Although the 13,000 glass panels that cover the tower appear to be identical, the glass varies in thickness and type according to its location. For the glass curtain wall, a different model was used to test wind pressure, some of which is relieved by the tower’s twisting profile. RWDI measured peak wind-induced pressures, concentrating their “instrumentation in areas where we expect the loads to be the highest. Typically, that’s close to the corners, the set-backs, close to any discontinuities in the geometry,” Galsworthy said. Additional testing simulated driving rain with a plane propeller and water, drenching the glass panels in order to evaluate their performance in tornado-like conditions. Ryan Kernan, project manager for Benson Industries, which engineered the curtain wall, enumerated the tests that were performed—wind, rain, thermal, structural, seismic, fire—basically “anything Mother Nature can throw at it.”
All buildings sway when exposed to wind. They are designed to move side-to-side relative to their height. Typically, a building is designed to sway as much as 1/500 of its height under a very strong wind event, which might happen only twice in a century. On a breezy day, the top of Tower One might move in the range of two to three inches (5.1 to 7.6 cm) in any direction, motion too small to be detected by most people. What shapes motion perception is the building’s movement in concert with the body’s own movement, particularly the head, where the organs governing equilibrium reside. Engineers quantify occupant comfort in terms of a milli-g, or one thousandth of gravitational acceleration. RWDI’s engineers measure the same forces fighter pilots experience, albeit in exponentially smaller quantities. When the building is moving, especially when it is changing its motion, it vibrates or accelerates. “People inside the building feel that acceleration as a small force that is pushing them back and forth. We can quantify those forces as a fraction of g. They start to feel motion when that acceleration is in the range of two to five milli-g. A more sensitive person would feel motion at two milli-g and nearly all would feel it at five,” Galsworthy said. Nausea usually doesn’t set in until acceleration reaches between ten and fifteen milli-g.
The “surroundings effect” also factors into aerodynamic design, especially in cities and particularly at the World Trade Center, which is one of very few clusters of supertall buildings in the world. The effect refers to the pressure that arises as wind flows around and above neighboring structures. When I asked Rahimian how the surroundings effect was calculated for towers Two, Three, and Four, which were barely a gleam in Larry Silverstein’s eye when structural planning commenced, he said any major building would “become an obstruction to the wind flow. That obstruction sometimes can be helpful, but it’s very difficult to figure out with hand calculations. Depending what the angle is, what the shape is, what the position is, you may shield the building from some of those pressures, but it could also create a slingshot effect, picking up speed while it goes around the building.” Since they didn’t know when or if the other towers would be built, they commissioned RWDI to test for multiple scenarios that considered One World Trade Center with and without the adjacent buildings and the maximum impact on the structure.
Large volumes of air also move inside a building due to temperature fluctuations: cooler air outside the building drives the warmer air inside upward through the building. While this movement, the “stack effect,” occurs naturally in structures of all sizes, it is magnified in skyscrapers. Stack effect is most commonly experienced as a rush of air into a building’s lobby, typically in winter, when a door is opened or, alternatively, as whistling elevator shafts and doors. While these might be minor nuisances, stack effect causes more significant problems related to energy, safety, and building operations. It can result in uncontrolled airflow into a building, called infiltration, which pushes out warm air and drives up energy costs. It is also capable of producing large pressure differences across doors, leading to slamming or difficulty opening them, a concern especially during an emergency. Finally, in some buildings, stack effect sometimes stalls elevators: elevator doors have safety mechanisms that are triggered by air pressure differences in the shafts, causing the elevators to stop until the doors are reset.
Another aspect of RWDI’s work involved managing the building’s exhaust—from mechanical, kitchen, and diesel generators—and that of its neighbors. Although venting systems remove and discharge air contaminants away from buildings, they can find their way back into the fresh air intakes, thanks to wind currents. RWDI also examined the emergency smoke exhausts at the bottom of the tower, including venting for a train-fire scenario in the PATH tunnel.
Just as water flows around an obstacle, so does wind. “Water and wind follow the same dynamic laws,” Rahimian said. “Obviously, water is fluid and air is a gas, both having their own nuances, but they follow the same laws. When your car breaks down in the middle of the road, what happens? All of the cars go around it, just as the flow of the wind goes around a building. That’s aerodynamic. Aeroelasticity, however, refers to a structure’s innate flexibility. Engineers consider the flexibility of each element individually and in tandem with other elements, and they study how both interact with many different types of loading.”
