One World Trade Center’s heroic structure evokes the engineering marvels of an earlier era: the Empire State Building, the Hoover Dam, and the George Washington Bridge, all erected against improbable odds during the Great Depression and willed into being by a determined belief in the future. The tower recalls, too, the legendary story of the spire of the beloved Chrysler Building, which was constructed secretly inside the tower in 1930 in order to take 40 Wall Street by surprise and wrest from it the title of world’s tallest building. It shares that magnificent hiddenness with One World Trade Center, whose groundbreaking structure is also concealed from view.

Realizing a structure on this colossal scale required nimble solutions to complex technical, political, and security considerations. The 104-story structure is a hybrid system consisting of a concrete core wrapped in a muscular steel perimeter frame that was designed to redistribute gravity loads in the event of an explosion or natural catastrophe. While the tower’s multifaceted planes are aerodynamically efficient, they made unique structural demands that necessitated the inclusion of special nodal elements to diffuse the forces of nature.

Although One’s timeless design transcends any one moment, its structural technologies bear witness to the tensions of the post-9/11 era. Greater security demands were made on it than on any other tower. Together, its engineers and designers conceived a new range of high-rise security and life-safety measures that were not available in any building standard at that time. Given the new normal in the United States—heightened security in all places at all times—many if not most of the arcane details of the tower’s extensive safety features will not be disclosed to the public, the majority of whom only want confirmation that it is safe and secure. By every measure and logistical miracle, it is.

Ahmad Rahimian of WSP USA led the structural engineering efforts. His collaboration with SOM, the architects, and Tishman/AECOM, the contractors, was immediate and intimate, as is standard with supertowers, where structural inefficiencies can mean millions in extra costs. Beyond the challenges that accompany the erection of any extremely tall building on a tight urban parcel, the tower’s location and the site itself imposed additional constraints. Libeskind’s master plan had placed it in the northwest corner, the most difficult and vulnerable place on the entire site. Below-grade conditions in that corner were complicated by existing infrastructure, which included the foundation walls in that area, which had sustained the most damage on September 11. Building there meant building over the four curving rail lines that serve the PATH system, which had to stay operational during construction, and next to the traffic-heavy West Side Highway, which increased security concerns. Additional constraints were imposed by the adjacent structures, with which One shares foundations and mechanical systems.

CONCRETE CORE

Because of One’s height and slenderness, WSP devised a reinforced concrete-core structure. By virtue of its strength and stiffness, a concrete core supports gravitational loads—the force of gravity, the weight of the building itself, and that of the occupants—and resists wind and seismic forces. WSP had long been a proponent of concrete-core structures, which are highly efficient. In New York, however, because of union jurisdictional issues, peculiar to the city, which govern the ways that steel and concrete crews work with each other, concrete cores were less frequently used. That changed after 9/11, when so many were trapped inside the Twin Towers, which had exterior load-bearing walls made of steel, with no means of escape. A concrete core protects exit routes, so everyone on the team—engineers, architects, and developers—was determined to construct a core that would establish new industry standards.