstate we had to drive the new bridge across was on a 4% slope with 2% cross-slopes. There also was a 5-ft elevation difference between northbound and southbound I-215 that the bridge had to traverse. A lot of geometry involved, lots of spreadsheets, to keep the loads on the trailer as the old bridge came off and new one went on. That’s why I had butterflies. We had to check on the stresses of the move, knowing that we would be lifting the bridge at places where it’s not normally stressed.”

The existing bridge was a four-span bridge with I-215 northbound and southbound traveling under the two middle spans, and slope paving continuing up to the abutments under the two end spans. The existing configuration allowed for new, full-height abutments to be constructed under the existing bridge while it remained in service. The slope paving was removed and the vertical cuts were retained with temporary soil nail walls to support the existing abutments. The new full-height abutments were constructed on spread footings to avoid driving piles under or through the existing bridge.

The new superstructure was constructed off-site in the I-215 southbound off-ramp gore area adjacent to the bridge. It was constructed on temporary abutments at an elevation that allowed it to be directly transferred to the final abutments. The orientation of the superstructure was approximately 90° to the final location.

The existing bridge was demolished using two techniques. The two end spans were demolished using traditional techniques of a hoe ram to remove the deck and a crane to remove the girders. Once I-215 was closed, self-propelled modular transporters (SPMTs) were used to remove the two middle spans. These spans were moved, one at a time, to an off-site demolition area where they were set on temporary abutments to be demolished later.

The superstructure was then lifted off the supports by the SPMTs, turned 90° while being “backed” onto I-215, and then moved forward into place on the new abutments after the existing bridge had been removed. The new abutments are semi-integral abutments on elastomeric bearing pads. The elastomeric bearing pads were already installed on the bridge. This allowed the bridge to simply be set on the new abutments, neoprene foam and waterproofing installed over the joint, and backfilling of the abutment to take place. Seismic bolsters were placed on the underside of the girders after the bridge was in place.

“This is the first one we did like this, using an accelerated bridge construction method,” said Kip Wadsworth, president of the Ralph L. Wadsworth Construction Co., which served as general contractor on the project.

UDOT decided to use a construction manager/general contractor approach on this job, hiring the contractor based on a qualifications proposal, not a formal project bid process. Wadsworth then worked with Baker to develop details of the project. After this initial design phase, Wadsworth submitted a non-competitive bid that fell within a “fair range” as determined by UDOT and was awarded the remainder of the job. Accelerating the process this way eliminated time required to advertise for bids and reduced risk because the contractor also had helped with the design. It enabled construction to begin while design was being finalized. Other advantages specific to this project were realized, as well.

“We’ve been working with Baker three or four years, and had done other design-builds with them,” said Wadsworth. “This was a very time-sensitive project to get it designed, constructed and open to traffic. It took a lot of cooperation to get something of this magnitude and level of technology done. Fortunately, we had that experience with Baker, so we didn’t have to work at establishing a relationship and figuring out how to work with each other.

“I give UDOT a lot of credit,” Wadsworth said. “They’re always looking for faster, better ways that serve the public well in the long run.”

Wadsworth’s impression is not only accurate, but reveals a core belief of the Utah agency.

“The department had been looking at this sort of approach for several years, trying to find a project to bring it here to Utah,” explained Lisa Wilson, project manger with UDOT. “The existing 4500 South Bridge was in such terrible condition that this project became the obvious choice. We’re always looking for ways to get things done faster that impact the traveling public less. This project achieved savings of $4 million in user costs by only having to close for one weekend, and saved a year in time. The project, in fact, was completed by the time it would only have been advertised for bid under the traditional approach.

“It was very well planned by Baker and Wadsworth,” Wilson continued. “Those guys were amazing. Nobody had experience with this type of project, but they worked countless hours and they really came through.”

Reaping the benefits

The payoff of all that coordination and brainpower, though, came during the removal and transport of the old bridge followed the next day by the transport and placement of the new one. Dutch-based contractor Mammoet—an international expert in the use of SPMTs, another way of saying specially equipped mobile trailers—handled the two “journeys,” neither one covering a lot of physical ground, but both representing a giant leap forward in achievement.

“When we first started talking with Mammoet, we said they’ve moved so many large objects like nuclear plant reactors and tanks, building frames, things like that, they’ll know what to do,” said Arens. “With the slopes involved on this job, it turned into a complicated geometric problem. It became probably the most complicated bridge move in the world, but Mammoet came through.”

“This was probably the most difficult project we’ve done to date,” acknowledged Bill Halsband, vice president of North America Business Development for Mammoet. “The pitch and slope made it difficult, but with the SPMTs everything gets engineered based on the parameters of the job. How much does it weigh and where does it need to go? That’s the info we need.

“The move in Utah was a learning curve for all of us on the project, with lots of give and take with Baker, Wadsworth and UDOT,” said Halsband. “It became an important trust issue and turned out to be a very good experience.”

Design began March 1, 2007, and by Oct. 31, 2007, the bridge was in place—a brief eight months from start to finish, versus the traditional process, which would have taken one full additional construction season. Using an accelerated bridge construction process also alleviated inconvenience to the traveling public. Under a traditional process, motorists traveling over the bridge would have had to contend with eight or nine months of lane restrictions during construction, while those traveling under the bridge on I-215 would have seen lane restrictions for that same time frame. Instead, traffic was diverted for one weekend and by Monday morning rush hour traffic was again flowing.

“It was pretty impressive,” Arens admitted. So what comes next for UDOT and Baker, fresh from this demonstration of engineering, construction, savings, safety and traveling convenience? More projects following a proven model, of course.

“UDOT awarded a Ralph L. Wadsworth-Baker design-build team another bridge project just like this one, the 3300 South Bridge over I-215, which was completed in August of this year with the same degree of success and speed as the 4500 bridge,” said Arens. “UDOT believes bridges replaced in this manner will perform better and last longer than those done using traditional bridge construction because the new structure is built off to the side as opposed to building it over live traffic. It’s expected to achieve better performance and more reliable maintenance.”

Accelerated bridge construction using SPMTs. Just like a child with building blocks. A simple concept, but a huge challenge—and hugely successful for UDOT and its traveling public.

For a more detailed discussion of the 4500 South Bridge project, see the technical paper written by Michael Arens of Michael Baker Corp. titled, “Accelerated Replacement of the 4500 South Bridge over I-215 in Salt Lake City, UT, using SPMTs,” from the International Bridge Conference 2008, IBC-08-24.

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