More Than One Way to Skin a Building  

Building facades have become more than the sum of their parts, particularly when curtain wall systems are used. Advances in materials, digital tools, fabrication techniques, and multidisciplinary collaboration mean that modern facades are being used to regulate entire building environments in terms of daylight, ventilation, and energy use. The cutting-edge technology of many curtain wall systems, coupled with good design principles, is saving energy while providing indoor environments that are focused on the needs of the users. The result is an impressive combination of design and performance coming together for new and renovated buildings.

Overview of Curtain Wall Systems

A curtain wall system is defined as a complete exterior envelope facade system which provides a non-structural, relatively lightweight, weather-tight covering on buildings. Being lightweight, it reduces both the load that must be supported and the manpower needed to erect it. In the case of small, low-rise projects, the system may be field fabricated or “stick built” and glazed using standard components similar to a storefront system. However, curtain wall components are notably different in design and performance characteristics with typically much better results compared to storefront components. Larger projects may justify full factory fabrication with panels prepared and glazed ready to be placed directly onto the building structure, minimizing the number of joints in the facade. Curtain wall systems are generally installed outside of the structural system of a building running past floor slabs and other structural elements. They are then attached via tiebacks directly to the building structure at floors, columns, and beams. This installation process means that all wind loads and dead loads imposed on the system are compartmentalized and transferred directly to the building structure. Hence the curtain wall system carries only its own weight and loading while the building structure absorbs all imposed loads.

Curtain wall systems, like this one at the Milstein Family Heart Center, can be used for a total building or just for a portion to meet a variety of design and performance needs. Photo by Paul Warchol, courtesy of the Ornamental Metal Institute of New York

The strength of curtain wall systems lies in its high overall performance, particularly when compared to a storefront system. This is true in terms of wind resistance, water management, and thermal performance. From a design standpoint they are easy to customize, are available with a variety of interior and exterior aesthetic appearances, and allow a virtually unlimited range of installation locations, configurations, and opportunities. Specific decisions can be made early related to anchoring options, accommodation of specified glazing thickness, and other details. Most curtain wall manufacturers also offer accessory items such as sun shades or light shelves to enhance daylighting approaches for the overall building.

Perhaps the single most important material common to curtain wall facades is glazing using all manner of glass products. Long marketed as architectural statements, the current generation of all-glass buildings is increasingly being promoted as an energy-efficient, environmentally friendly solution that enhances occupant experience as well as building performance. Technological advances are responsible for this performance boost with glazing currently available that can manage heat and glare, feature low-emissivity, and provide user privacy without compromising light transmission.

In concert with the pace of technological innovation, it has become increasingly important that good architectural design respond to a variety of basic human needs. In particular, recent investigations into the effect of the indoor environment on people suggest that basic design choices regarding the building facade—notably natural daylighting and ventilation—can dramatically affect the performance of a building, and by extension its users. No longer viewed as just an assembly of materials providing environmental separation between conditioned space and the exterior environment, the curtain wall facade is now recognized as an integral component of high-performance building systems.

With all of the above in mind, we will explore four very different buildings that have successfully achieved the appropriate balance of technology, design, installation, and user satisfaction that each of these well-designed projects required.

1 Milstein Family Heart Center
Designing for Daylight and Energy

Project Credits

Location: 165th Street and Fort Washington Avenue, New York, NY
Owner: New York-Presbyterian Hospital, New York, NY
Architect: Pei Cobb Freed & Partners Architects, New York, NY
Associate Architect: daSILVA Architects, New York, NY
Structural Engineer: Thornton Tomasetti, New York, NY
Mechanical Engineer: Syska Hennessy Group, New York, NY
General Contractor: Bovis Lend Lease, New York, NY
Curtain Wall Consultant: R.A. Heintges & Associates, New York, NY
Structural Steel Erector: Empire City Iron Works, Long Island City, NY
Miscellaneous Iron Fabricator and Erector: Post Road Iron Works, Greenwich, CT
Architectural Metal Fabricator and Erector: Post Road Iron Works, Greenwich, CT
Ornamental Metal Fabricator and Erector: Precision Glass and Metal Works Co., Inc., Maspeth, NY
Curtain Wall Erector: W&W Glass, Nanuet, NY

The Vivian and Seymour Milstein Family Heart Center at New York Presbyterian Hospital in New York City is one of the world's leading pioneers of cardiac treatment. In order to maintain this edge, the institution recently built a $240-million, 125,000-square-foot addition to its 165th Street hospital complex.

