Easy as ABC
June 30, 2011
Utah County’s I-15, located 30 miles south of Salt Lake City, is in the midst of a major overhaul. The existing pavement was crumbling, and the highway also needed to be expanded to address future traffic capacity for the fast-growing region.
Under the direction of the Utah Department of Transportation (UDOT), the $1.725 billion I-15 Corridor Expansion (I-15 CORE) calls for complete reconstruction of the existing roadway and the addition of two new lanes in each direction. The 24-mile project is set to wrap up by December 2012, and construction is now more than 30 percent complete. At the current pace, design/build contractor Provo River Constructors will finish on that aggressive schedule, says Robert Stewart, UDOT’s construction manager for I-15 CORE.
One of the reasons UDOT and its contractors are confident they will meet the deadline is because they are using accelerated bridge construction (ABC) methods on the project. With ABC, a bridge superstructure is built in a precast yard or area that is off the main roadway but near its permanent location. Equipment called self-propelled modular transporters, or SPMTs, then move the entire bridge, from the girders up through the deck and parapet walls, into its permanent position in one huge piece. SPMTs are segmented into six-axle and eight-axle modules that link together in long trains. Powerful hydraulic jacks are built into the axles to raise the bridges off temporary supports so they can be carried to their permanent abutments and piers.
Stewart says this offline method of bridge construction offers less disruption of traffic, increased worker safety and faster construction. In the past, UDOT has used ABC to slash an entire construction season from a project schedule. “We’re always looking for new and better ways to accelerate the pace of highway construction,” Stewart says.
For the I-15 CORE project, Provo River Constructors is using ABC to build four new bridges offline. Two of those bridges carry cross streets over I-15, and two of them are mainline interstate bridges. One of those is a two-span steel girder bridge that will carry interstate traffic over Provo Center Street. Each span is being prefabricated separately--one on the north side of Center Street and one on the south side in locations adjacent to I-15. The longer of the two spans measures 141 feet, 4.25 inches long; the shorter is 123 feet, 5.5 inches long. Both bridges are 88 feet, 4 inches wide.
For the ABC method to work, precise measurement is crucial. The survey work for the project is being handled by DEI Professional Services, a 29-year-old surveying and civil engineering firm headquartered in Phoenix and a subcontractor to Provo River Constructors. Dan Leslie, PLS, survey area manager for DEI, says the survey crews calculate the position of the pier and abutments in the office and then upload the data onto a Topcon FC-2200 data collector before heading into the field with a Topcon Series 9000 robotic total station. Once onsite, “we lay out the corners of the abutments and wing walls, and for the center bent we lay out the foundation corners and the centerline of the columns,” he says. “We shoot the centerline of the girders with the proper bearing seat elevation so they can build the tops of the abutments to the right elevation. And then after concrete is poured for the abutments and center bent, we lay out the girder lines one more time. Then the contractor grinds down the concrete on the abutments to the correct elevation. Once it’s ground down, they’re ready to move the bridge.”
Shipping containers are used to support the superstructures of the two spans in their temporary location next to I-15. Each span has 10 steel I-beams, each measuring 5 feet 2 inches deep. A blocking system of short I-beams and a concrete block, all resting on the shipping containers, supports the ends of each girder. “The blocking reaches to the proper elevation that we want for the bottoms of the big girders,” Leslie says. “Then, after we set the concrete block on top of the stub beams, we check that elevation with our Topcon digital level.”
For the Provo Center Street bridge, UDOT’s I-15 CORE contractor, Provo River Constructors--a joint venture of Fluor, Ames Construction, Ralph L. Wadsworth Construction and Wadsworth Brothers Construction--is prefabricating the superstructure at exactly the same elevation it will be in its final permanent position. That eliminates the chance that somebody will neglect to calculate a change in elevation when converting the numbers from the superstructure to the abutments.
Leslie says they take the measurements of the girders after they are set to give the concrete contractor the proper elevation for the 8-inch precast deck. Steel diaphragms join the steel girders in the transverse direction. Leslie says DEI measures the girders at 8-foot intervals along their length, and then makes a map of that “surface” on top of the girders.
