By Shari Held
The 81-year-old, two-span bridge, located southwest of Carlisle, Pa., on state Route 11, was “structurally deficient” and slated for a full bridge replacement. The only question was which method to choose – conventional or accelerated bridge construction.
“It’s all project specific,” said Kristin L. Langer, P.E., assistant chief bridge engineer for Pennsylvania Department of Transportation. “ABC is a better option where traffic impacts are a huge factor and we need to get in and out fast and keep construction workers safe.”
If conventional construction with a road closure was selected, the estimated time frame ranged from three to six months. If one-lane staged construction was used, the project would likely have lasted six months to a year.
Ultimately PennDOT District 8 chose to build its first completely precast bridge using ABC techniques. The old bridge was shut down July 29, 2017, and reopened 13 days later as a new, modular, single-span bridge.
PennDOT has been using ABC techniques since 2011. To date, it has built or is in the process of planning 100 bridges using ABC.
The perfect ABC project
The 45-foot-wide and 56-foot-long bridge fit the bill for a successful ABC project. The project required a speedy execution, since SR 11 is the main emergency route for Interstate 81, a heavily travelled major corridor connecting to the turnpike. But catering to motorists’ needs wasn’t the only reason Prefabricated Bridge Elements and Systems, one type of ABC, was the best choice for this bridge.
With ABC and precast concrete, PennDOT could close the bridge for a brief two weeks. Using this strategy meant cranes could be placed on the roadway, protecting the wetlands close to the bridge. Property owners on all four corners of the bridge didn’t have to see a crane in their backyards for months and construction workers were safer due to the shortened construction time frame.
“Any other technique, in this particular situation, would have increased the impacts to all of these,” said Eric Gogola, a project engineer for HNTB Harrisburg, an infrastructure solution firm headquartered in Kansas City, Mo. “In addition, precast is cast in a controlled environment so the product is high quality. We know there won’t be any issues with it.”
PennDOT District 8 and HNTB decided to use a spread foundation rather than driving piles and making a deep foundation.
“It’s a smaller bridge so the foundation didn’t need to resist a lot of load,” Gogola said. “And the underlying rock and soil was in good condition, so it was able to support the bridge.”
PennStress fabricated the components for the substructure – two abutment walls, four wing walls and six footings. Northeast Prestressed Products in Cressona, Pa., fabricated the five Northeast Extreme Tee (NEXT) beams for the superstructure.
PennStress used 6,000 psi self-consolidating concrete reinforced with 60 ksi mild steel epoxy-coated rebar to fabricate the substructure elements. The concrete mix was poured into custom wood and steel forms in a “lay-flat” orientation.
The largest footing measured 6-feet-5-inches-wide by 37-feet-9.5-inches-long. It weighed 39 tons. The largest wing wall weighed in at nearly nine tons and measured 1-foot-6-inches-wide by 10-feet-8.25-inches-deep by 8-feet-4.5-inches-long. The two abutment walls measured 1-foot-7.5-inches-wide by 52-feet-1/16-inch-long, and 51 feet 10 9/16 inches long, weighing 42.5 tons and 43.9 tons, respectively.
“We were able to attain that length largely because the depth was only about seven feet, making it still a legal load to ship on the road,” said Russell Dickson, vice president, engineering and production, for PennStress, located in Roaring Spring, Pa.
Dickson noted architects and engineers often struggle with placement of joints on precast bridge pieces and said his biggest piece of advice is this: “Be open to suggested changes from precasters that may help improve the overall structure. It’s a new type of construction, and the best way to do it is not yet completely known.”
The fabrication process presented several challenges, the biggest being the three-piece footers had to be match cast. The center piece was cast first, treated with a release agent, and then each side piece poured flush to it.
“When you pour the other piece adjacent to it, it fills in every nook and cranny, and the contour is a perfect match,” Dickson said.
Another challenge was correctly placing the connections between the walls and footers using a grouted splice coupler system. Two pieces of rebar were spliced together and spaced from 6-to-12 inches apart. One of the large abutment walls used 61 No. 8 couplers and 48 No. 6 couplers. They had to line up within 1/4 inch of each other. PennStress used a plywood template to ensure everything fit together perfectly.
The elements were produced and ready to go within three weeks. But before they left the plant, PennStress dry-fitted the bridge to make sure everything lined up precisely before it got to the job site. Through planning and precision, it did.
As easy as A-B-C
All elements were shipped in the same orientation as they were poured. It took 15 truckloads, with the larger loads requiring planned alternate routes. Two 450-ton cranes, one on each side of the bridge, then set the elements.
The installation process for a precast bridge is as simple as putting Legos together, but there is a learning curve, such as how to angle the large pieces for vertical orientation. Dillsburg, Pa.-based Lobar Site Development, the prime contractor for the project, had never installed a precast bridge before. The biggest challenge for Lobar was learning how to do it efficiently and get everything right the first time.
First, workers set the three-piece footers. Next came the abutment walls, and finally the superstructure. The NEXT Beams were placed and finished with closure pours of extra-high-performance concrete.
Lobar ran split shifts working around the clock and on weekends to meet the deadline.
“It’s a bit hard to imagine something of that size and scope to be completed in eight days,” Daubert said. “But we did it.”
Shari Held is an Indianapolis, Ind.-based freelance writer who has covered the construction industry for more than 10 years.