Little remains of the original twin towers. According to the 9/11 Memorial & Museum, the buildings once featured 200,000 tons of structural steel, 425,000 cubic yards of concrete, 43,600 windows and more than 600,000 square feet of architectural glass. What endures are a handful of steel remnants, a 38-step concrete and granite staircase, the slurry wall and the “survivor glass,” a lite of glass that fell from the 82nd floor of the South Tower, recovered from the debris and preserved at the 9/11 Memorial & Museum.
Twenty-five years later, the recovered lite is a rare physical reminder of the towers’ curtainwall system, offering a connection to one of the most significant glazing projects of the 20th century.
Much has changed in the construction and glass industries since American Airlines Flight 11 struck the North Tower and United Airlines Flight 175 flew into the South Tower at 8:46 a.m. and 9:03 a.m., respectively. In the aftermath, security became a larger design priority throughout the wider construction industry, with owners and designers placing greater emphasis on blast resistance, hardened entrances and controlled access, along with accelerated conversations regarding protective glazing.
For the glazing industry, the September 11 attacks translated into renewed attention on how façades perform during extreme events, particularly the role of glass in protecting occupants after extreme impact.
The Pentagon Test
Thirty-four minutes following United Airlines Flight 175’s collision with the South Tower, American Airlines Flight 77 was deliberately flown into the west side of the Pentagon, impacting the E Ring at the first deck level between corridors four and five. The attack—which preceded the fatal crash of United Airlines Flight 93 into a field near Shanksville, Pennsylvania, at 10:03 a.m.—claimed the lives of 184 people.
Before September 11, the Pentagon had undergone a series of renovations to wedge one (defined as a section of the five-sided building), including the addition of Viracon laminated windows, roughly 1.5 inches thick, according to Brett Eaton, a former spokesperson for the Pentagon renovation project in the 1990s. Ken Hays and Rob Grosze, partners of Bessemer, Alabama-based Physical Security, worked for Masonry Arts at the time. The company had crews at the Pentagon installing windows and other protective systems the day of the attack as part of the building’s renovation project.
Hays explains that the challenge behind the renovations centered on replicating the exact appearance and profiles of the Pentagon’s existing windows. The only products on the market at the time were heavy steel frame punched windows, with the glass held in place by large gaskets in deep glazing pockets, designed for high embassy blast loads. Hays notes they did not look like the Pentagon’s existing 1940’s steel windows, leading Masonry Arts to develop unique replacement windows. “Because the Pentagon’s original windows were large, with two lites of glass separated by a center horizontal bar/mullion, the center bar had to be designed to take the blast loads transferred from the top and bottom lites of glass,” says Hays. “To meet the requirements of the National Capital Planning Commission, the bar and all the trim and exposed pieces had to be milled to recreate the historic profiles, and the center bar also had to have ‘tabs’ to anchor into the perimeter steel window framing.”
Hays says that, when viewed from the exterior, the finished product, 1,755 windows in total, is an exact replication of the Pentagon’s original steel window, including screen clips and a handle atop the simulated lower sash. The new window weighed roughly 2,000 pounds and was anchored at 16 points into support steel framing that transfers loads into the structure.
While the renovated window systems maintained the historic aesthetic of the building, they were also able to mostly withstand the tremendous forces of the plane crash, largely staying intact during the collision and the ensuing fire. Those renovations improved survivability for the building’s occupants and saved lives, noted Garret Henson, Viracon’s vice president of sales and marketing, in USGlass magazine’s coverage of the 20th anniversary of 9/11.
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