Falsework is coming down this month after supporting the construction of what officials believe is the only transit bridge in the world to cross over an active taxiway. The 740-ft-long cast-in-place box-girder bridge is the centerpiece of a two-mile-long transit system that will connect two of Phoenix Sky Harbor International Airport’s terminals with off-airport transportation facilities.

Photo: Courtesy of Austin Bridge & Road

Officials think mass transit guideway at Phoenix airport may be the only one of its kind to cross through active airspace.

The Phoenix office of Irving, Texas-based Austin Bridge & Road holds an approximately $6-million subcontract to build the three-span bridge as part of an overall $318-million construction manager-at-risk contract held by Hensel Phelps Construction Co., located in Greeley, Colo.

The airport’s linear nature required the system to cross a 2,000-ft-long taxiway, says Jay DeWitt, airport special projects administrator. The Federal Aviation Administration required a minimal underclearance of 75 ft, for taxiing planes, and a maximum bridge height of 115 ft, to avoid obstructing air traffic control tower views, says David Burrows, structural engineer with Gannett Fleming, Camp Hill, Pa., the designer.

Wedged between a busy access roadway and a terminal for Southwest Airlines, the bridge needed a design that could be built quickly with a minimal footprint and require little maintenance yet look nice, says Mark Pilwallis, Gannett Fleming project manager. Designers rejected a transit tunnel because of poor soil conditions and the proximity of airport facilities. They also scrapped the idea of a steel bridge. “We worried about the need for detailed inspection of welds,” he says, noting the location over an active taxiway. “With a post-tensioned box-girder bridge, once it’s in place, you have no bearings on the main piers, just the joints at the ends. And concrete is very durable out here in the desert.”

The team also considered precast segmental construction, but the CM-at-risk proposed saving $1 million by shutting down the taxiway for six months. “Since the savings were substantial relative to the little additonal time needed, we agreed to the change as long as we could keep the closure under six months,” says DeWitt.

Gannett Fleming’s original design called for the two 200-ft-long end spans to be built on falsework and post-tensioned; the falsework would be moved to build the 340-ft main span over the taxiway. Austin proposed building the entire bridge on falsework all at once. “We felt that it would be easier in terms of economies of scale,” says Tim Muller, Austin regional manager. “We could reduce some concrete quantities. … When you build the ends first, some temporary concrete sections would have had to be built.”

Working mostly at night within limited allowed periods of time, crews built 21 temporary bents as heavy as 100,000 lbs, towed them to the site by dollies, then erected them with the help of sand jacks, which are basically bags of sand that can be emptied or filled to adjust for height. The falsework then supported the box girders, which have depths of up to 17.5 in. The columns are supported on 8-ft-dia drilled shafts as deep as 90 ft, topped by 10-ft-thick pier caps.

The taxiway reopened Oct. 11. Work continues on three stations and the rest of the two-mile guideway. “We go from a simple at-grade section alongside a street … across a parking lot, make a hairpin turn between two garages, hit the taxiway, then go on the airfield past gates and concourses,” says Pilwallis.

Steel, precast and cast-in-place are used at various points along the route, set to open in 2013. A three-mile extension to the third terminal and a rental car facility, scheduled to open in 2020, could be accelerated if Congress passes a pending measure to increase passenger facility charges to $7 from $4.50.