Design Program: Improved Patient Care Through Daylight

Seeking to provide more than just room for the latest in medical advances, the hospital leadership and key donors, including the Milsteins, wanted a building that would buoy its patients' morale—giving the gift of hope to those facing life-threatening illness.

Design Approach–A Very Transparent “Climate Wall”

Ian Bader, the project's lead designer for architects Pei Cobb Freed & Partners, knew immediately how to deliver this kind of reassurance in the form of architecture: The site, a cramped plot at the southern edge of the hospital's upper West Side campus, overlooked a breathtaking panorama of the Hudson River and Palisades. Bader intended to bring this view to the interior, opening up the Milstein Family Heart Center facility to the soothing effects of unfiltered daylight in the process. The problem was how to do so without also causing wild swings in temperature. His solution was to enclose the building in a four-story-high glass climate wall—a dynamic double curtain wall that tracks the diurnal course of the sun, controlling incoming daylight while keeping the hospital's occupants in immediate touch with the glory of the natural world. “Looking outwards becomes an event of hope,” explains Bader. Thanks to superb thermal performance and unrivaled craftsmanship, the facade system also helped the project to earn a LEED Gold rating.

A dynamic double curtain wall delivers energy performance and optimism for the cutting-edge medical community at the Milstein Family Heart Center. Photos by Paul Warchol, courtesy of the Ornamental Metal Institute of New York

More than just an expansion of the hospital's facilities, the addition creates a new entry sequence to the Milstein Heart Center, ushering visitors in from Fort Washington Avenue along a curving passageway that opens into a naturally illuminated four-story atrium. Glass-floored bridges cross the atrium, spanning between the addition and the existing Irving Pavilion and linking directly to corridor waiting areas that abut the curving climate wall. Constructed of custom steel box beams, the bridges support a structural glazing floor system that allows daylight to pass freely through the space's water white glass curtain wall and skylight. A single, mid-span vertical cable suspended from the atrium roof above is used only to control deflections and vibrations. The bridges connect the neighboring Irving Cancer Center with the floors of the new building, facilitating continuity between medical departments.

Structural System Coordination–Cables Tie into Building Structure

The four-story atrium is constructed with both a glass ceiling and an approximately 45-by-70-foot glass facade. The paramount aesthetic goal was that the structural support of the atrium facade be as willowy as possible in order to leave views of the Hudson River and beyond unobstructed. An efficient single-plate steel girder system spanning between the new addition and the existing Irving Pavilion supports the atrium roof. The gravity load of the atrium wall is supported by small-diameter bright drawn stainless steel 316 S 1x19 strand cables hung from the atrium trusses above. The lateral support of the vertical wall is provided by pre-stressed, Vierendeel horizontal trusses, constructed of a thin plate and pre-stressed horizontal cable.

Curtain Wall System–Cable Support System and Cavity

The glazing is supported from the structural systems with stainless steel glass point support hardware. Rising to the west of the atrium, the addition's climate wall is composed of a 22-millimeter-thick laminated glass outer wall and a 44-millimeter-thick insulated glass inner wall separated by a 3-foot cavity. The layer of air in the cavity mediates solar heat gain in warm months and acts as insulation during the winter. When outdoor temperatures rise, the building's exhaust air is drawn into the lowest level of the wall by convection currents, allowing built-up heat to rise naturally to the top elevation where it is expelled through a rooftop vent. In winter, this vent is closed off, holding in the air and creating a thermal blanket for the building.

The wall's double-laminated glass panels are a variety of sizes, though they are generally 5 feet wide and 16 or 17 feet high depending on floor-to-floor heights. The wall does not rely on a mullioned framing system, but upon structural glazing and custom-designed point supports. The point supports attach to a system of crisscrossing post-tensioned fine ground brush drawn 304 stainless steel rods that hang from the ceiling and are drawn down by coil springs at the wall's base. This system kept the ⅜-inch-diameter rods as slim as possible, as the structures in tension require significantly less material to handle the applicable loads. Rods also tie the tensioned system back to the floor plates to absorb lateral forces, primarily wind loads.

