Saturday, June 23, 2007

Curtain walls

Curtain walls

Traditionally, the wall of a building served both structural and environmental purposes. That is, it carried to the ground the weight of the building and its contents and, while admitting air and light through openings, protected the interior from extremes of weather, noise, and other undesirable intrusions. The introduction of structures in which the loads are carried by beams and columns liberated the wall from load bearing, allowing it to function solely as an environmental filter—a relatively thin, light curtain, so to speak. This was first seen in the later medieval cathedrals with their vast stained-glass windows, but it would not be widely developed until the nineteenth century, with the advent of metal-framed architecture and, subsequently, reinforced concrete. The metal-and-glass membrane supported by the building frame, known as the curtain wall, is principally associated with multistory office buildings after about 1880.

Seagram Building, New York City; Ludwig Mies van der Rohe, architect, 1954–1958. Exterior, photographed in 1997.

Although the first skyscrapers, such as the Rookery (1885–1886) and Monadnock Building (1889–1891), both in Chicago and both designed by architects Burnham and Root, had thick conventional load-bearing walls, the twin economic necessities of getting buildings up quickly and optimizing the quantity and quality of interior space soon led to buildings whose outer walls consisted almost entirely of windows supported by perimeter columns and beams. This was a first step toward the development of a true curtain wall, that is, a continuous wall in front of the structural frame. The earliest example was Albert Kahn’s Packard Motor Car Forge Shop in Detroit (1905). A curtain of glass in steel frames allowed more space

and light in the factory, just as it would in an office tower, and Kahn again employed it for the Brown-Lipe-Chapin gear factory (1908) and the T-model Ford assembly plant in Highland Park, Michigan (1908–1909). This rational industrial architecture drew the admiration of Europe and was emulated in Peter Behrens’s A. E. G. Turbine Factory (1909–1910) in Berlin and Gropius and Meyer’s Fagus Works in Alfeld-an-der-Leine, Germany, of 1911.

It is widely accepted that the first office block with a curtain wall was Willis Jefferson Polk’s eight-story Hallidie Building (1917–1918) in San Francisco. Although it was cluttered in places with florid cast-iron ornament, the street facade, suspended 3 feet 3 inches (1 meter) in front of the structure by brackets fixed to cantilevered floor slabs, presented an unbroken skin of glass. Elsewhere, others dreamed of crystal prisms in which the building’s whole external membrane was glass: the serried towers of H. Th. Wijdeveld’s Amsterdam 2000 (1919–1920) and Le Corbusier’s Ville Contemporaine (1922) and—probably best known—the skyscrapers Ludwig Mies van der Rohe projected between 1919 and 1923. But dreams and visions they remained, because the technology was not yet available to turn them to reality. One exception was the A. O. Smith Research Building in Milwaukee (1928–1930) by Holabird and Root, the first multistory structure with a full curtain wall (rather than a single facade) of large sheets of plate glass supported on aluminum frames.

Spin-offs from defense technologies after World War II paved the way for tall curtain wall buildings. Important among them was cost reduction in the production of aluminum, whose corrosion resistance could be improved by a process known as anodizing. This lightweight metal could be extruded into the complicated profiles needed to frame the glass and strengthen the wall against wind loads. Reliable cold-setting synthetic rubber sealants had also become more widely available. These advances were combined with more efficient sheet glass manufacture, especially polished cast glass and, after 1952, the much flatter float glass. Wall elements could be fabricated off-site to exacting tolerances and then transported, assembled, fixed, and glazed with none of the “wet” processes that impede building contracts. Relevant engineering developments included reverse-cycle air-conditioning—available since 1928—and fluorescent lighting, first demonstrated at the 1938 Chicago World’s Fair. All these technologies were exploited in Pietro Belluschi’s twelve-story Equitable Building in Portland, Oregon (1944–1948), described by one historian as “an ethereal tower of sea green glass and aluminum.” Another writer asserts that it “set styles for hundreds that came after.”

