Redefining New York City's skyscrapers

Architect David Childs lost a colleague in the attacks of Sept. 11, a calamity he watched unfold from his office nearby. Mr. Childs, tapped almost two years later to design the signature skyscraper at the new World Trade Center, had that memory always in his mind as he grappled with a conundrum that has bedeviled construction professionals over the past decade: how to build an office tower that is safe and secure yet doesn't look and feel like a bunker, and that is economically viable.

“It had to be a proud gesture to our resilience,” said Mr. Childs, a consulting design partner at Skidmore Owings & Merrill. “It had to be a great place to work, and it had to be safe.”

One World Trade Center must be worthy of its place on the skyline, and serve as a symbol of the city's rebound from tragedy and as a marker for the nearby memorial. And, given its inevitable allure as a target, the 1,776-foot tower must be among the safest skyscrapers in the United States.

The attacks changed how New York's prominent office towers are designed, built and protected. Police, fire officials and security experts now play a role, working alongside architects, developers and engineers. Together, they have forged a new paradigm for safety that employs both physical and psychological tools.

Though property owners are reluctant to go into detail for fear of providing blueprints to terrorists, some elements of the new normal are plainly visible: 3-foot-high bollards ringing major towers, platoons of security guards and a plethora of cameras and X-ray machines. Others are less obvious: cores and columns composed of extra-thick, steel-reinforced concrete; backup sprinkler systems; and acres of shatter-resistant glass.

In the aftermath of the disaster, developers found few answers to questions about what changes should be incorporated into design. The city's building code, which hadn't been updated since 1968, provided no help.

Planning process transformed

When Larry Silverstein, who leased the World Trade Center from the Port Authority of New York & New Jersey, began planning a replacement for 7 WTC, he called in Mr. Childs and a team of engineers, as well as police and fire officials. Many of the features they used were integrated into the building code, which was updated in 2004, and then into other high-profile properties.

Among the crucial enhancements at 7 WTC: Elevators, emergency stairs and mechanical systems, located in the building's center, are wrapped in a 2-foot-thick, steel-reinforced concrete shell. Before, such core elements were often enclosed with drywall and rebar and in some cases were even on the building perimeter.

The tower's core also includes “repeaters,” devices that let emergency responders communicate during a disaster—something they were unable to do on Sept. 11. Similarly, to help prevent the gridlock as firefighters race upstairs into a stream of fleeing tenants, the staircases at 7 WTC are 66 inches wide, 20% wider than the building code requires. There's also a system to suck smoke out of stairwells, and strips on each stair that glow if the building's three systems that keep the lights on fail.

To protect the building from a bomb blast, the glass panels in the four-story lobby are treated to bend rather than break.

High-profile projects elsewhere in the city also must balance security and aesthetics.

“You don't want to create architecture that gives in to fear,” said Richard Cook, a partner at Cook+Fox Architects, which designed the Bank of America Tower at One Bryant Park. “Our idea was to create a transparent building that focuses on nature and had a clear view of the park,” he said.

The tower is surrounded with 149 stainless-steel, yard-high bollards placed about every five feet along the curb. Light flows through specially treated glass into a soaring, 43-foot-high lobby, whose support columns can bear the extra weight if one becomes damaged.

“I think the building still feels open and airy,” said Vishaan Chakrabarti, director of real estate development at Columbia University's Graduate School of Architecture, Planning and Preservation. “I didn't love the bollards, but I've gotten used to them.”

There are other, nondesign security measures, including uniformed security guards and turnstiles patrolled by dogs. “So much of this is psychological,” said Louis Esposito, chief operating officer of The Durst Organization, co-owner of 1 Bryant Park. “You want the bad guys to know we are watching.”

Visible or invisible, security is expensive. Experts estimate that it can add 5% to 20% to construction costs, which can translate into hundreds of millions of dollars. For example, shatter-resistant glass is about $9 a square foot, 50% more than an untreated pane. Wider staircases and thicker cores eat into rentable space.

Extra effort, extra cost

Similarly, the lobby at 11 Times Square, the 1.1 million square-foot tower that opened in 2010, was made roughly 30% larger than it otherwise would have been to accommodate the turnstiles.

The bollards at One Bryant Park cost $16,000 each, and the building has a backup sprinkler system. “Is three better than two?” Mr. Esposito asked. “Yes, and four is better than three, but at some point you have to think of the building's economics.”

The need for safer buildings coincides with the city's effort to create inviting, lively streetscapes. For example, thoroughfares removed to make way for the original WTC are being reintroduced.

Despite any concessions, some wonder if security concerns influenced 1 World Trade's design to the point that it looks overly fortified, particularly at its base.

In 2005, the NYPD demanded that the tower design be changed to make it safer. It is now an office building sitting on a 187-foot-high concrete box with foot-thick, steel-reinforced concrete columns designed to bear up to 14,000 pounds of pressure a square inch—55% more than normal. It is also set back 40 feet from the street instead of the 25 feet initially envisioned.

One WTC will likely end up being a “forbidding presence,” wrote Blair Kamin, in his Pulitzer Prize-winning book, Terror and Wonder: Architecture in a Tumultuous Age.

Only when the building is finished in 2013 will we know if the designers succeeded in humanizing it, but Mr. Childs says he has improved the 2005 design. For example, the base will now be clad in glass rather than metal.

“The glass will create an openness,” Mr. Childs said. “It will reflect the people, the street, the trees.”

Still, creating that open feeling has had challenges. After spending about $6 million testing the prismatic glass Mr. Childs had hoped to use, the Port Authority realized recently that the material kept breaking. Now the architect is tweaking the design to use a different type of glass.

“We're hoping it will be as close to the original design as possible,” a Port Authority spokesman said. “We hope it will still be beautiful and distinctive.”