SeaTac IAF Pedestrian Walkway

Seattle-Tacoma International Airport (SeaTac) dove into a rarely used method for passenger cross-taxiway transit and set the standard for it.

Most airports move passengers from terminal to terminal with an underground walkway, a subway tram, or an elevated tram that hugs the terminal buildings. Port of Seattle, which owns and operates the airport, chose the most direct but difficult concept: an overhead bridge crossing an active taxiway.

Only two other airports in the world have previously built a bridge over a tarmac. Seattle’s new bridge outdid both. Its 780-ft walkway is the world’s largest structure over an active taxiway. Its 610-ft span also represents the longest clear-span structure at an airport. If stood vertically, it would be the second highest structure in Seattle, 150 ft taller than the Space Needle. It’s a direct and efficient route for international passengers to reach the International Arrivals Facility, which houses customs. Stunning views of the Pacific Northwest and Mt. Rainier greet travelers as they ascend its escalators and walk across it.

The bridge took eight years to plan, design, and construct. Steel was the material of choice due to the clear span length, seismic criteria, and need for innovative design and construction methods. The walkway contains 3,000 tons of steel, while the cores use another 160. It has 800 tons of rebar and a 2,200-ft cable length. Its 191-ft escalators are among the 10 longest in the United States. Steel V-Piers support the pedestrian bridge at either side and allow the bridge to have an 85-ft clearance from the bottom deck to the tarmac, enough room to fit a Boeing 747.

The aerial walkway was designed as a cable-stayed bridge and built with the Accelerated Bridge Construction (ABC) method. Unique design and geometry created complex, heavy weld joints.

Erecting a 780-ft walkway and a 320-ft, 1,565-ton center span on site would have meant an untenable closure length for a portion of a taxiway and several gates. Instead, the walkway was built in one of the airport’s cargo areas in 17 major prefabricated components, including the center span. The ABC method minimized the project’s impact on airport operations and allowed for simultaneous construction of various walkway components. The V-Piers were erected and welded on site while the taxiway remained open.

Once completed, the center span moved three miles on four remote-controlled self-propelled modular transporters (SPMTs). The SPMTs operated at walking speed down a closed center runway in the early morning hours when airplane traffic is at its lowest. The three-and-a-half-hour move took months of planning between general contractor Clark Construction, airport operations, and the Federal Aviation Administration.

Upon arrival, the center span was lifted into place and connected to the pre-erected structural V-piers. That connection clicked without issue after several hours of site checks to ensure the pieces lined up. Structural engineer KPFF adjusted the side spans to account for the center span’s weight and projected a 7¾-in. deflection upon final placement. A fast and efficient installation and welding of the center section closed the taxiway for only a week.

Project Team

• Owner: Port of Seattle, SeaTac, Wash.

• General contractor: Clark Construction, Seattle

• Structural engineer: KPFF, Seattle

• Steel fabricators: 

  • Thompson Metal Fab, Vancouver, Wash. *AISC full member; AISC-Certified fabricator*

  • Jesse Engineering, Tacoma, Wash. *AISC full member; AISC-Certified fabricator*

  • Transco Industries, Portland, Ore. *AISC full member; AISC-Certified fabricator*

  • Greenberry Fabrication, Vancouver, Wash. *AISC full member; AISC-Certified fabricator*