The long-span roof hovers about 30 feet above the floor of the departure hall. It is a folded plate made from multiple trusses oriented in an inclined position. Trusses share either a top or bottom chord. The horizontal structural lid rests on 22 treelike columns spaced 100 feet on center. While the geometries of the column grid and building shell relate to existing site-plan features, the primary axis of the roof trusses was rotated about 45-degrees in deference to the sun. Sloped, north-facing roof surfaces are sheathed with glazed panels to welcome abundant diffused daylight into the hall below. The roof cantilevers 50 feet over the roadway elevation to provide some weather protection along this edge. The plate tilts up about 5 degrees at this point so that it will appear flat to the eye-a technique that Helmut Jahn learned years ago when working on the McCormick Center in Chicago, says Cook.

The base of each support structure consists of four vertical tubes positioned in a square formation and braced horizontally near their tops by another set of tubes. Each side of the square measures about 8 feet across. All vertical tubes terminate about 8.5 feet above the departure hall floor on Level 5. Those along the roadway facade, however, start at a lower level, so their vertical components rise to 43 feet.

More slender, tapered branches extend diagonally from the tops of the vertical elements to the roof above, at which point they engage the lower chords of the trusses with 2-inch-thick steel plates and pin connections. The greatest distance between the uppermost ends of a structural canopy emanating from a single support column is 105 feet. The designers specified seamless tubes of cast steel for both roof and column members to achieve a structural system that is both highly efficient and visually elegant.

Fresh air is supplied, and return air pulled in, through ducts that are integrated within the base of these open-framed supports, and additional fan-coil units are installed at the building perimeter. Only the pedestrian level of the expansive departure hall is conditioned, assuring passenger comfort while minimizing energy consumption. Rather than installing communication and lighting systems from the ceiling far above, the architects also relied on the column assemblies to serve as platforms for such technologies. This took a lot of coordination among the various disciplines, recalls Cook, but was necessary to ensure that the equipment would be in easy reach for ongoing maintenance and repairs.

According to Cook, one of the most challenging aspects of this roof design was its lack of expansion joints. Normally, a span of this magnitude would have such details to accommodate the movement caused by inevitable changes in temperature. But the architects wanted the top covering of this terminal to appear as a continuous plane-an expression that could not be achieved if it had to be interrupted periodically by joints. Instead, explains Raymond Crane, principal of Arup's Boston office, the engineers used computer modeling to analyze the forces that would be exerted due to thermal expansion and treated them as if they were yet another type of load that had to be handled by the structural design.