By Sue McCraven
It’s not easy to create ground, especially over a freeway,” explains Duane Milligan, TxDOT construction engineer for a 5.2-acre urban park that is part of a green initiative for downtown Dallas. Drivers rushing along the eight-lane Woodall Rodgers Freeway can see the increasing number of precast concrete beams spanning the airspace above them. A park is being born where there was no land.
“A phenomenal engineering design”
Called a “very meticulous project” and a “hybrid between a bridge and a building” by project engineers, the two Texas precasters for the Woodall Rodgers Deck Park also found the beam specifications for spanning the freeway to be out of the ordinary. Texas Precast Concrete Inc. in Waco and Speed Fab-Crete of Fort Worth produced the precast concrete components for the project. “This is not just your ordinary TxDOT bridge,” says Harvey Smith of Texas Precast, “nor a standard DOT precast, prestressed box beam.”
The Waco plant fabricated the 54-in. by 52-in.-wide precast, prestressed box beams with lengths ranging from 100 ft to 210 ft. Minimum beam depth was required by the design so that Dallas residents and visitors would not have to “step up” when entering the Park. Precast concrete delivered the durability and maintenance-free structure that the DOT required for this one-of-a-kind construction. The heaviest beams top out around 150,000 lb and are erected by two large cranes.
Support for the concentrated live loads of trees (38,100 to 17,400 lb) relied on the strength of odd-shaped precast trench panels. “Basically they are huge planter boxes,” said Carl Hall of Speed Fab-Crete, the manufacturer of the panels. “The trenches themselves were 10 ft long, and the width varied from 8 ft 10 in. down to 4 ft 10 in. – all designed out of 5,000 psi concrete. The trenches will butt end-to-end, and then they’ll be filled with dirt.” Soil for the trenches is mixed with geofoam to lighten the loads.
The outside box beams of each three-unit, post-tensioned deck assembly support the trench panels on ledges that run the width of the overpass. Weight of the tree bulbs and water for irrigation are concentrated loads that made the precast design extremely complicated.
There was no precedent for this new precast system. “It’s so different,” said Hall. “You sit down and go, ‘Who thought this up? Is it really going to work? Did they take account for this or that?’ And yes they have! They thought about how much that tree is going to weigh when it’s mature, how much water it’s going to take and the weight of the water, and people out there. It’s very amazing.”
Precast is the only solution
Ali Hadi, P.E., structural engineer and engineer of record for the Park, said that, “because of the random location of heavy concentrated loads such as trees, column loads from the band shell and the restaurant, and the varying span lengths, each beam had to be designed separately.”
Precast, prestressed box and precast trench beams were the only materials able to meet the demanding project specifications, including:
- Strength to carry heavy live and dead loads
- Minimum beam depth
- Rapid erection over a busy freeway
- Durability, sustainability and maintenance-free design
- Ability to blend aesthetically with existing highway structure
- Integration with existing highway walls without modification
- Controlled deflection under heavy loads
- Design flexibility to accommodate large tree bulbs
Measurable green benefits
New construction projects across the county often boast of the “green” advantages they afford. But in reality, it is often difficult to accurately measure these intangible benefits in quantifiable terms. This is not the case for the 1,500-ft-long Woodall Rodgers Deck Park. This precast-supported Park is defined by:
- 5.2 acres of urban parkland
- 40,000 sq ft of green grass
- More than 300 trees and more than 4,000 plants and shrubs
- Carbon emission offsets from new landscaped acreage (reduction of 7 tons of CO2 annually)
- Water conservation due to an underground, high-efficiency irrigation system
For the time being, smelly exhaust fumes, noisy traffic and dismal acreage-eating freeways remain a “necessary evil” for high-speed travel through big cities. But thanks to the design flexibility and strength of precast concrete systems, urban residents finally have a viable option for creating beautiful new parks without impeding freeway traffic and, even more remarkably, without existing land.
Sidebar: Precasters Discuss Production for the Park in Dallas
Texas precasters Carl Hall of Speed Fab-Crete, Fort Worth, and Harvey Smith of Texas Precast Concrete Inc., Waco, share their views on the project.
Q. What was most unusual about this project from a precaster’s viewpoint?
Hall: Probably the most unusual was the amount of reinforcing that was in each piece. A standard piece had more than 1,400 lbs of rebar in it. I think there was more than a million pounds of rebar in this project. For the fabrication and the tying of the rebar, TxDOT was very particular about getting everything just right. There was just so much of it, we couldn’t do it ourselves. We went through quite a process of finding somebody that could do this and do it right.
Smith: Our concrete was a special design developed for the requirements of this job. We needed a mix design that would remain plastic for about 1½ hours and would achieve 6,500 psi for release of tension and 9,000 psi for 28-day strength. The casting process began with pouring concrete to the bottom layer of reinforcing steel and 0.6-in. prestressed strand. At this time the Styrofoam voids were placed and the top mat of steel was secured in place. Then the remainder of the concrete was placed.
Q. What did your production team find most challenging about fabricating the precast pieces?
Hall: Because of the design, there were areas in the mold that trapped air when we poured it. No matter what we did with vibration, we couldn’t get that trapped air out, and we were getting some air voids and bug holes in the finished piece. What we ended up doing was drilling a lot of holes in the mold and letting the air bleed through those holes – and that actually solved the problem. It kind of pained me to spend all that money on those molds and then start drilling holes in them. But it did work.
Smith: The most difficult of the beams were the dapped1 ends with the ledge. The reinforcing steel at the end was so tight that special consideration was needed to ensure proper consolidation of the concrete mix.
Q. How did you handle transportation of the products?
Hall: Actually, they stored easily because they were stackable, and they loaded easily. The only thing about deliveries is they had to take place at night and on weekends. They’ve been under a microscope through this whole thing trying to keep traffic flowing through there.
Smith: No special consideration was needed, because we already had the necessary equipment (and trucking contractor) to handle the beam weight and length. The logistics of moving these loads into the downtown area as required by Archer-Western Contractors were very challenging. The only time given for transport work was 12:01 a.m. Sunday until 12:00 p.m., and arranging for enough trucks to meet expectations was very difficult. Traffic had to be completely shut down on Spur 366 during this 12-hour time frame to erect the structure.
Q. Did you have any similar prior experience that helped prepare you for this DOT project?
Hall: We have a good relationship with TxDOT, and that has helped us on this project. (We have a) bridge division, and we do a lot of MSE walls. We’re used to them (TxDOT) being around.
Smith: We have had a long-term relationship with TxDOT, and they were very familiar with our operation. DOT inspectors in our plant were very precise about (compliance with) all phases of production.
1 Dapped beam end: the steel plate that transmits the load beneath a beam end; the plate is recessed so that the anchorage will sit at the same level as a beam base without any reinforcing.