It can take the blast from a truck bomb, or the fiery impact of a jet airplane — and yet, at 1,776 feet in height, the supertower rises higher than the terror-hit twin towers that once stood on the site.
Safety first, then symbolism: That's the message conveyed by New York's One World Trade Center.
Twelve years after the attacks that made Sept. 11 a date that will live in infamy, on a par with Pearl Harbor, the construction of the $3.8 billion building is providing the focus for a flurry of books, articles and TV shows. All that attention is justified, due to the skyscraper's high profile on the New York skyline, its huge price tag, and its symbolism for the past and the future.
Each milestone in the building's rise, leading up to its official opening next year, serves as an opportunity to reflect on 9/11 — but not just on the day's death and destruction.
"This building is a strong, visible image of our resilience, coming back after this terrible tragedy," architect David Childs says in a "Nova" documentary airing Wednesday on PBS, titled "Ground Zero Supertower." Looking beyond the Ground Zero symbolism, One World Trade Center serves as the template for a new generation of soaring but safe skyscrapers.
Lessons from 9/11
The building's wide base, concrete core and blastproof facade were designed to address the weaknesses that helped terrorists succeed on Sept. 11, 2001. Forty years ago, architects built the 1,368-foot-tall twin towers of the original World Trade Center to withstand wind loads and all the other stresses they could imagine. They never imagined that hijackers would fly jet airplanes into the structures, setting off fires so hot they melted the central steel supports.
Almost 3,000 people perished when the buildings burned and collapsed, triggering an international crisis that still reverberates from New York, Washington and Shanksville, Pa., to Baghdad and Kabul. While America went to war, New Yorkers also considered how to refill the empty spot on the skyline — and in their hearts.
From the start, planners struggled to strike a balance between safety and symbolism. The first concept was edgy and asymmetrical, with an off-center tower that evoked Lady Liberty and her arm. That design underwent several revisions, however. At one point, the New York Police Department said the building would "become the prime terrorist target in New York City as soon as it was occupied" — and determined that, as designed, it just wouldn't be safe enough.
Childs went back to the drawing board and came up with a design meant to make One World Trade Center as secure as a U.S. Embassy in a foreign country — while rising 104 floors above street level.
The revised design set the building farther back from the curb, with a bombproof, 200-foot-high concrete-and-steel base. To decorate the facade, Childs at first specified a shatterproof prismatic glass, but the glass was found to be not quite shatterproof enough. A different kind of latticework, made from shimmering "fins" of safety glass, was created to take its place.
The key to the building's safety is a boxy core, constructed with reinforced concrete that has been likened to "liquid steel." In contrast to the sheetrock-covered steel girders that formed the backbone of the original twin towers, this fire-resistant core was created using a special concrete mix that set fast and set solid, to a thickness of up to 6 feet.
The mix posed a challenge for the construction crews: They had only 90 minutes to pour the concrete into forms on the site before it set up — but as shown in the "Ground Zero Supertower" PBS special, a high-speed pumping system proved equal to the task.
Inside the core, the staircases were built 50 percent wider than the stairs in the twin towers — with the aim of heading off the overcrowding that contributed to the chaos on 9/11. The stairwells are pressurized, so air streams out the doors rather than letting a fire in. Redundant water systems feed the sprinklers, with tanks at the building's top and bottom. Core-protected, fireproof elevators are provided for the use of first responders.
Beauty vs. bunker mentality
"One of the things I've learned about safety is the appearance of safety," Childs says during the PBS program. Building in all the safety features, as well as a sense of solidity, meant getting rid of the artsy, asymmetric Statue of Liberty look. Some feared that a super-safe One World Trade Center would look like nothing more than a 1,776-foot-high concrete bunker — and in a newly published book titled "Battle for Ground Zero," urban anthropologist Elizabeth Greenspan says it's debatable whether the finished design steered sufficiently clear of the bunker image.
Childs softened the monolithic look by having the tower's vertical lines lean gracefully toward each other. That provided an engineering payoff as well: The angled edges reduce wind turbulence, easing the everyday stresses on the structure.
As huge as it is, One World Trade Center is just one part of a 16-acre complex that will include several other office towers as well as the National September 11 Monument and Museum. Two square reflecting pools already have been placed in the footprints of the old twin towers, and a museum built beneath the plaza will preserve artifacts that evoke the tragedy and heroism of that infamous day.
Trident-shaped girders from the old towers' facade will be put on display, along with a cross of steel beams that served as an inspiration to rescuers. Other artifacts include a "Survivors' Stairs" that allowed hundreds of people to escape the towers before they fell, and fire trucks that were recovered from the wreckage. The museum, which will open to the public next year, contributes to the balance between the tragic past and a hopeful future at Ground Zero.
Paula Grant Berry, who lost her husband in the 9/11 collapse and serves as a board member for the memorial, echoed Childs' words about the symbolism of the site as she surveyed the museum for "Ground Zero Supertower."
"This, to me, is resilience," Berry told the filmmakers. "This is resilience."
More about the World Trade Center site:
- 9/11 museum to balance grief with remembrance
- Cheers erupt as spire tops One World Trade Center
- Report: WTC naming rights were sold for $10 in 1986
Check out the PBS.org website for more about "Ground Zero Supertower."
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.