When Orange County, Fla., needed a new interchange to relieve traffic congestion and provide fully directional access between two state expressways, they turned to the Florida Department of Transportation (FDOT) for help. FDOT agreed to provide a Transportation Infrastructure Finance and Innovation Act (TIFIA) loan, which is designed to give credit assistance for qualified projects of regional significance.
The loan would help Orange Country deliver the Central Florida Expressway (CFX) with savings of approximately $260 million through bond interest payments. However, the project team had to meet the completion deadline to get the money.
It was one of many challenges the project faced, says Susan Gratch, vice president of transportation and civil practice engineer for Atkins North America. Her team used a precast concrete solution to mitigate many of the risks they faced. “Precast concrete provided cost-effectiveness, shorter fabrication delivery time frames, reduced maintenance, and enhanced durability,” she says.
The design team had considered a steel alternative; however, the reduced long-term life-cycle costs and durability benefits led the team to move forward with precast concrete.
A first time for U beams
The project required construction of eight precast concrete girder bridges, and the engineers soon decided to use haunched, curved, post-tensioned concrete U-beams for the design. It would be the first use of these haunched U-beams in Florida. The decision provided the designers with a structurally efficient solution that delivered a pleasing aesthetic appeal within the allotted budget. “Precast concrete offered the ability to deliver the haunched feature on the girders and the greater spans through the use of post-tensioning,” Gratch says.
The precast concrete U-beams were post-tensioned to extend the spans up to 150 ft, minimizing the number of piers in this environmentally-sensitive corridor. The use of a post-tensioned pier for the mainline crossing of Ramp K saved 12 ft in bridge height compared to conventional expansion piers. It also allowed lower column heights and walls, and reduced the footprint of the project.
The design also accommodated future widening requirements by using a single precast concrete U-beam to provide an additional lane on each side.
During design, the engineer worked closely with local precast concrete manufacturer to develop standard details for repetition that mimic colors, textures, and shapes from the natural environment. The designers also worked with the precast concrete producer to optimize the forming, shipping, and handling of the curved and haunched concrete beam segments, as the units required custom formwork and setup. “Working closely with a local and well-respected precaster was key to meeting all of the owner’s goals,” Gratch says.
Once forms were established, the box width and web slope were made consistent to increase repetition of details and speed up the precasting processes. “This interchange design represents a holistic design approach encompassing form, function, safety, aesthetics, and maintenance,” she says. “It demonstrates to the public that transportation infrastructure can be beautiful.”