Daylight Control Strategies–Adjustable Vertical Shades Move with Sun Patterns

As with the atrium, the climate wall's glass panels are composed of clear insulated glass units, meaning that the light that comes through is unadulterated by high-performance coatings, which tend to taint the sun's full-spectrum rays. “We live in a filtered world,” muses Bader. “I tried to avoid that here.” The tradeoff, of course, is that the glazing offers little in the way of shading. To make up for this, Bader and his team designed a system of computer-controlled fabric louvers, essentially motorized vertical blinds that track the trajectory of the sun. In the evening, the system is completely open, at midday it is closed, and between these two extremes the panels adjust accordingly, optimizing the amount of light passing through at any given time. “Typically, a high-performance glass would have a shading coefficient of .6 to .45,” says Bader. “The shading coefficient of this system is very low—.1 or .05. You would have to have a virtually opaque wall to get that.” In addition to the blinds, the airspace features a stainless steel catwalk system, a custom grating supported by small-diameter pipe members integrated into the climate wall support structure that allows easy access for maintenance.

Computer-controlled louvers track sunlight throughout the day, optimizing the amount of light entering the four-story atrium at the Milstein Family Heart Center. Image by Pei Cobb Freed & Partners, courtesy of the Ornamental Metal Institute of New York

Behind its sophisticated stainless steel and glass corset, The Milstein Family Heart Center now stands as a hopeful refuge for patients and their families, with enlarged details of Hudson River School painters' landscapes on the walls of the waiting areas and lobby imbuing a natural vibrancy and depth to the center's interior and reiterating the addition's strong ties with the natural world. Luminous between New York Presbyterian's older masonry structures, “all the elements of the center were carefully engineered like pieces of jewelry—each item has its own special identity and purpose,” says Bader. “And while the existing buildings are not architecturally distinguished, they are of archaeological value, allowing a layering of stories to happen. The dialogue is alive and well here.”

2 Harlem Hospital Patient Pavilion
Making a Statement

Project Credits

Location: 506 Lenox Avenue, New York, NY
Owner: NYC Health & Hospitals Corporation, New York, NY
Developer: Dormitory Authority of the State of New York (DASNY)
Architect: HOK, New York, NY
Associate Architect: Studio JTA, Bronx, NY
Structural Engineers: Robert Silman & Associates, New York, NY Trevor Salmon Associates, New York, NY
Mechanical Engineers: Kallen & Lemelson, New York, NY
Lakhani & Jordan, New York, NY
Construction Manager: TDX Construction Corporation, New York, NY
Curtain Wall Consultant: Ove Arup & Partners, New York, NY
Structural Steel Erector: Brooklyn Welding Corporation, Brooklyn, NY
Miscellaneous Iron Erector: Capco Steel Co., Providence, RI
Architectural Metal Erector: Brooklyn Welding Corporation, Brooklyn, NY
Curtain Wall Erector: W&W Glass Systems/Metal Sales, Nanuet, NY
Metal Deck Erector: AC Associates, Lyndhurst, NJ

The new patient pavilion at Harlem Hospital is proving that modernization need not be at the expense of historical significance.

Design Program–Create a Unified, Modernized Complex

The modernization of this 100-plus-year-old teaching hospital was needed to make room for improved patient care and community outreach in the Harlem section of New York City. The recently completed project will house new emergency and surgery departments, diagnostic and treatment services, a critical care suite, and a modern radiology center—all centered around an expansive full-height atrium to create a unified health care complex out of seven disparate structures spread over two city blocks.

Design Approach–A Unifying Facade

The project's design, undertaken by HOK's New York office in association with Bronx-based architect Jack Travis of Studio/JTA called for constructing a new six-story, 150,000-square-foot pavilion to connect to the existing Martin Luther King Pavilion and the existing Ron Brown Ambulatory Care Pavilion. Though the plan is rooted in bringing the most up-to-date medical care and teaching facilities to Harlem, at its creative center is the restoration of the hospital's historic WPA-era murals painted by some of the most famous African American artists of the 1930s, including Charles Alston and Vertis Hayes. While the original artwork, depicting themes of medical science, life in Harlem, and experiences of black people in America, will be displayed in the new pavilion's lobby gallery, Hayes's work Pursuit of Happiness is replicated on the building's six-story curtain wall facade in the form of a 180-by-65-foot glass mural mounted on a concealed steel structure.