“The engineers give us road elevations, also called screed elevations, plus a planned dead-load deflection of the deck,” Leslie says. “So we have a design surface that takes into account the dead-load deflections of the deck after the concrete is poured. We compare the design surface to the actual elevations of the tops of the girders and give the carpenters a design cut or fill in the concrete over the girders.”
The “cut or fill” is also called a haunch in the concrete deck. “Suppose the design elevation of the top of deck is 4,550 feet,” says Bryce Jaynes, project engineer over accelerated bridge construction for Provo River Constructors. “And the designer calls out a 0.2 foot dead-load deflection, which is figured into the 4,550. The deck is a constant 8 inches thick, or 0.66 foot. So we subtract 0.66 from the design elevation, and get 4,549.34 feet. But the as-built shot of the girder at that point is 4,549.14. So the difference between the as-built and the design elevation is 0.2, and that is the haunch. The carpenters form the deck with a 0.2 extra thickness of concrete at that point to bring the deck up to design elevation after deflection.”
Jaynes says they measure both sides of the I-beam’s flange. “You’ll get a different haunch on each side of the girder because of the cross-slope of the road,” He says
The design surface comes to DEI as measured in 10 segments along the girders. “Then I can shoot the girders at 8-foot intervals, and I can provide fills at 8-foot intervals based on the tenth-point surface,” Leslie says. DEI uses digital modeling software to create its surface of the girders as measured with the Topcon robotic total station.
Jaynes says surveyors do an as-built survey of the entire bridge after the deck is poured. “We shoot elevations and alignment at the corners of the superstructure, and then we create a perimeter line for the bridge” Jaynes says. “Then we give all that information to our designers, and they take those points to our permanent location and make sure the bridge fits. We know the as-built of the superstructure and the as-built of the substructure, and we know the design is right. If there is a modification, we modify the substructure. We shim up the abutments or piers or grind them down.
“We’re talking hundredths [of a foot] here,” he continues. “On Provo Center Street, I’ve got shims at 0.02 to 0.04 and grinds of 0.02 to 0.03. That’s so the substructure matches the as-builts of the superstructure. That way we don’t put any undue stress on the structure.”
It took Belgian contractor Sarens, working with Provo River Constructors, just 1 hour and 28 minutes to move the shorter span of the Provo Center Street bridge into place. Sarens used two trains of SPMTs, each with 18 axles, to move the bridge. There are four wheels per axle, and the wheels swivel under the SPMT rigging. Pipe-like structures joined the two trains of axles to create a large undercarriage for the bridge.
When the bridge was moved, the SPMTs maneuvered the superstructure to line up its girders with marks DEI had made on the abutment and pier. Then the SPMT operator, using a remote controller, lowered the axle-mounted hydraulic jacks to settle the bridge into place. Jaynes says the ABC process saved more than two months on the bridge, compared to a cast-in-place structure.
Jason Kack, a principal and survey division leader with DEI, says what satisfies him most about the Provo Center Street Bridge is that it fits precisely into place. “We had not done an ABC bridge prior to this bridge, so we learned the process,” Kack says. “Our biggest challenge was to make sure that our quality controls were 100 percent, because if you move the bridge and it doesn’t fit, it’s a problem. So we’re very pleased that the bridge fit and that we met the project schedule. And we’re proud to be a part of the Provo River Constructors team.”
Three Steps to Good Quality Control• Step 1: Office Calculations. Calculations are completed by a survey tech and recalculated by a crew leader, and both are checked and compared by DEI Survey Area Manager Dan Leslie, PLS.
• Step 2: Field Verification. An initial verification is completed by the primary crew. A second survey crew does a complete re-layout and verification, and a third crew dedicated to internal quality control also verifies the field work.
• Step 3: Outside Consultant QC. Both steps one and two are done independently by an outside survey firm dedicated to quality control.
This quality control process is applied to each stage of layout and calculations for every task in the project to ensure a perfect bridge fit.