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South Terminal

June 10, 2004

Seismic research shows in Sea-Tac's new look

  • Engineers turn to moment frames for an innovative structural system

    Photos courtesy of the Port of Seattle
    Moment frames fell out of favor after the 1994 Northridge earthquake in California. Recent studies have led to new guidelines for their use in earthquake-prone regions such as Seattle.

    The South Terminal expansion project (STEP) began with a bold idea from the Port of Seattle: present a distinctly Northwest style while defining the state of the art in aviation. In short, transform the look and feel of Sea-Tac Airport.

    The finished project embodies the Port's vision.

    Architect NBBJ designed an office tower with sweeping views of the airfield, a 14-gate concourse with a butterfly roof that invokes the feeling of flight, and a grand glass hall where passengers can meet and greet each other.

    The terminal expansion adds ticket counters and significantly increases bag-handling capacity. It features the first power-walks at the airport, and new retail, food and beverage concessions. The office tower houses the Port of Seattle's aviation division. The gates in the new 2,100-foot-long Concourse A can receive large-capacity airplanes.

    The South Hall

    The South Hall features a 70-foot high glass-enclosed ellipse with a roof approximately 330-feet long by 100-feet wide.

    To develop the architect's vision for the South Hall, over 20 framing schemes were explored. By utilizing 100-foot-long trusses (exposed steel double-channel top chords with double-rod bottom chords) the goal of a clean and simple roof expanse was met.

    Ladder trusses of pipe steel support the glass wall. By coordinating the layout of the roof and ladder trusses, the need for interior columns for the roof was eliminated, creating an open area.

    Moment frames

    Due to seismic activity in the Puget Sound region, STEP needed to meet stringent seismic criteria. To achieve the Port's desire for an open lateral system, KPFF chose to install moment frames.

    The use of moment frames had fallen out of favor with structural engineers due to connection issues that were discovered following the 1994 Northridge earthquake in California.

    KPFF decided to utilize a moment frame after participating in a study to improve the performance of steel moment frames. The study — funded by the Federal Emergency Management Agency and undertaken by a consortium that included the Structural Engineers Association of California, Applied Technology Council and California Universities for Research in Earthquake Engineering — created guidelines for moment-framed structures in earthquake-prone regions such as Seattle.

    The study resulted in a moment frame lateral system capable of meeting the most stringent seismic requirements. Due to KPFF's participation in the study, we were able to take advantage of this innovative lateral system two years ahead of the publication of the formal FEMA documents and deliver an obstruction-free structural frame.

    Juggling interests

    arrivals hall
    The arrivals hall is supported by 100-foot-long trusses that achieve a clean and simple roof expanse. Pipe steel ladder trusses support the glass wall.

    The construction planning of STEP included taking into account a wide array of other ongoing projects: an expansion of the Central Terminal, completion of the parking garage, construction of the Federal Aviation Administration air traffic control tower, restroom renovations, an asbestos abatement, and a major infrastructure improvement package that included seismic, utility, airfield and communication systems upgrades.

    The design team had to be mindful of these other improvement projects and schedules. The Port of Seattle's mandate to maintain 24-hour operations and minimize hassles for the public required close collaboration between the Port, the design consultants and the contractor.

    Prior to construction, the airlines with gates on the existing Concourse A were relocated to the South Satellite without disrupting their operations. Further, the subway tunnel that lies directly beneath the new STEP project and carries passengers to the South Satellite could not be taken out of service.

    Restricted cranes

    Had STEP been constructed at any other site, a regular crane could have been used for construction. However, airport construction requires abiding by FAA regulations. One FAA regulation precluded the use a stationary tower crane — a typical construction site fixture — because it could potentially interfere with radar.

    To prevent interference, STEP designers planned for the use of a “knock-down” crane, movable and able to be quickly lowered. If the crane interfered with radar, the FAA could contact the STEP site to request that the crane's boom be lowered or moved within 10 minutes. This happened at the STEP site, mostly in bad weather, and the crane came down or was moved within five minutes of the FAA's request.

    Challenges met

    It is a marvel to see STEP — the embodiment of bold ideas and challenges met. More than a beautiful new terminal to those of us lucky enough to have been a part of the design and construction, it stands as a monument to the ability of the Port, NBBJ and the rest of the design team to achieve something truly spectacular.

    David Arndt is an associate with KPFF Consulting Engineers and structural project manager for the South Terminal expansion project. Caroline Weiss, KPFF associate, was the structural project designer. Robert Riley was KPFF's project engineer for construction.

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