Curtain walls are non-structural exterior building walls. They protect the interior of the building from the elements but since they carry no structural load beyond their own dead-load weight, they can be made of lightweight materials. They transfer lateral wind loads to the main building's structure through connections at floors or columns. Curtain walls may be designed as "systems" that integrate frame, wall panel, and weatherproofing materials. Steel frames have largely given way to aluminum extrusions. Glass is typically used for infill because it can reduce construction costs, provides a pleasing look, and allows natural light to penetrate deeper within the building. However, glass makes the effects of light on visual comfort and solar-heat gain in a building more difficult to control. Other common infills include stone veneer, metal panels, louvres, and operable windows and vents. Unlike storefront systems, curtain wall systems are designed to span multiple floors, taking into consideration building sway and movement and design requirements such as thermal expansion and contraction; seismic requirements; water diversion; and thermal efficiency for cost-effective heating, cooling, and interior lighting.
History
Historically, buildings were constructed of timber, masonry, or a combination of both. Their exterior walls were load-bearing, supporting much or all of the load of the entire structure. The nature of the materials resulted in inherent limits to a building's height and the maximum size of window openings. The development and widespread use of structural steel and later reinforced concrete allowed relatively small columns to support large loads. The exterior walls could be non-load bearing, and thus much lighter and more open than load-bearing walls of the past. This gave way to increased use of glass as an exterior façade, and the modern-day curtain wall was born. Post-and-beam and balloon framed timber structures effectively had an early version of curtain walls, for their frames supported loads that allowed the walls themselves to serve other functions, such as keeping weather out and allowing light in. When iron began to be used extensively in buildings in late 18th-century Britain, such as at Ditherington Flax Mill, and later when buildings of wrought iron and glass such as The Crystal Palace were built, the building blocks of structural understanding were laid for the development of curtain walls. Oriel Chambers (1864) and 16 Cook Street (1866), both built in Liverpool, England, by local architect and civil engineer Peter Ellis, are characterised by their extensive use of glass in their facades. Toward the courtyards they boasted metal-framed glass curtain walls, which makes them two of the world's first buildings to include this architectural feature. Oriel Chambers is listed in the Guinness Book of Records as the earliest such building. The extensive glass walls allowed light to penetrate further into the building, utilizing more floor space and reducing lighting costs. Oriel Chambers comprises 43,000 sq ft (4,000 m2) set over five floors without an elevator, which had only recently been invented and was not yet widespread. The Statue of Liberty (1886) features a thin, non-load-bearing copper skin. Extensive use of glass became required for large factory buildings to allow light for manufacture, sometimes making it seem like they had all glass facades. An early example of an all-steel curtain wall used in the classical style is the Kaufhaus Tietz department store on Leipziger Straße, Berlin, built in 1901 (since demolished). Some of the first curtain walls were made with steel mullions, and the polished plate glass was attached to the mullions with asbestos- or fiberglass-modified glazing compound. Eventually silicone sealants or glazing tape were substituted for the glazing compound. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The landmarks of curtain wall design as it came to dominate construction were the very different systems used by the United Nations Headquarters and the Lever House completed in 1952. Ludwig Mies van der Rohe's curtain wall is one of the most important aspects of his architectural design. Mies first began prototyping the curtain wall in his high-rise residential building designs along Chicago's lakeshore, achieving the look of a curtain wall at 860-880 Lake Shore Drive Apartments. He finally perfected the curtain wall at 900–910 Lake Shore Drive, where the curtain is an autonomous aluminum and glass skin. After 900–910, Mies's curtain wall appeared on all of his subsequent high-rise building designs, including the Seagram Building in New York. The widespread use of aluminium extrusions for mullions began during the 1970s. Aluminum alloys offer the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease. The Omni San Diego Hotel curtain wall in California, designed by architectural firm Hornberger and Worstel and developed by JMI Realty, is an example of a unitized curtain-wall system with integrated sunshades.
Systems and principles
Stick systems The vast majority of ground-floor curtain walls are installed as long pieces (referred to as sticks) between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop, but installation and glazing is typically performed at the jobsite.
Ladder systems Very similar to a stick system, a ladder system has mullions which can be split and then either snapped or screwed together consisting of a half box and plate. This allows sections of curtain wall to be fabricated in a shop, effectively reducing the time spent installing the system onsite. The drawbacks of using such a system is reduced structural performance and visible joint lines down the length of each mullion.
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