By Sue McCraven
Precast concrete beams that are forming the foundation for a Dallas park in the air space above a busy downtown freeway may be structural “magic” but there’s nothing “presto” about the precision engineering involved in this $110 million undertaking. “This is a very meticulous project,” said Duane Milligan, TxDOT construction engineer. “It’s not easy to create ground, especially over a freeway.”
“This is not just your ordinary TxDOT bridge nor a standard DOT precast prestressed box beam,” said Harvey Smith, president of Texas Prestressed Concrete Inc. in Elm Mott, Texas. “It’s a phenomenal engineering design.” Texas Prestressed Concrete’s plant in Waco fabricated the 54-in.-deep (1,370 mm), 52-in.-wide
(1,320 mm) precast, prestressed box beams with lengths from 90 ft to 100 ft (27 m to 30.5 m) to span the eight-lane Woodall Rogers Freeway in downtown Dallas. These precast beams don’t just span the freeway; they will support the very high live and dead loads of a unique urban parkland, a multi-venue community and cultural space currently under construction.
Woodall Rogers Deck Park
Significant structural loads from The Woodall Rogers Deck Park (The Park) vary and include a restaurant, 18-ft-tall (5.5-m) trees, a performance pavilion, gardens, water features and children’s playground – all supported on a series of three precast, prestressed box beam units that are post-tensioned together. “Public attendance at a major event in the performance pavilion, for example, could impose a live load of a couple thousand people,” said Keith Bjerke, program manager and owner’s representative for the project. “Depending on the expected dead and live loadings, about 25 different beam designs were engineered for the deck tunnel.”
Construction of the 1,500-ft-long (757-m) green space began in 2009 and is expected to be complete in 2012. The project will create a 5.2-acre (2.1-hectare) urban green space for cultural and recreational venues. This unprecedented Texas project is funded by an unusual public-private partnership: $20 million from the sale of bonds by the City of Dallas, $20 million from state and federal highway STEP1 funds through TxDOT, and nearly $53.3 million from private donations. Another $16.7 million is from ARRA stimulus funds2 that are specifically for transportation enhancement construction (visit www.theparkdallas.org).
Nighttime construction and exceptional design
At any given time, the number of workmen on the site is “anywhere from 80 to 100 men,” said Shane Harp, TxDOT project manager. Workers erect the beams at night and on weekends to minimize traffic disruptions on the Woodall Rogers Freeway during the day. Construction was held up during the Super Bowl, but otherwise “it’s 24/7, round-the-clock,” said Sharp.
Two large cranes lift the immense precast box
beams in tandem from truck beds to their designated positions in the deck tunnel. Concrete piers constructed outside the walls of the existing freeway support the ends of the box beams. A center concrete wall that runs in the median space between the opposing lanes of traffic will also help carry the beam loading. The heaviest beams top out around 150,000 lb. (68,040 kg) or about 75 tons (68 tonnes). Once the designated lanes of the freeway are closed, erection crews get down to business. “They average around 35 minutes from the time the truck pulls up,” says Harp, “and they hook it up and set the beam and release it from the crane.”
The engineered design of a trench panel between the beams that will support the soil, trees and vegetation is unique (see the sidebar “Extraordinary Design Challenges for Structural Engineer”). The outside beams of each three-unit, post-tensioned beam assembly have a ledge running the width of the overpass to carry precast concrete trench panels (the middle beam has smooth sides) manufactured by Speed Fab-Crete of neighboring city Forth Worth. The trench panels are supported on both longitudinal sides by the precast box beam ledges and serve as structural containers for the park’s soil, trees and vegetation.
To minimize the eventual and extreme loading forces of the park’s earth and irrigation water, engineered lightweight soil alternating with geofoam (a material similar to Styrofoam) blocks will fill the precast concrete trench panels. Every tree that will be planted and its soil loading have been taken into account in engineering the precast beams and trenches. The trench plan is to have deeper depths of engineered soil to accommodate tree locations and only about 12 in. (305 mm) of soil over the geofoam blocks for the rooting layer for grass lawns and park gardens.