The curtain wall facade on the Harlem Hospital New Patient Pavilion features a digitally applied artistic mural that provides light control while still allowing vision out. Photo by Dominick Reda/TDX Construction Corp., courtesy of the Ornamental Metal Institute of New York

Structural System Coordination

The unusual project posed several challenges to the design team, who worked with structural engineer Robert Silman & Associates to realize the new pavilion's structural design. Pavilion spaces needed to be organized to flow into existing portions of the hospital campus. Perhaps most importantly, the design had to anticipate plans to build two additional floors atop the new pavilion in the future. Using steel for the structural frame provided the necessary flexibility in both cases, but the team found that the unique mural curtain wall design approach prevented them from using more efficient cross-braced frames to address lateral loading. Therefore, the building's lateral system (with the exception of a double-height mechanical space) is designed as a series of moment frames. This required large W14 x 342 and W24 x 117 girders, both of Grade 50, A992 steel, to keep lateral drift to acceptable limits. The double-height mechanical space at the seventh floor created a “soft story” where moment frames were inefficient, so the design team instead used perimeter-braced framing for better lateral load resistance.

With the facility's expansion goals in mind, the structure is designed to carry two more floors in the future. In order to anticipate the additional stresses on the building that those future floors presented, the team developed several structural models to determine appropriate seismic and wind requirements both before and after their addition. Because the hospital is set back one bay at the sixth floor, large W44 x 335 transfer girders support the new floors and the potential floors above.

Column layout also presented a challenge to the design team. Because of the hospital's desire to create open, light-filled spaces in line with modern health care standards, the architects set column lines on the north and west sides of the building 9 feet 6 inches back from the building edge. The design team used cantilevered stub beams moment connected to the columns to create corridor areas free of vertical structural elements.

Curtain Wall System–Digital Printing on Glazing

The unique space created by the structural alignments was designed to accommodate the hospital's new Pursuit of Happiness glass mural onto Lenox Avenue. Aside from visible horizontal mullions at each floor line, the atrium's west wall is structurally glazed, presenting an uninterrupted canvas for Hayes's work. Each of the curtain wall's 429 panels was printed with a new digital, direct-to-glass colored ceramic printing technology much like putting ceramic frit on glass. The digitized artwork is printed on the #3 surface of the six-layer insulated glass units, directly under a PVB interlayer laminating it to the outer lite. To accommodate the building's show-stopping curtain wall, bent plates of ⅜-inch-thick A36 steel form the edge of the slab along the west facade. Three quarter-inch-diameter, embedded head steel studs (attached to the inside face of the bent plates) provide the additional capacity necessary at the slab edges for the curtain wall attachment. Because the printed curtain wall was part of a separate design-build contract, Silman's engineers worked with curtain wall engineer Arup to develop connections to the bent plates that could accommodate the necessary vertical deflection at the slab edge, as well as the lateral movement of the building. As with many design-build curtain wall systems, the hospital's steel sub-frame needed to be designed for maximum flexibility at the attachment points. Because the slab edge at the west mural wall was cantilevered out as much as 1 foot – 10-½ inches, the spandrel beam design uses HSS members connected with full-height stiffeners to keep them from rotating.

Strategy at Night–Backlighting

Because interior corridors pass behind the glass, it will remain unobstructed by furniture. At night, the fluorescent back-lit image comes alive thanks to the illuminated column-free space behind it, highlighting the historic scene from the past—the hospital's new face—for all who pass by.

3 Learning Spring School
Controlling Light

Project Credits

Location: 247 East 20th Street, New York
Developer: The Learning Spring School, New York, NY
Architect: Platt Byard Dovell White Architects, New York, NY
Structural Engineer: Leslie E. Robertson Associates, New York, NY
Mechanical Engineer: AKF Group LLC, New York, NY
Construction Manager: Cauldwell Wingate Company, New York, NY
Curtain Wall Consultant: William G. Young of Axis Facades, New York, NY
Structural Steel Erector: Metropolitan-Walters LLC, New York, NY
Miscellaneous Iron Erector: Metropolitan-Walters LLC, New York, NY
Architectural Metal Erectors: Metropolitan-Walters LLC, New York, NY; Jordan Installation Services, East Northport, New York
Ornamental Metal Erectors: Metropolitan-Walters LLC, New York, NY; Jordan Installation Services, East Northport, New York
Curtain Wall Erector: Jordan Installation Services, East Northport, New York
Metal Deck Erector: AC Associates, Lyndhurst, NJ

The Learning Spring School (LSS) is not your average New York City independent educational institution. Established by a group of concerned parents in the fall of 2001, LSS was conceived, built, and staffed for children with high-functioning autism spectrum disorders.

