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DuPont™ SentryGlas® Interlayer Provides Unrivaled Strength for Thin, Frameless Glass Canopy

At the Bowling Green subway station in New York City, a fritted glass canopy made with SentryGlas® helps keep the weather out and the daylight in.

Photos ©DuPont

Sixteen identical glass panels create the arched canopy. Five glass panels on each sidewall have the same thickness and width as the canopy, but in progressively smaller lengths to accommodate side elevation and radius.
Glass support hardware is 316SS with an as-machined finish. The black sealant ages better than white, which can discolor over time. Backer rope is an open cell, non-gassing polyurethane.
Ceramic frit helps control heat gain and diffuse light. The canopy's glass panels consist of: 10-mm Optiwhite T-Plus glass; 1.52-mm SentryGlas® interlayer; and 10-mm Optiwhite T-Plus with 40% white dot ceramic frit on Surface 2.
The canopy assembly is engineered to withstand structural loads per A-36042 specifications.
Wind load, any direction: 30 psf
Snow load: 30 psf
Point load: 300 lb

When a subway location is more than just a subway station.

Among the many landmark locations in New York City, Bowling Green stands out for both its history and its architecture. It is where Native Americans sold Manhattan to Peter Minuit in 1626. The U.S. Custom House, now The National Museum of the American Indian, is located here. The beaux arts granite building is on the National Register of Historic Places and it was one of the earliest designations of the New York City Landmarks Preservation Commission. The site is also home to Bowling Green Park, a small oasis that is the city’s oldest existing public park and the stomping grounds for the iconic Charging Bull bronze sculpture designed by artist Arturo Di Modica. So, when MTA New York City Transit wanted to add a protective canopy to the subway station’s entrance, to say there was an architectural sensitivity is an understatement. After much soul searching, the project team at Dattner Architects designed an innovative canopy that complements the surroundings without competing with them. Its curved shape leads you into the entrance and almost has motion to it. The unique, frameless, laminated glass architecture is made possible by DuPont™ SentryGlas®, which enables the large glass panel size, thinness, clarity and stiffness. In fact, this functional work of art would have been impossible if using ordinary laminated glass.

Escalators and elevators need protective canopy

Bowling Green is the southernmost station on Manhattan’s Lexington Avenue Line. The busy terminal has added many commuter-friendly features over the years, including elevators for compliance with the Americans with Disabilities Act and escalators to facilitate the comings and goings of tens of thousands daily passengers who use the subways on a 24/7 basis. Studies over many years had proposed covering the open entrance to shield people and machinery from precipitation and wind. There was no question that the entrance of this heavily used station needed to be protected with some kind of canopy. When it rained or snowed, the stairs got slippery. The escalators weren’t warranted for performance in exposed weather conditions, and they kept breaking down. When MTA New York City Transit installed new elevators in 2007, the decision was made to move forward with covering the entrance. William Stein, AIA, principal architect at Dattner Architects, NYC, says, “The real question, considering this highly visible location, was not if the station needs a canopy, but what the canopy should look like and what it should be made of.” He adds, “The city’s transit office overlooks the plaza, so the canopy would be a constant reminder of good choices, or bad, to everyone involved in the decision-making process.”

Decision Time

Stein recounts, “When we were brought into the project, we discussed the need for design sensitivity for this landmark location. Should the canopy be Victorian? Traditional? Classical? Contemporary? We decided early on to recommend a transparent structure that wouldn’t obstruct views of the surrounding sights. We didn’t want to compete with or hide the surrounding designs, we wanted to enhance and work with them.” A laminated, glazed glass structure seemed the obvious direction to head in. Stein explains, “Glass would allow us to design a canopy that was light, transparent and open. At one point we discussed using framed glass, but a frameless design is much more elegant. The exposed edges on frameless glass are a more natural fit for the canopy’s graceful, arcing shape.”