You carry all the ingredients To turn existence into joy, Mix them, mix Them!
HAFIZ (c. 1320–1389) To Build a Swing, trans. Daniel Ladinsky
CONCRETE
In conjunction with the Port Authority Materials Division and its concrete producers, WSP specified for the superstructure a custom-designed concrete that was durable and extremely strong, ranging from 10,000 to 14,000 pounds per square inch (psi) (68.9 to 96.6 MPa) of compressive strength, the latter the strongest concrete ever used in New York City. For comparison’s sake, concrete of 3,500 to 4,000 psi (24.1 to 27.6 MPa) is used on many buildings up to ten stories in height. As heights rise, demand increases because of additional loads; a 40-story tower might use 6,000-psi (41.4 MPa) concrete, while very tall towers require even greater strength. When WSP engineered the Trump Tower in 1980, for instance, they used 8,500-psi (58.7 MPa) concrete, the first time that strength was used in Manhattan; in 1998, they successfully used 12,000 psi (82.8 MPa) on Trump World Tower, which, until One World Trade Center was designed, was the highest compressive strength ever used in the city.
The strength of the concrete does more than support the structure; it increases safety while using less material. While it would have been possible to achieve the tower’s safety requirements using concrete that wasn’t as strong, doing so would have required a different design and reinforcements. The 14,000-psi concrete permitted a minimal design that created more leasable floor space. After all, Rahimian said, if we built only with the materials that were available two thousand years ago, we would still be building pyramids. The higher the strength of the concrete, the less concrete is needed. Generous with metaphors, he also mentioned the Empire State Building. Although it was designed by brilliant engineers, “with the same material, today we could make another three buildings out of it.”
To illustrate just how strong this concrete is, Rahimian told me that if my big toe was made of 14,000-psi concrete, a five-ton elephant could stand on it without a problem. When I ask why only five tons, Rahimian said, “Those are the biggest elephants I could find.” His quip calls to mind the hidden elephant in Saint-Exupéry’s Little Prince, which isn’t surprising, given Rahimian’s childhood penchant for spending time in the cool darkness of the library that his father, a poet and a novelist, had built in their home in Tehran. “My father had a huge library, just thousands of books, so what did I do in the summers? Yes, I could play football with my friends, but a lot of afternoons it was too hot, so I was in the library, picking up books. Many of my father’s books weren’t for me, not for a ten-year-old kid, but there were a lot of fantastic titles. I started reading them. I read John Steinbeck, Hemingway, Dostoevsky, many others.” He also read books by the great Persian poets Rumi, Hafiz, and Khayyám, with their experiential considerations of the self in relation to the physical world. If architects can be compared to novelists, who tell a sweeping story, then engineers surely are poets, finding beauty in economy.
For an engineer, economy is the sum definition of beauty, which arises from finding the most minimal solution to a given condition, using as few materials as possible to create an optimal structure. Rahimian said, “Anybody could do engineering, if you remove the safety and economic criteria. Pour enough cement in a site, that could do it. It only becomes an engineering problem when you bring safety in light of economy.” One World Trade Center’s beauty is expressed in the economical way it meets all the challenges that were thrown at it. Its engineers had to prioritize and address each of many considerations coherently in order to arrive at a solution that works as a whole. Utter reliability has an invisible beauty about it. 
Ahmad Rahimian, a director of building structures at WSP USA, led the structural engineering efforts. Rahimian is soft-spoken and urbane, and his approachability belies the string of letters—Ph.D., P.E., S.E., F.ASCE—that follows his name. An internationally recognized expert in tall buildings, Rahimian has implemented innovative designs for supertowers and sport facilities. He has played a key role in the structural engineering of numerous landmark skyscrapers in Manhattan and around the globe, including Torre Mayor, the tallest building in Mexico City; Trump World Tower in Manhattan, at one time the world’s tallest residential tower; and the London Shard, the tallest building in Europe. He has been honored with many awards for his pioneering structures and holds multiple U.S. patents for seismic-protective design.
An aerial view shows a PATH train running through the construction site. Because the PATH trains had to remain operational during construction, building strategies were a key component of the design of the below-grade structure. Work often had to be performed late at night and on weekends.
A construction view of the podium, partially clad in glass, reveals the belt truss system that provides the tower’s structural support. A temporary service elevator runs up the center.