The Learning Spring School curtain wall facade integrates with the rest of the building. Photo by Fredrick Charles, courtesy of the Ornamental Metal Institute of New York

Design Program–Educating Children With Autism

Not long after opening up in a small commercial building, the school realized that its facilities were inadequate to meet the very specialized needs of its student body. Adding to the challenge, as early intervention for children in the autism spectrum became more and more prevalent, admissions applications began pouring in, and enrollment quickly exceeded the capacity of the space. To address both of these shortcomings, LSS commissioned New York architectural firm Platt Byard Dovell White (PBDW) to design a dedicated building that would meet the school's growth projections and create an environment conducive to educating children with autism.

Design Approach–A Sheltering Curtain Wall

Whereas in some projects the purpose of a curtain wall system is to maximize views and daylight, the objectives in this specialized urban school were to mitigate harsh sunlight and minimize visual stimuli. With complete transparency not an objective, the architects initially explored a window wall system because of cost, but changed to a curtain wall system in order to ensure a more reliable enclosure. The design was made of glass, zinc, and an aluminum sunshade system that lets filtered daylight in while keeping autistic students calm and focused on the lessons at hand. What makes the facade unusual is that such systems are not generally used to enclose educational settings. However, to help foster the school's mission of enabling autistic children to succeed academically as well as socially and emotionally, PBDW laid out an extensive 34,000-square-foot building accommodating occupational therapy, drama and music, lifestyles, culinary arts, fine arts, science and computer labs, plus a library.

Each of these varied spaces was geared to fit the unique qualities exhibited by children with autism. Among the most critical of these is the tendency to become overwhelmed by sensory stimuli. “Generally when we design spaces for kids with autism we try to play down the environment,” explains Matthew Mueller, an associate architect at PBDW. “A lot of kids have sensory issues with their visual surroundings and others have issues with things that are too tactile or too rough. We tried to make the interiors calming, using materials that are not too distracting to help keep the students focused.”

Curtain Wall System–Filtered Light

Nowhere was this rationale of minimizing stimulus more conspicuous than in the design of the cladding enclosure system. “The building facade allows nice light in, but you feel there's an internal focus to rooms” says Mueller. “It's not about views, it's about mitigating light, eliminating glare, and making an environment that's comfortable so students can focus internally.”

PBDW began designing the facade as a window wall system with framed spandrel panels, thinking that it would be less expensive to construct than curtain wall. However, an integrated curtain wall system represented a better option during the construction and for the lifetime of the building. “We all felt that utilizing a curtain wall assembly would ensure a more reliable wall in the long run” says Mueller. “When you transition between different wall assemblies, it creates more opportunities for leaks and defects. Building the wall as essentially one type of assembly lessened this risk and still enabled us to create the expression we wanted.” This solution lent itself to improved construction, better coordination, and more control over the finished product as the sequencing involved only one primary trade. In the end, while the curtain wall was more expensive, the client and the design team felt that the upfront expense would minimize maintenance costs in the future, providing a long-term return on investment. The architects developed a scheme using zinc spandrel panels, operable windows, and aluminum louver solar shades, all designed to make the interiors feel both well-lit and sheltered.

The louvers on the south side of the building were carefully studied and designed to create the desired shading and limit the amount of daylight entering the interior spaces. Photo by Frederick Charles, courtesy of the Ornamental Metal Institute of New York; sun diagram courtesy of Platt Byard Dovell White

