Janno Lieber, director of World Trade Center development for Silverstein, boasts that 7 WTC's curtain wall system is "unparalleled in terms of occupant health benefits, energy conservation and resiliency."
The building illustrates the advances of modern glass curtain wall in both energy conservation and security. At the same time, when it comes to the prospect of a terrorist attack, even the best is vulnerable, its strength more symbolic than practical, according to experts.
The lobby curtain wall is primarily a two-way cable-net wall with heavily laminated and heat-strengthened glass. Carl Galioto, partner in charge of SOM's technical group in New York, describes it as "resilient like tennis racquet." The structure has been tested to meet federal criteria for blast resistance, he says.
Such material advances can go some distance to reassure building occupants, visitors, and owners that the buildings are safe from calamity — laminated glass is helpful in reducing flying shards, for example. But there should be no illusion that any building is safe from determined terrorists, says Gordon Smith, an exterior wall consultant and forensic engineer based in New York.
The protective potential of glass can vary depending on whom you talk to. Bruce Fowle, of Fox & Fowle Architects in New York — who are working as the associate architects on the new, TK-story headquarters of the New York Times with Renzo Piano under developer Forest City Ratner — says the company is working on a high-security government-related building project and is contemplating the use of two-inch-thick glass that he refers to as "blast-proof."
Experts say to really do a so-called blast-proof building, immense amounts of supporting steel and devices are required to keep the glass in place.
On the other hand, curtain wall has been around for a while that can weather the most vicious storm. Smith engineered Carnival Cruise Lines' headquarters in Florida 10 years ago with a combination of glass, frames and anchors that can withstand winds of 200 miles per hour, well beyond the range of the worst hurricane winds, which can reach up to 140 miles per hour.
Along with its sheer strength, the glass being used at Seven World Trade, Galioto explains, offers very real energy efficiencies. It combines low-iron and low-emission coating and a ceramic frit to deter ultraviolet rays and heat while letting sunlight pour in.
Part of the lighting benefit comes because low-iron glass has less of a greenish tint than normal glass. Because of its cost, it has been used sparingly — in high-end showrooms, for example. But Silverstein's building uses it abundantly, to control energy costs and create a more pleasant work environment. "Low-E" is a clear glass with a microscopic coating of metal oxide that allows heat and sunlight to pass into the building. Fritting also lessens direct heat gain, but applied in the correct patterns, the densest location looks no more obtrusive than would an insect screen, says Galioto.
The glass will reduce energy consumption and help the building qualify for LEED certification. That could help to lure environmentally conscious companies — and European firms are especially so — to a building that is intended to be part of a cosmopolitan complex of buildings.
The cladding promises to be pleasing to the eye from outside as well because of collaboration between SOM and New York-based James Carpenter Design Associates, experts in the interaction of glass and light in architecture. Thanks to a reflective technology developed by Carpenter, sunlight, both direct and reflected off surrounding buildings will create color accents on the façade that shift as the observer moves.
The fast-tracked building topped out in late October, and the curtain wall is on its way, meaning we will see for ourselves what the new age of glass has to offer to the eye. Nonetheless, the specter of terrorism has raised the awareness of glass curtain wall beyond aesthetic issues.
