One World Trade Center Structure A product of both historic Viñoly’s recently topped-out 432 tions. At the lower levels, multi- loss and future goals for Park Avenue, also in Manhattan— floor “super diagonals” encircle Lower Manhattan, the which, at 1,396 feet, surpasses the building like a belt. At the One WTC’s 1,368-foot-high uppermost floors by the mechani- building’s structural steel roofline—can fit inside of it. cal levels, outriggers connect the frame maximizes safety, Notably, the “tube” design perimeter columns to the core. construction efficiency, of the original Twin Towers took The structural system at the pe- and tenant experience. much of the structural duty away rimeter allows for load shedding, from the buildings’ center and or the transfer of loads to other Editor’s note: Because of security placed it with the exterior walls. In locations. “On a theoretical level, requirements for One World Trade One WTC’s hybrid structure, both the moment frame creates a rope Center and surrounding proper- are equally important. “Having a out of all of the perimeter beams ties, certain information on the robust core has a lot of value,” so that if the thing that is sup- grade, size, and type of structural says Ahmad Rahimian, Director porting that rope goes away—two steel has been omitted. of Building Structures at WSP columns, for example—that rope Group, who worked with the goes into tension and pulls the SIMPLY PUT, ONE WORLD TRADE architects at SOM on both One forces back to the core, which is Center’s structure consists of a WTC and the previously com- where they want to go,” explains concrete core and a steel perim- pleted Seven WTC, in addition to SOM’s Kenneth Lewis. eter moment frame. But there is several other structures at Ground At One WTC, life safety is nothing simple about the design Zero. “The core brings stiff- paramount. “The engineering of this 1,776-foot tall crystalline ness and strength, and securely of the original Twin Towers was icon—the most significant of the houses all the mechanical distri- brilliant,” Lewis says. “It created new construction at the 16-acre bution, and especially all means robust buildings against forces of World Trade Center site both from of egress—the inadequacies of wind and earthquake that were a symbolic point of view, and in the Twin Towers’ stairs were a incredibly light. However, it did terms of sheer size. problem.” not anticipate what happened to At its base, the structure’s The building’s perimeter them—I don’t think anyone could massive 110-by-110-foot con- features a framing system that have.” To provide optimum egress crete core, whose walls are as uses approximately 45,000 tons and firefighting capacity, the new much as 4 feet thick, is bigger of high-strength structural steel, building features extra-wide pres- than most towers being built used along with the prequalified surized stairs, concrete protection today. By comparison, Rafael ductile beam-column connec- for all sprinklers and emergency 26 Metals in Construction Spring 2015 One World Trade Center Structure 27 Left The tower under construction in 2011 amid massive infrastructure improvements adjacent to the site. Facing top An aerial view of One WTC and the National September 11 Memorial site. Facing bottom From left: typical low-, mid-, and high-rise floor plans illustrate the tower’s tapering form. risers, interconnected redun- dant exits, additional stair exit locations at all adjacent streets, and direct exits to the street from tower stairs. To safeguard the building against the threat of vehicle-delivered explosives, the lower portion of the fortified base features 28-inch-thick concrete walls that shelter the area of the 60-foot-high lobby, though this is an added feature and does not contribute to the tower’s struc- tural integrity. “We really pushed the con- crete system to produce a 14,000 psi concrete—that was the first time it was done in New York City,” says Rahimian. “One thing we had with this project that was nice is that the Port Authority has its own material testing lab. We worked for months to streamline the mixes.” As the over three million- square-foot building tapers, some of the cells within the core drop off, and its walls decrease in thickness to a minimum of two feet. The narrowing of the building geometry not only accommo- dates the project’s gross area requirement, but also creates an aerodynamic shape that reduces the wind effect on the tower. “There’s a practical reason why the top of the building is smaller 0 7 R O O L F F S R 3 6 7 , 0 4 than the bottom, not just the M Q S 7 8 7 , 3 : Y E K usual notions of load,” explains E L B A L I A AV L A C I N HA C E M Lewis. “In the old World Trade S R O T A V E L E Center, you’d get to the higher floors and there were these 70- to 90-foot spans from the core, which was relatively small, to the perimeter, leaving large areas with T no daylight. You want the floor E area to be smaller so people are Taken together, the perimeter closer to the glass.” and core systems make The floor system within the concrete core is a formed cast- One World Trade Center safer in-place concrete beam and flat slab system, while the floor area than either system could make outside the core is concrete on ONEWTC.COM EMAIL BROKER S N A L P R O O L F E V I T A C I D N I composite metal deck supported it on its own. on steel beams and connected via shear connectors. That column-free floor system spans Center Trade This page and opening page: James Ewing/OTTO; facing top: Iwan Baan; bottom: One World 28 Metals in Construction Spring 2015 One World Trade Center Structure 29 Left Concrete walls more than 2 feet Rahimian, that mat is strong order to achieve a higher bearing ONE WORLD TRADE CENTER STRUCTURE thick shelter the lobby; slots allow day- enough to be the foundation for a capacity of up to 114 tons per light into the space. 40-story building. square foot. Rock anchors/tie Location: 1 World Trade Center, New York, NY Above Most members of the tower’s de- sign and construction team also worked The tower structure extends downs extending 80 feet into the Architect: Skidmore, Owings & Merrill LLP, New York, NY together on 7 World Trade center, a 70 feet below grade passing rock were installed to resist the Developer: 1 World Trade Center LLC (a wholly owned corporation by the Port project that allowed them to test many through four subterranean levels overturning effect from extreme Authority of New York and New Jersey and the Durst Organization), New York, NY of the safety strategies they would later where some of its structural wind events. Structural Engineer: WSP Cantor Seinuk, New York, NY use on One WTC. components required reposition- The symbolic significance Mechanical Engineer: Jaros Baum & Bolles, New York, NY ing to clear the PATH com- of One WTC, rising to a record- Spire, Communications Rings, Cable Net Wall Engineer: Schlaich, Bergermann und muter train tracks that cross the breaking height from the ashes Partner Gmbh, New York, NY between the core and the perim- building at the lowest basement of the worst attack on American Protective Design Engineer: Weidlinger Associates, New York, NY eter—with a maximum span of 47 level. The foundation and below- soil, forced designers to create Construction Manager: Tishman Construction Corporation (An AECOM Company), feet—for construction efficiency grade structure are founded on a heavily reinforced tower with New York, NY and flexibility of tenant use. Manhattan bedrock using spread structural and life safety redun- Curtain Wall Commissioning Consultant: Israel Berger and Associates, New York, NY At the building’s apex, a and strip footings with bearing dancies that exceed existing Structural Steel Erector: DCM Erectors Inc., New York, NY 441-foot-tall spire—comprising capacities of 60 tons per square codes to withstand a potential Architectural and Ornamental Metal Fabricator: Airflex Corp., Inc., Farmingdale, NY three circular communication foot or better. Space constraints similar attack while preserving Architectural and Ornamental Metal Erector: Tower Installation, New York, NY platform rings and a needle- due to the proximity of exist- life and the building itself. Taken Curtain Wall Fabricator and Erector: Benson Industries, Inc., New York, NY like mast—is mounted atop a ing train lines—which remained together, the perimeter and core Metal Deck Erector: DCM Erectors Inc., New York, NY reinforced concrete mat that is operational throughout construc- systems make One WTC safer directly supported by the tower’s tion—required excavating deeper than either system could make it concrete core. According to James Ewing This spread: into the rock at select locations in on its own. 30 Metals in Construction Spring 2015 One World Trade Center Structure 31.