Page 66 - Colorado Construction & Design Magazine - Spring 2023

“The girders cure overnight as far as release strength.
We get 28-day strength in about a day. But for this project, the engineer of record has dictated that we need to wait 30 days before posttensioning to mitigate creep and shrinkage.” Werner says.
“After we had our initial conference call earlier this year, we did our shops, then we had to wait for the post tensioner to get his shops to us to incorporate what they needed in our girders,” Kovac recalls. “When we're talking with Kiewit,
and post tensioner, is when we found out about the 30-day rule. We knew at that point the schedule would be impacted. Ultimately, I estimated to pour these girders about one every third day, because they're just labor intensive. But we had to move that to every other day to meet a tight schedule.”
While the actual setup for the girders isn’t all that difficult, it’s the actual pouring that makes these girders labor intensive.
“One challenge comes from the fact that they have three-to- four-inch ducts running through the bottom web, so when we pour it, we pour the bottom 10-inch slab and then we set the formwork to form the U. We set those after we pour that 10- inch bottom and then we fill the webs up after that. There are a couple challenges with this. The mud on the bottoms needs to retard so it can mix with the mud that's going into the webs and not have a cold joint.”
In addition, there is a lot going on in these forms and things get a little crammed. The casting crew had to deal with confined space.
“We only have 5-inch webs that we're pouring concrete into, and it's about 170 pounds per foot of reinforcing, so they're very congested,” Kovac continues. “We’ve got to get mud in there and get it to flow. Those were those were probably the biggest challenges.”
Learning Curve
The plan was to start setting girders in mid-May, and Plum Creek started pouring in early March. Unpredictable and up and down spring Colorado weather created some challenges, but the team kept moving forward. And for these beams, there was certainly a learning curve, as well.
“The last time we had poured girders like this was in 2009,” Kovac says. “I think we have two guys with us who were around for that. So, the first four or five pours were difficult. The form itself is just basically two sheets of steel and there’s bottom iron work, and in that you build a floor, the soffit. So everything is built from the ground up. Helser Industries was our form manufacturer and they did an excellent job.”
It was a team effort to get to the best setup for these girders. At Plum Creek, operations manager Ken Smith and plant
66 | Colorado Construction & Design
manager Salvador Mendoza have been integral in bringing this project to life. The precaster also had lots of help from longtime partners in the industry.
“Our cement was supplied by GCC, and BASF helped us with the mix design,” Werner says. “They assisted us in getting
it more floatable without adding water. The cool thing you can’t see about the form is that the metal is on the one-to- four slope. The metal bends for the curve. You can imagine it bending in the weak dimension, but this actually bends in the strong direction. The top and the bottom of the side form stays level. There are two plates. You can see there’s a plate in the middle so they can bend it in the strong direction. That’s why
it takes a lot of force to do this.”
Moving On Up
Casting is its own challenge, but once the girders are complete, it’s also no small task to move them into place. Transportation of the heavy girders, each weighing roughly 210,000 pounds, was also a challenge. Only three girders per day can be transported due to the weight and the number of axles
needed. APEX Transportation, an expert in moving heavy precast parts, was brought on to make it happen.
“We have worked with Plum Creek for 23 years and have a good working relationship,” says Walt Schattinger, president of APEX Transportation. “The weight of a tub curved girder with offsetting on trailer for center of gravity, cantilever, and height for getting through the Eisenhower Tunnel and two mountain passes on I-70, spread out over fifteen loads make this a very difficult job. The way we deal with these challenges is through good communication between Plum Creek, Kiewit, CDOT and the whole engineering and design team."
Adjustments also had to be made to the girders themselves to allow for their transportation and placement.
“Normally lifting eyes are at quarter points on the girder, but because they're precast they're roughly 30 feet from each
end, which is where the bearing is now,” Kovac says. “So, once they're posttensioned, the bearings move out to the very edges of the girder. It’s unique from an engineering perspective.”
In the face of any engineering and logistical obstacles, the team is working together to ensure the successful completion of the project by October, when Kiewit aims to be off the mountain. The ingenuity of the design and construction team, combined with the durability, strength and flexibility of the precast components look to add up to a bridge that will serve residents and visitors for many years to come.
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