The issues facing the project team on the $968 million International Arrivals Facility project at Seattle-Tacoma International Airport could hardly have been more extreme.

Topping the list of challenges on the job was a middle-of-the-night installation of a 3 million-pound glass and steel walkway during driving rain while Boeing 747s landed and taxied nearby.

The 780-foot-long walkway, with 85 feet of vertical clearance and a 610-foot-long clear span between footings —​enough room for wide-body aircraft to taxi underneath — was designed to connect the airport's South Satellite building to the new International Arrivals hall.

Moving and hoisting the walkway's 320-foot-long center section into place was a complex undertaking that required months of planning, high-tech tools and coordination among all stakeholders.

In order to optimize safety and minimize the impact to ongoing airport operations, the span was fabricated at a nearby airport cargo area. This approach enabled the team to streamline fabrication efforts, troubleshoot potential challenges and perform sensitive operations, such as the installation of the walkway’s stay cables, in a controlled environment, according to Brad McDermott, project executive at Clark Construction, the project's design-builder. 

When the massive structure was complete, Clark utilized self-propelled modular transport devices (SPMTs) operated by heavy transport consultant Mammoet to move it onto the tarmac. The specially designed hydraulic trailers can move in any direction and feature axles that telescope independently of one another to help evenly distribute weight and keep loads flat while moving over uneven terrain, McDermott told Construction Dive. Mammoet operating engineers controlled the SPMTs via remote, walking directly behind the transport during the move.

Spotters, in a convoy of vehicles in front and back of the transport, as well as walking on foot, also accompanied the center span as it navigated the circuitous 3-mile route from the cargo area down the airport’s center runway to the IAF ​at walking speed. The operator and all members of the move team maintained ongoing radio communication throughout the entire operation, McDermott said.

The painstaking three-and-a-half-hour operation took place just after midnight during periods of heavy rain and as airplanes taxied, landed, and took off on nearby runways. 

“We knew that any major movement to the steel assembly could result in stress to the structure,” he said. “We needed a transport solution for the walkway’s center span that provided a safe, stable ride from beginning to end. The SPMTs helped us achieve that goal.”

Tech takes off

To smooth the way for the transport, the firm leveraged what it says is cutting-edge survey technology to analyze existing conditions on the site, including laser scanning the walkway’s center and end span connections and the airport’s center runway. The scans allowed the team to identify key stake points along the runway that presented heavy cross-slopes or elevation changes. 

During the transport, the team monitored more than 100 strain gauges, which were attached to the center span steel to measure strains on the span such as bending or stretching. The gauges delivered real-time data that the team measured and compared to the threshold limits predicted by structural engineers from KPFF.

"It was essential for the project team to identify the variations in elevation and cross-slopes of the runway, particularly at the taxiways between runways, in order to fully understand how the transport path would impact the center span during its move," McDermott said. "Following extensive analysis of the survey data, the project team elected to stiffen the temporary steel support design even further, making the center span more rigid."

Once the transport was complete, the hoisting operation began. Crews leveraged the walkway’s end spans to support the weight of the lift. Using four strand jacks anchored to each end span connection, they hoisted the walkway to its final position 85 feet above the airport’s taxi lane. The strand jacks provided the steady strength needed to move the massive structure upward and provided greater control over the lift. 

With engineering tolerances between the center and side spans as small as 1 inch, there was no room for error, McDermott said. 

“We knew we had to get this right the first time,” he said. 

Clark's virtual design and construction and field engineering teams relied on laser scanning and animation to help visualize the entire process ahead of time, from fabrication and transport to the lift operation. The laser scans created using Autodesk/Navisworks and Recap Pro provided what the team said ended up being precise as-built data, which it used to verify that the various phases of fabrication remained within the tolerances of the design.

Animations using Bentley Systems' Synchro software detailed the sequence of work ahead of time. This was crucial because the team's goal was to limit the impact to airport operations, so there was very limited laydown area to work within, McDermott said.

"The Synchro animation helped the team establish and test their sequencing logic to ensure they minimized the impact to the airport and airlines," he said. "The animation also served as a tool to communicate their plan to project stakeholders."

Other software used on the project included Leica Cyclone laser scan registration and Strain Gauges micro-measurements from Vishay Precision Group (VSG). Because the IAF project and pedestrian walkway are being constructed at an active airport, drones were not an option, he said. 

World record

Thanks to advance planning, technology and collaboration among all stakeholders, the structure came together seamlessly without the need for further adjustment, McDermott said. Last week, the taxi lane beneath the walkway reopened, enabling aircraft traffic full access below the massive structure.

Now fully supported, the walkway is the world’s longest pedestrian bridge over an active airport taxi lane, eclipsing London’s Gatwick Airport’s Pier 6 Connector by 120 feet. Clark and trade partner The Erection Co. will continue welding operations on the walkway and begin exterior cladding work this spring with the goal of completing the walkway structure and International Arrivals Facility this fall. 

In the end, technology was an important factor in the success of the project but it did not replace the collaborative work of the team, McDermott noted.

“Technology served to heighten planning, quality, and safety, but it was the team's culture of relentless planning that ultimately ensured the precise execution of work,” he said.