More Than One Way to Skin a Building

Four curtain wall case studies show different ways to address daylight while achieving superior aesthetics and performance in new and renovated buildings
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Sponsored by the Ornamental Metal Institute of New York
Peter J. Arsenault, FAIA, NCARB, LEED AP

4 United Nations Secretariat
Renovating for Modern Needs

Project Credits

Location: 1 United Nations Plaza, New York, NY
Owner: United Nations, New York, NY
Design Architect, Architect of Record: HLW International, New York, NY
Original Architects: Le Corbusier; Oscar Niemeyer; Wallace Harrison
Architect of Record, Facade: R.A. Heintges & Associates, New York, NY
Engineering Consultant: Ove Arup & Partners Consulting Engineering, New York, NY
Construction Manager: Skanska, USA, Queens, NY
Curtain Wall Consultant: R.A. Heintges & Associates, New York, NY
Miscellaneous Iron Fabricator and Erector: Empire City Iron Works, Long Island City, NY
Curtain Wall Fabricator and Erector: Benson Industries, New York, NY

Completed in 1952, the iconic 30-story United Nations (UN) Secretariat was the first time ever in the world that a curtain wall system was used on a high-rise building. Like many other buildings of that era, the UN Secretariat had suffered from years of stopgap repair measures that compromised the curtain wall's intended aesthetic. The UN undertook a post-9/11 campus-wide renovation that included replacement of the curtain wall to not only increase blast resistance but bring its energy performance up to contemporary standards. Replacing it required a solution that was above all respectful of the original design. As part of an ongoing $1.87-billion renovation of the entire UN compound in New York City, the wall that started it all has been succeeded by a contemporary unitized system that brings the secretariat into the 21st century, while maintaining its mid-20th century looks.

Completed in 1952, the UN Secretariat building was the first skyscraper in the world with a unitized curtain wall enclosure.

Photo by UN CMP/John Woodruff and Peter Brown, courtesy of the Ornamental Metal Institute of New York

Design Approach–Update Performance but Keep Original Aesthetic

The challenge was to recreate the original transparency intended by the original architects, Le Corbusier and Oscar Niemeyer. A new panelized system was therefore designed that creates that look, benefitting from modern glazing's thinner profile. “The new curtain wall has been designed to look like it did in 1952,” says Michael Adlerstein, UN assistant secretary-General and executive director of the capital Master Plan, “though it's greener, more sustainable, and safer against blast.” When the UN staff started planning the renovation in 2000, it first considered repairing the facade of the secretariat rather than recladding it entirely. The decision to replace the wall entirely came about for a couple of reasons. For one, the existing system had deteriorated significantly. “The original wall was primitive,” says Robert Heintges, founder of curtain wall consultancy R.A. Heintges Associates, which worked on the recladding project along with architecture firm HLW International. “It leaked water and air right away. Over the years they put up patch plates and smeared it with every variety of sealant that came along.” In spite of these stopgap measures, the water intrusion led to varying degrees of rusting in the steel members that connected the wall system to the floor slab. Then came September 11, 2001, and the world changed forever. “Whether to repair or replace became a moot question post 9/11 when the UN knew that the facade would have to be security-enhanced,” Heintges continues. “There was no way to make the existing wall bomb blast safe.” Once the decision to do a full re-clad was made, the team unanimously agreed to recreate the look of the original wall as closely as possible.

Curtain Wall System Used

The result turned out to be somewhat different from how the curtain wall had looked for most of its life. New Yorkers had grown accustomed to an east river view of Manhattan dominated by the minimalist slab of the secretariat with its white stone-clad shear walls and green bottle fly-colored glazing. Most people didn't know, however, that the tower's iridescent glass is in fact a perversion of the transparent operable windows put in place by the building's original design team. What today's viewers experienced was the result of “after-market” tampering to improve energy performance. Since the original wall was a unitized frame system, it meant that the frame was erected first and connected to the structure, with the double-hung windows installed in the frame later. After erecting the framing, it was discovered that the insulating glass the designers had wanted to use—a brand new product at the time—would be too heavy for the frame to sustain the load. So, in an early example of value engineering, the team decided to use ¼-inch monolithic glass in the windows instead. While this created an admirably transparent facade, it also left something to be desired in terms of insulation and energy performance. As a result, a series of reflective films was applied to the glass over the course of the 1950s to cut down on solar loading, which was over powering the building's HVAC system and turning the offices into sweatboxes—thus was born the well-known green facade.

The new curtain wall is a panelized system, factory built and installed on site. The panels match the dimensions of the original wall almost exactly, each featuring two roughly square glass modules 4 feet wide and 3 feet, 10 ½ inches high. Although the new windows are not operable (if a window were open during a blast event it would essentially negate any protection offered by the system), they were designed with an intermediate horizontal mullion that creates the look of the original double-hung window. The two glass modules in each panel are laid in plane, as opposed to offset, as would be the case in a true operable window. This was done in order to match the profile of the original mullion while adhering to current codes for wind-loading.

Offsetting the modules would have required a thicker mullion, and the team determined through full-scale mockups that the panels satisfactorily matched the original in appearance. The most difficult part of the process was getting the new glass itself to look like the original. “In the new system you have a really thick laminated IGU,” says Heintges. “To get this thick of an assembly to resemble ¼-inch monolithic glass is challenging.” Heintges' team performed a spectral analysis of various types of glass, studying their reflection patterns, and came up with a formula to represent them. They then developed a computer model and plugged in the different formulas, allowing them to analyze how the different types of glass might look at different times of day and light conditions. The team also took various material candidates to the site and compared them to an existing window whose film had been removed. Once the field had been narrowed down to four choices, they built a full-scale mockup on UN headquarters grounds in front of the Secretariat for final scrutiny. In the end, they selected a product with a low-e coating that did not cause a purple shift in reflected light.

 

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Originally published in Architectural Record
Originally published in May 2013

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