Phoenix Sky Train Stage 2 is an extension project to connect the Phoenix Sky Harbor International Airport’s existing elevated train to its consolidated rental car center and new ground transportation center. The goal of the extension was to streamline transportation for passengers and to support growth of the Arizona airport, which is expected to serve 58 million travelers by 2024. It also helped meet the airport’s sustainability goals by reducing the daily vehicle count by 20,000.
The project included construction of more than 2.2 miles of elevated bridge, along with two aircraft taxiway bridges that will carry future airplanes over the below-grade portions of the guideway. Precast concrete was selected as the original design material for all of the bridge superstructures because the designers knew its use was the best way to meet the aggressive delivery schedule required for the 2022 opening.
“Precast concrete was cost effective, and it met the aesthetic goal of matching earlier sections of the project,” says Mark M. Pilwallis, vice president at Gannett Fleming. “Most importantly, the speed at which precast concrete could be constructed was a big factor since we were working in a constrained airport environment where maintaining operations was critical.”
Corbel brackets and drop-in girders
To support the new guideway bridge superstructure, precast concrete U-girders were placed on cast-in-place columns and caps. The taxiway bridges were designed using precast concrete voided rectangular box girders. A facilities access road bridge was also constructed using precast concrete voided slab girders.
The U-girders maintain the overall aesthetic of the guideway structure while meeting all of the structural and serviceability requirements, Pilwallis explains. The precast concrete producer modified its forms to accommodate a deeper U-girder section, allowing for continuous spans up to 197 ft 8 in., and to maintain the standard web slope with filler forms to increase the versatility of the forming system.
Erecting the post-tensioned U-girder was the biggest obstacle the project team faced, as the unit spanned over an active airport terminal building during construction. “The challenging site constraints and the operational need to have small- to medium-size aircrafts taxi below our guideway led us to use longer-than-typical bridge spans for precast concrete,” Pilwallis says. “We used spliced precast concrete girders to extend the spans and fit the site.”
The tight site conditions did not allow the team to use temporary falsework supports in the span adjacent to the terminal building, so the precast specialty engineer designed embedded corbel brackets to support the drop-in girders from the adjacent pier girders. The corbels allowed for vertical adjustment with shim plates and a simple field connection of two bolts per corbel. With this accelerated construction method, the girders could be erected in the four-hour time slots allotted for the work.
Given the splice location and pier geometry, several of the post-tensioned pier girders did not have a large enough factor of safety against overturning during intermediate stages of construction prior to casting splices and applying the post-tensioning. To ensure the girders remained stable and to maintain worker safety, the precast concrete producer and precast specialty engineer worked together to design and cast temporary ballast blocks to sit inside of the U-girders during the intermediate construction stages. The girders were erected with the ballast in place to ensure stability when the cranes were disengaged from the girders. After the cast-in-place splices were cast and the girder stability was ensured, the ballast blocks were removed.
To further accelerate delivery, the precast concrete producer installed electrical conduits on the exterior girder webs at the precast concrete plant before girders were shipped. This allowed for easier access with the girders at ground level and saved the contractor weeks of field installation. It also created a safer work environment by eliminating the need to install thousands of feet of conduit, which would have required laborers tied off in man lifts to work in an active traffic zone at an airport.
“The project is the latest segment of a multiphase transit system that creates significantly more landslide capacity at the airport, while reducing congestion and greenhouse gases,” Pilwallis says. The train is scheduled to open to the public in early 2022.