Post-tension 'Trail' bridge one of the first in Iowa
Wednesday, June 20, 2012 8:11 AM
Decorah's new $2.1 million Trout Run Trail Bridge that will connect the 12-mile recreational loop has the state's attention.
Above, a Croell Redi-Mix truck and Protivin Builders pump truck were on hand for the concrete pour of the Trout Run Trail Friday. (Photo courtesy of John Hjelle of Erdman Engineering in Decorah)
"This will be one of the first longitudinally post-tensioned bridges in the state of Iowa," said project engineer John Hjelle of Erdman Engineering in Decorah
"There's a lot of interest in it from the (Iowa) Department of Transportation from the standpoint it's not done on a regular basis. They're watching what we're doing very closely to see if they can replicate it on other projects throughout the state."
"Post-tensioning" of the bridge, located over Highway 9 east of the Highway 52 intersection, is tentatively scheduled for today, Thursday, and Friday.
The bridge's post-tensioning system is composed of six hollow plastic ducts embedded into the concrete deck slab, which was poured last week. High-strength steel wire ropes, called tendons, are run through the hollow ducts.
In the work scheduled for this week, the six tendons are "tensioned" using hydraulic jacks that place the entire deck slab into compression.
"It's an interesting procedure. People from the DOT are going to come out and watch as they do it," said Structural Engineer Stan Stallsmith of WHKS Engineers in Mason City.
The general contractor for the project is Cramer and Associates of Grimes, and the post-tension subcontractor is Freyssinet Inc. of Sterling, Va.
WHKS and Erdman Engineering performed a feasibility study to evaluate various bridge options for the site. After considering several types, the city selected the concrete, post-tensioned frame bridge.
"John really was the one who did all the main leg work. He looked at several different options and asked us to narrow them down, putting costs to different options," Stallsmith said. "I think when it's all done, the city will be happy with it. It's something the city of Decorah can be proud of."
Hjelle said city officials wanted a bridge that wouldn't require extra land.
"We were trying to have a slender bridge so we wouldn't have to purchase excess right of way or extra fill material in order to build a higher bridge. The bridge tensioning system is going to allow it to be located in that particular crossing and to be able to achieve the minimum height clearance the DOT requires over a state highway," Hjelle said.
The combination of the post- tensioning and the high-strength concrete allowed the bridge to be narrow for a concrete structure, according to WHKS.
The bridge consists of three spans for a total of 235 feet. The main span over the highway is 125-feet long. The concrete is poured using "falsework" support made from timber and I-beams. The falsework supports the structure until construction is completed and the bridge can stand on its own.
"It will be pretty exciting to see what it will look like when they start unwrapping the falsework right after the Fourth of July ... it will be like unwrapping a present. Everybody has been anticipating it coming for months ... they'll get a chance to see what it really looks like," Hjelle said.
Decorah's trail bridge was featured in the WHKS February newsletter.
"Unique structural features of the bridge include the thin concrete deck slab with curved arches and the post-tensioning system in the slab. The superstructure and piers are made with higher-than-normal strength concrete," the article said.
"In addition to the unique structure, the bridge also incorporates several key aesthetic features. The railing system is comprised of curved steel posts with tensioned stainless steel cables as the horizontal rails.
"A lighting system featuring high efficiency LED lights will be placed on the structure to illuminate the bridge and portions of the trail ... using a concrete sealer the bridge will be stained a buff limestone color to match the mechanically stabilized earth retaining wall on the south end of the bridge," the WHKS newsletter said.
"It's going to be a very solid, long-lasting bridge. It's going to be a good thing for Decorah," Hjelle said.
The contractor is on schedule to complete the bridge in the next couple of weeks, according to Hjelle.
However, the contract allows construction to continue into mid-August for removal of falsework, shouldering, finish grading, seeding, painting and electrical work, he said.
The bridge could be open to the public by the end of next month, Hjelle said.
Financing for the bridge includes a Vision Iowa grant, a DOT Enhancement grant and local financing.