Before the trench panels are filled with the lightweight soil and geofoam, the precast concrete tunnel deck will be covered with a waterproof membrane. Traffic beneath the tunnel must be protected from water runoff from the park soils that could create ice on the road surface and unsafe driving conditions (as Super Bowl news coverage showed, Texas can have frigid weather). Precast concrete unistrut panels beneath the box beams provide space and anchoring for the mechanical ductwork for tunnel lighting, fireproofing and electrical runs.
A big structural challenge was to design precast crossover beams that would be short enough (minimum beam depth) to allow the new concrete deck and park surface to be level with the adjacent existing roadways. Designers didn’t want Dallas residents and visitors to have to “step up” to enter the park. Beams also had to be of minimum depth to maintain the TxDOT clearance under the tunnel of 16.6 ft. (5 m) for truck traffic. It is a testament to precast concrete’s design flexibility that engineers were able to craft a box beam that was both strong enough to support extreme loads and geometrically adaptable (minimized beam depth) to accommodate adjacent surface parameters with no noticeable grade change for visitors entering The Park.
Precast’s green potential
Chicago, St. Louis, Cincinnati and Los Angeles have similar precast deck designs that create, essentially, acreage out of thin air. In these congested, highly developed urban areas, real estate is at a premium and development space is scarce. Providing the solution of choice for these urban open space projects is only part of precast concrete’s “magic.” That precast concrete structures can create significant environmental enhancement by providing the foundation for land, trees, grass, trails and the public’s outdoor enjoyment is a truly remarkable engineering achievement. Public amenities aside, new urban parks are green space that measurably offset carbon emissions and decrease noise levels from busy freeways.
Sue McCraven, NPCA technical consultant and Precast Solutions editor, is a civil and environmental engineer.
Sidebar: Extraordinary Design Challenges for Structural Engineer
Mir Hadi Ali, P.E., discusses some of the intricacies about The Park in Dallas.
Determining the design weight of fully grown trees
Ali: The deck park will have diverse amenities and features like large trees, green space, pathways, a children’s park, a dog park, a band shell and a restaurant. The challenge was to estimate the design weight of a fully grown tree and its roots. The deck is designed for all these dead loads and a live load of 250 psf (1,220 kg/m2) in open areas. The tree loads varied from 38,100 lb. to 17,400 lb. (17,280 kg to 7,890 kg).
Separate design for each beam
Ali: 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. The precast, prestressed box beams are made continuous for the superimposed dead and live loads at the middle support.
Deck structural system is one-of-a-kind
Ali: The Dallas Deck Park is not a bridge. The structural system is unique; the loading is nothing close to bridge loading. The deck itself is designed for trees and amenity loads while the deck plaza is designed for live loads. The software used is CONSPAN, which is written for the prestressed concrete bridge beam design.
First use of unique design
Ali: The thing about this project is that it is unique in many ways. It seems like it is hybrid between a bridge and a building, and I am not aware of a similar structural system elsewhere. We had to come up with a structural system to accommodate the tree root bulbs within the structural depth so that the trees can grow at the deck level. All the construction is over a busy highway, which cannot be closed for more than a few hours at a time during the night or on weekends. We have created a tunnel, so all the fire protection requirements of NFPA 502 were applied to this project.
Precast concrete was the only material to meet demanding project requirements
Ali: Based on the project requirements and constrains, we needed a structural system that can deliver the following:
- Carry heavy loads
- Have minimum depth of structure
- Accommodate root bulbs of large trees
- Be constructed over running traffic with minimum interruptions
- Have controlled deflection under heavy loads
- Be durable and maintenance free
- Blend aesthetically with the existing highway structure
- Integrate structurally with the existing highway walls without the need for wall modifications
The only structural system that can meet these requirements is precast, prestressed concrete butted box beams. One of the particular challenges during design was keeping up with the ongoing architectural changes.
Mir Hadi Ali, P.E., is the structural engineer and engineer of record for the Woodall Rodgers Deck Park Project. He works for Jacobs Engineering as the structural principal for the South Central Region and can be reached at email@example.com or (214) 920-8103.