The project team added to their “wish list” frameless glass that was as light and thin as possible. W & W Glass Systems, Inc., the New York metropolitan area’s largest architectural glass and metal contractor recommended a structural glass system made with DuPont™ SentryGlas®. Jeff Haber, W & W managing partner, says, “It was important to fulfill the architect’s goal for a material that was thin and light, but that also would perform under physical stresses including high winds, snow loads, and, let’s face it, whatever New Yorkers can throw at it.” Laminated glass with a PVB interlayer has a tendency to wick moisture, discolor, and not age well. But, the Pilkington Planar™ SentryGlas® System is a high performance glass without any of PVB’s downsides. Haber confirms, “SentryGlas® is more structurally stable and stiffer than PVB. It has improved post-breakage performance over PVB laminated glass. Plus, the Pilkington Planar SentryGlas System uses an OptiWhite glass that stays crystal clear over time with no green tinge.”

Low maintenance and energy savings.

The canopy, completed in 2007, consists of sixteen number of pieces of frameless, corner-bolted, segmented laminated glass panels. The roof and side wall panels are supported by five stainless steel ribs that project over the entrance. A small but significant detail is the black sealant used at the glass joints. “Black discolors less than white,” according to Stein. “We simply walked around the city to look at other bolted glass structures, and saw that, over time, white sealants get dirty.” The canopy virtually self-washes with rain water, and only periodic maintenance will be needed for the sealants. The lifespan of the canopy is expected to be approximately 40 years with little to no other maintenance.

Another design detail is the ceramic fritting located between the underside of the glass and the SentryGlas® layer. The 40% dot pattern coverage is enough to enhance maintenance without negatively affecting the adhesion of the glass and interlayer. Stein confirms, “The MTA New York City Transit is very maintenance-oriented and the fritting is a critical dirt-hiding feature.” In addition to maintenance benefits, the project team at Dattner liked the fact that fritted glass lets daylight in. Daylighting is a sustainable way to reduce lighting energy costs and improve safety. And, although no empirical studies have been conducted by the MTA New York City Transit, Stein and Haber agree that fritted glass creates an effective UV shield that helps control solar gain and increase comfort for commuters using the subway entrance. “The fritting also subtly distinguishes the overhead roof from the vertical glass on the canopy’s sides, and changes translucency throughout the day,” observes Stein. “With changing weather conditions and with various viewing angles, the canopy can look crystal clear, opaque or practically white. It’s a very elegant effect, befitting the location.”

Many companies and public authorities working together to realize the goal.

The city, private companies and many dedicated professionals collaborated on this successful project. The architectural team included Stein, Richard Dattner, FAIA, lead designer, and Steven Frankel, AIA, project manager at Dattner Architects. The glazing was provided by W & W Glass Systems, Inc Engineers at MTA New York City Transit designed the structural system of the canopy. David Foell, AIA, design manager at MTA New York City Transit, managed the project, and Parsons Brinckerhoff provided consultative and administrative services. The general contractor was Citnalta Construction.

Although most large municipalities greet new concepts at best with guarded optimism, there was total top-to bottom endorsement by the lbMTA New York City Transit for this project. Stein explains, “The project involved close coordination with various public agencies and community organizations, as well as the General Contractor and all the subcontractors and materials suppliers. We really thank New York City Transit for their support and collaboration on this project, considering the unique design, demanding shapes and complex geometry. They went out on a limb with us to do something a little different and everyone’s faith in the outcome has paid off.” Validating their decision is the fact that the canopy is receiving much favorable feedback from the city, the community and fellow architects.

Another measure of the canopy’s success is that MTA New York City Transit is using Bowling Green as the prototype for additional projects. A proposed new subway station at the city’s West 34th Street terminal will be modeled on the Bowling Green design, but with a slightly different shape containing more glass spans and built on a larger footprint. Haber confirms, “Thin, floating glass is gaining in popularity because of the look that can be achieved using the Pilkington Planar™ SentryGlas® System. Architects can rest easy about the structural integrity of their designs, even those using larger-sized panels, thanks to innovations in laminated glass technology.”