Early Solutions

Long before engineers began planning One’s structural design, a vast number in their field had worked at Ground Zero. In the immediate wake of September 11, engineers provided the technical expertise and critical assessments that allowed massive, entwined structural members to be removed in ways that would protect rescue and recovery workers and avoid triggering destructive slides or falls. They included experts from WSP USA, which designed One, Two, Three, and Seven World Trade Center, and Mueser Rutledge Consulting Engineers (MRCE), the geotechnical and foundation experts that designed the below-grade foundations on which One World Trade Center stands.

A second below-grade engineering challenge was rebuilding the PATH train tunnels. However, the first task after 9/11 was stanching the flow of water that was flooding them. “We immediately began laying sandbags at the western end of the Exchange Place station to prevent water from advancing further into the system,” said Michael DePallo, PATH’s former director. “We then instituted a pumping operation to extract water flowing from the tunnels into Exchange Place. It took forty days to pump all the water out.” The flooding was caused initially by the millions of gallons of water pumped from the Hudson River to extinguish the fires at Ground Zero. The tunnels, located at the bottom of the World Trade Center basement, functioned as a natural drain but were clogged by building dust, powder, and debris. The slurry walls had not yet been stabilized, so sixteen-foot-thick (4.9 m) concrete plugs were inserted into the tunnels at Exchange Place to make sure that if they did collapse, the rest of the PATH system wouldn’t be inundated with water. The flooding damaged most of the equipment inside the tunnels, requiring that it be completely rebuilt. By the time the recovery effort was finished, in May 2002, the Port Authority already had begun reconstructing the two PATH train tunnels along with a temporary station to serve New Jersey commuters. At the same time, the Metropolitan Transit Authority had begun the reconstruction of a tunnel to accommodate IRT subway lines 1 and 9.

Architect Nicole Dosso, a technical director at SOM, represented the architects on the day-to-day execution of the project and managed the below-grade portion of the tower, which, at a half-million square feet (46,451.5 m²), is enormous. Dosso is a rarity in her field: young, only thirty-eight when SOM named her to a senior leadership position in 2013, and female in a profession that is still visibly male. After working on Seven World Trade Center, she started on One World Trade Center in 2006, resolving field issues and managing upward of thirty architects working on the site. To oversee literally hundreds of construction documents, she worked three-dimensionally, using Revit software to coordinate the placement of structural steel nodes at the tower’s base and transitions; it was the first large-scale use of this now industry-standard CAD program. Coordinating everyone’s efforts was her biggest and most time-consuming challenge. “There were multiple parties, multiple owners and contractors, and everybody had a stake,” she said. “We were not dealing with a single owner, a single set of architects, and a single set of engineers—there could be fifteen architects to coordinate one opening in a wall.… Reaching ground level was one of the highlights because it took us so long to get to that point.”

THE FORCES OF NATURE

Loads, or forces, that are exerted on a structure are diffused throughout the structure. Axial shortening, or compression, becomes more critical in hybrid structures. The design and construction have to accommodate the differing natures and behaviors of the materials being used, such as steel and concrete, which expand and shrink over time at different rates. Axial shortening studies were performed to anticipate the deformation of the concrete core wall and perimeter steel framing during and after construction. The floors were leveled and positioned at theoretical elevations. Then, as compensation for shortening, the contractor adjusted the elevation of both the perimeter steel columns and the concrete core walls. For the structural steel, for instance, this was achieved both by making the columns longer than the theoretical elevation and shimming them during construction.

Structures are in constant motion, due to winds, shifting foundations, thermal effects from the sun, or crane loads. Often imperceptible, all movement has to be monitored and accommodated, since a small miscalculation can compromise the structure’s many interconnected parts. To make sure One rose straight and true, DCM Erectors relied on a structural monitoring and positioning technology developed by Leica Geosystems and first used at the Burj Khalifa. Using a network of ground controls in conjunction with GPS antennas that were attached to structural steel, the system tracked the vertical position of the tower’s beams and walls to within a few centimeters. This system was used in tandem with inclinometers, devices installed on the core walls that measure tilt variations to the structure’s main axis.

“Every structure interacts with the wind. Imagine a blade of grass, a long reed. Even though it may have an extremely small sail area, it tends to be very compliant in the wind and can deflect a lot.”

JON GALSWORTHY Wind Engineer, Rowan Williams Davies & Irwin (RWDI)

AERODYNAMICS

Wind is a skyscraper’s nemesis. At the top of a very tall building, even on a calm day, winds can gust to speeds greater than 50 mph (80 kph). Taming its force is key to designing the most efficient structure, one that will stand years into the future. To design the most aerodynamic structure possible, Tower One’s architects and engineers had to consider wind action outside and inside the tower. As the tower tapers and turns, it deflects wind forces, redirecting the wind rather than blocking it. Lessening wind resistance translates to greater efficiency, economy, and safety. Architect Lewis said, “Inefficiency in the design of a building of this scale has incredible repercussions.… The building will actually want to pull out of the ground.”

Wind testing, conducted during various stages of the design process and completed before construction, assessed how the tower, along with its spire, glass curtain wall, and mechanical systems, would respond to wind under normal conditions and during natural disasters such as hurricanes. They also measured how the tower met human comfort criteria, predicting people’s experience on different floors and at the sidewalk level.

To make sure One would perform well in every conceivable kind of weather, RWDI built a customized wind tunnel, approximately eight feet (2.4 m) tall, eight feet wide, and 100 feet (30.5 m) long, that re-created in miniature the structures and wind conditions at the site. Blocks inside the tunnel create turbulence the same way a building does.