The thirty-nine-story United Nations Secretariat Building in New York City followed in 1947–1952. The final design was developed from a proposal by Le Corbusier, and Wallace Harrison acted as executive architect in consultation with him. The curtain walls of the Secretariat Building’s east and west facades are all glass, cantilevered 27 inches (80 centimeters) from the line of the perimeter columns; black-painted glass spandrels hide the between-floor spaces. The blue-green tinted windows are of “Thermopane,” a special glass that absorbs radiant heat, preventing it from reaching the interior, thus reducing the load on the air-conditioning system. The only breaks in the sheer curtain wall are full-width air-conditioning intake grilles at four levels. Because of its innovation, and no doubt because of its associations, the U.N. Secretariat, together with Mies van der Rohe’s Lake Shore Drive Apartments (1951) in Chicago and Skidmore, Owings, and Merrill’s Lever House (1952) on Park Avenue, New York, contributed to the universal standard for high-rise buildings.

The latter building, a twenty-four-story, green-tinted glass and stainless steel tower, designed by Gordon Bunshaft, marked a change of direction in American corporate architecture and in the way New Yorkers built. In keeping with the wishes of a client who made household cleaning products, Bunshaft produced an immaculate, clean-lined tower. The architectural critic Lewis Mumford called it “an impeccable achievement.” The top three floors are reserved for mechanical services. A mobile gantry carries a window cleaners’ platform that serves all faces of the building; such devices became standard for the curtain wall office buildings that followed. Lever House was the first skyscraper to exploit the allowable plot ratios in city planning regulations. By

occupying only a quarter of the site, it allowed much more natural light to enter the offices than conventional stepped-back skyscrapers that covered the whole allotment. Lever House is a New York historic landmark, and in November 1999 a $10.7 million contract was let to renovate its curtain walls, designed by Skidmore, Owings, and Merrill under the supervision of the New York City Historical Society.

That leads us to the inherent problems in curtain wall construction, for all of its advantages. In forty-five years, the pristine facades failed in a number of ways—water penetration and consequent damage, corrosion, and broken glass panels. Since their inception, curtain wall systems have been continually revised, most changes geared toward reducing weight while retaining strength. Stiffened sheet aluminum, enameled steel laminated with insulation, and later even thin sheets of stone were used for spandrel panels. The design of joints—problem spots for leaks—was improved and more durable sealants were invented. More recently, the availability of reliable adhesives has allowed architects to indulge in so-called “fish tank” joints between glass panels, doing away with framing bars. Glass technology has also been refined. Double glazing, first manufactured in the 1940s, improves both the sound and thermal insulation of curtain walls. Heat-absorbing glass, already available in the 1950s, evolved in the following decade into reflective glass with thin metallic coatings, also used to reduce heat gain within buildings. In 1984 heat mirror glass was developed; when combined with double glazing, its insulating value approaches that of masonry, but the esthetic effect seems to be a denial of the form of the building: all it does is reflect what’s around it.

Given that the two significant advantages of curtain wall construction are the reduction of weight and speed of erection, it might be concluded that it costs less than conventional work. That is not necessarily true, because its behavior as an environmental filter, especially in relation to heat flow, may result in higher air-conditioning costs. Often, the preciousness of the architect’s detailing increases costs, as evidenced by Mies van der Rohe’s bronze-and-brown-glass Seagram Building (1954–1958) in New York City. It cost $36 million, approximately twice as much as office towers normally did.

The tall glass prism was the major contribution of the United States to the so-called International Style of modern architecture. But its glorious day passed with the rise of postmodernism, and the crystal towers that Frank Lloyd Wright dismissed as “glass boxes on stilts” were replaced with less anonymous designs. Even Philip Johnson, Mies van der Rohe’s most ardent disciple, forsook the minimalist forms of curtain-wall architecture in favor of a more congenial architecture.

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