The expressed steel framing for the grand-scale sculptural transit hall of the WTC’s Transportation Hub, designed by architect-engineer Santiago Calatrava to evoke a dove of peace, already has been simplified to keep it from literally flapping its wings. Yet it still is going to be “as challenging a steel project as it gets,” says Dan Payea, vice president of operations for Skanska Koch, a Carteret, N.J.-based division of Skanska Civil USA. SK won a $221-million contract to fabricate and erect the dove’s skeleton.
The dove is the only part of the hub visible at grade, and it curves, slopes and tapers. It looks bilaterally symmetrical but isn’t. The heaviest member, 200 ft long, will weigh 85 tons, and no two pieces are the same. The only consistent dimension is the 1-ft thickness of the steel skeleton’s “bones”—a giant ribcage with splayed wings. Most of the skeleton remains the same, in concept, as the original (ENR 9/11/06 p. 26). But instead of a pair of piggyback arches, the redesign calls for parallel arches that spring from east and west abutments at grade and span 340 ft along the longitudinal axis of the hall. Instead of the upper arch, a skylight between the arches will open and close. Each wing is made from 55 tapered rafters, 30 to 180 ft long, that cantilever from a tapered rib.
SK and its team now are working out the geometry and detailing connections. Plans call for fabricating the bones into one long piece, consisting of a rib, its arch segment and a rafter. Italian fabricator Cimolai, Pordenone, which has experience with Calatrava structures, faces the huge quantity of welding and welding stainless steel to carbon steel, says Payea. Some 600 pieces will be shipped to the U.S. and trucked to the site at night.
The framing will not be self-supporting during construction. The goal is to minimize erection steel and bracing, but even the ribs will have to be guyed because of their inward slope.