Structural Coordination

This curtain wall system is not unitized, but stick-built on site with aluminum mullions attached to building anchors. These anchors handle gravity loads on every other floor, while on the intervening floors clips that only manage wind loads are used. The operable windows are structurally glazed to the framing and consist of insulated glass units with ¼-inch inner lites, ½-inch gaps, and ¼-inch outer lites with low-e coatings on the #2 surface. The 1-millimeter quartz-zinc spandrels are a rainscreen system with an 18-gauge galvanized steel back panel, insulation, and vapor barrier, an air space, and then the exterior cladding. The back panel is glazed into the curtain wall pockets at the spandrel areas. Then vertical aluminum girts fasten to flanges on the back pane and the zinc panel attaches to these vertical girts. The exposed zinc panels are interlocking with concealed fasteners and feature cutouts allowing the supports for the aluminum sunshade system to penetrate through and be welded directly to the steel substructure. The sunshades, which are built from 5/16-inch-thick aluminum plates, span the 18- to 20-foot column bays. They hang from their own dedicated support system, which delivers all loads back to the main building structure rather than the curtain wall assembly. The span proved to be a bit extreme for the aluminum plates, which deflected from their own weight, raising concerns about how the louvers would react to wind loads. In response, the designers placed stainless steel tension rods running vertically through the sunshade system at the midpoint of the spans, holding them rigid in the face of wind and gravity. The rods are fabricated from solid ¾-inch-diameter 316 Grade stock. This primary wall assembly is accented by terra cotta rain screen details at the building's corners, as well as a channel glass feature that bookends the corners.

The Learning Spring School uses the curtain wall to shade and filter light in spaces where autistic children are being educated. Photo by Fredrick Charles, courtesy of the Ornamental Metal Institute of New York

Environmental Benefits

In addition to its well-designed appearance from the outside, and calming effects on the inside, the cladding design had an environmental payoff. The building's corner site at Second Avenue and 20th Street faces southwest, putting it right in the line of fire of New York's most punishing daylight. The 's aluminum sunshades, low-e coated insulated glass units, and zinc rainscreen spandrels all help to cut solar gain significantly. Along with the building's other environmentally friendly features, such as operable windows for natural ventilation, low-flow fixtures for water savings, and high-efficiency equipment for energy savings, the project received LEED for Schools Gold Certification in August of 2011.

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.

Matching the look of the original aluminum mullions also proved to be challenging. When the UN was designed in the 1940s, the use of aluminum in architectural windows was relatively new. Stainless steel, on the other hand, had been used quite a bit. So when the original design team specified aluminum they tried to make it look like stainless with a No. 4 finish. “That's not a good finish for aluminum,” says Heintges. “All the scratches get filled with atmospheric contaminates and it starts to get dirty and pitted.” The recladding team couldn't find a manufacturer who would finish aluminum in that way, so it compromised and gave the extrusions a gentler brushed look. Inside the building, the anodized aluminum was left exposed, as it was in the original. On the outside, it was given a protective coating with two types of fluoropolymer paint, one in silver to resemble the look of exposed metal, and the other black, recalling the mullion caps of the original wall.

Construction Process

The old wall was stripped off, the asbestos abated, and the new system installed in three zones. Ornamental ironworkers with the local union worked from a three-story Beeche work access system suspended from the roof via a dedicated aluminum structure, which allowed work to go on in a safe, contained environment. They removed the old glass sashes from the aluminum and steel frames then unbolted the frames from the existing anchors. Reciprocating saws were used to cut frames into more manageable pieces and the threaded studs that remained on the structure were ground smooth. Once this was complete, the ironworkers attached the new panels with connections that go directly to the Secretariat's steel structure. Demolition of the old curtain wall and installation of the new went from the bottom up in three sections starting with the top third of the building, then the middle third, and finally the storefront and bottom third. The new curtain wall followed behind the old by approximately three floors. “Zipper” units were installed at the mechanical floors to join the sections of the facade.

The recladding of the Secretariat is part of a $1.87-billion renovation of the UN’s New York City compound that included the complete replacement of the curtain wall system with much better performance and faithfulness to the original design. Photo by William Rivelli, courtesy of the Ornamental Metal Institute of New York

The result of the project team's careful design and planning, and the ironworkers' fine craftsmanship, combined with new open-plan interiors by HLW, is a reborn UN, at once a spitting image of its younger self and an example of how far technology has evolved since 1952. “The look of the exterior is pretty consistent with what it was,” says John Gering, AIA, managing partner of HLW, “except for the fact that the original Secretariat had enclosed offices. When you looked inside you saw a wall. Now when you look through the glass you look through an open space.”

Peter J. Arsenault, FAIA, NCARB, LEED AP, practices, consults, and writes about sustainable design and practice solutions nationwide. www.linkedin.com/in/pjaarch

Established in 1972 to advance the interests of the architectural, ornamental, and miscellaneous metal industries, the Ornamental Metal Institute of New York sponsors programs to help architects, engineers, builders, developers, and construction managers transform design into reality. www.ominy.org