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Nestled along the East River in New York City, the United Nations Headquarters has long been admired as the physical embodiment of global diplomacy and collaboration.  Born from the ashes of World War II, the United Nations Headquarters was designed in 1947 by a collaborative, international team of esteemed architects including Le Corbusier, Wallace K. Harrison, and Oscar Niemeyer.  The campus’ resulting architectural achievements have endured as one of the most widely-recognized examples of 20th century International Style architecture, including the first glass curtain wall skyscraper in the United States.

After decades of wear and weather exposure, the buildings’ envelopes were performing poorly, and the iconic green glass of the Secretariat Tower had lost its transparency due to the application of reflective film.  In 2001, the United Nations started developing a master plan to renovate the campus for the next century.  Heintges & Associates was tasked with restoring the integrity of the facades for all six of the campus buildings.  To date, renovation has been completed on four: the Secretariat, General Assembly, Conference, and North Lawn Buildings.

At the time of its completion, the United Nations was considered a highly-innovative facility, employing the best technologies of the era.  The crown jewel of the campus was the 39-story Secretariat building, which had a revolutionary glass curtain wall design that was the first of its kind on a tower.  The major east and west elevations are comprised of glazed curtain walls with double-hung windows, spandrels of blue-green tinted glass, and a grid of louvered frames at the mechanical floors and the parapet level.  The curtain walls were bookended to the north and south by white, monolithic Vermont marble cladding.  The Secretariat’s sleek, slab-like form was a departure from New York’s common blocky towers and its sparkling glass curtain wall articulated the post-war era’s aspirations of modernity and transparency.  It became a precedent for subsequent curtain wall skyscrapers.

Preserving a Modernist Icon

Serving as both Design Architect and Specialist Consultant for the facades, Heintges & Associates developed criteria that guided the decision on whether to restore, remediate, or replace the facades at the onset of the renovation project.  Because the United Nations Headquarters Campus is sovereign territory, it cannot be protected by landmark status.  By choosing to renovate the historic structures rather than demolish and replace them, the enlightened client demonstrated a strong moral commitment to voluntary preservation.  The goals of the project were to upgrade and modernize the campus to meet or exceed current building safety and energy codes, while maintaining and restoring the original character of the historic structures.  Sustainability, security, performance, and history were considered with each step.

The process began with research and review of archival material in order to develop a good understanding of each building’s unique history.  The curtain wall of the stately Secretariat building had significantly deteriorated and was altered by poorly matched replacement glass, patches on the aluminum mullions, layers of remedial caulking, and highly reflective solar control film.  In addition to corrosion of the exterior aluminum, less apparent deterioration was discovered through a rigorous inspection program from suspended scaffolding that included forensic probes.  From a statistically meaningful sampling, a large percentage of curtain wall anchors were found to be compromised by corrosion to the extent that they could not provide adequate support against the wind loads required by current building code. 

Beyond the deteriorated condition of the facades, the U.N. faces a very different security landscape today than it did in 1949.  This was addressed by the incorporation of stringent security requirements for the design of the renovated facades.  Another challenge that arose was a broad gap between the thermal performance of the original building envelopes and current standards of energy efficiency.  Additionally, the stonework in many locations had moderate damage from exposure to freeze/thaw cycles, urban pollution, and a mildly acidic environment.

Our work on the Secretariat curtain wall highlights the dilemma of purist preservation standards versus contemporary performance expectations.  We decided to pursue a faithful reconstruction that is compatible with the aesthetic ideals and original intent of its designers, while modifying the design to meet modern standards.

Technical Interventions

Glass design and specification for the new high performance, low-e coated, insulating glass entailed a 12-month process that began with computer simulations of the original single-pane glass from measured spectral data.  Optical properties were compared for dozens of potential combinations of insulating glass with lamination and high-performance coatings.  Samples were reviewed at various sky-dome and time-of-day conditions to ensure there were no unforeseen color shift polarizing effects from the coatings, and the final selection was based on a full-size mock-up erected in front of the Secretariat building.  The new curtain wall has returned the Secretariat to its original tint and transparency.  

The replacement includes thermally broken extruded aluminum framing and dual seal, pressure-equalized systems which were unitized and pre-assembled off site.  This state-of-the-art curtain wall performs substantially better than the single seal, thermally unbroken original with its steel channels and extruded aluminum cladding.  The original wall had been pre-assembled into story-height “ladders” prior to installation and then infilled with Aklo blue-green tinted wired glass at spandrels and aluminum window sashes with single-pane PPG Solex green tinted glass at the vision zones. 

Unlike the Secretariat’s original curtain wall, which had been attached to the concrete floor slabs, the new system utilizes outrigger plates to connect the wall to the now-reinforced building frame.  The aluminum profiles adhere as closely as possible to the original, with slight modifications in mullion depth to accommodate the thicker insulating glass, and structural requirements.  The finish of many of the original anodized aluminum extrusions attempted to emulate brushed stainless steel.  Various sample trials were undertaken to identify a contemporary finish that would best match the original, and a clear anodized finish was developed with close quality control to include faint die lines that mimic the original “brushed” appearance. 

At the General Assembly building, the original north façade alternates vertical bands of marble and single-pane patterned glass, which was a custom photo-sensitive product developed by Corning Glass.  This unique product, referred to in historical documents as “Glass Ceramic Fotoform,” was unable to be replicated.  Heintges & Associates worked to develop a micro-patterned glass (usually produced for the photovoltaic industry) to replicate the surface texture of the original product.  This patterned glass was combined with acid etching and custom-patterned ceramic frits to replicate the original grid densities.  The thickness of the glass was increased to meet security and energy performance criteria, and the final prototype was tested and approved on site and successfully replicates the essence of the original.

Lastly, the renovation of the Conference Building required close coordination to accommodate its five bespoke curtain wall types, including a long-span canted curtain wall with integral door portals.  All of the wall types were replaced with new framing and glass that faithfully matches the profiles and appearance of the original design.

With the expertise of Integrated Conservation Resources and Atkinson-Nolan & Associates, a conservation approach was adopted for stone restoration and stabilization.  Solutions were developed to be physically and aesthetically compatible with the original stonework.  The stonework on the facades involved cleaning, repointing, resetting, Dutchmen and composite repairs, pinning, installation of new anchors, and the design and installation of new flashing.  This approach respected the intent of the original design.

Conclusion

An unprecedented preservation opportunity was met with solutions that set new standards for the renovation of early modern aluminum and glass curtain wall buildings.  The design team’s knowledge of materials, fabrication techniques, and procurement strategies enabled them to successfully realize solutions to the complex challenges presented by this project.  The result maintains the original appearance of the United Nations Campus, with restored stone and new glazed facades that incorporate a broad spectrum of contemporary performance criteria.