The Metal And Glass Curtain Wall And Its Deterioration Mechanisms

New technologies resulting from World War II had a great influence on the acceptance of the machine-made metal and glass curtain wall. Given the abundant postwar supply, aluminum was reasonably priced. There was experimentation with mild steel, stainless steel, and bronze as well. Extruded components were suitable for standardization and could be prefabricated for delivery to the site. This was important because labor had become a significant part of construction costs. The new curtain wall technology further decreased building weight and construction cost, and increased usable floor area. Prefabricated construction was less limited by cold temperatures which prohibited erection of "wet" walls of brick and mortar. The invention

Figure 4. The Lever House in New York where the prefabricated metal and glass curtain wall realized its potential (SOM, 1952).

and development of float glass by Alistair Pilkington in the 1950s would make large panes of glass afford-ably available. American architectural philosophers of the day lauded the fact that craftsmanship had been transplanted from the site to the factory.

One of the first post-war buildings to be constructed with a glass curtain wall was the Equitable Building (Pietro Belluschi, 1948) in Portland, Oregon. Belluschi was able to take advantage of leftover aluminum stockpiled for World War II by smelters and to utilize assembly techniques derived from West Coast airplane plants. The 860-880 Lake Shore Drive buildings in Chicago (Figure 1) were among the first residential buildings in the United States to be sheathed entirely in glass, and were the realization of Mies' 1920 proposal for a glass skyscraper. The steel, aluminum, and glass skin was assembled on the buildings' roofs in two story high units, and then lowered into place on the facade.

At the Lever House (Figure 4), the curtain wall has an interior frame of mild steel clad with stainless steel. At the United Nations Secretariat Building (Harrison and Abramovitz, 1950), curtain walls were conceived as an assembly of aluminum windows held in place with a grid of reinforced mullions. At both buildings, the lower portion of the curtain wall at each level was backed up by a concrete masonry wall to provide the fire rating that code officials felt was not provided by the curtain wall. The masonry wall could not be immediately abandoned though it became hidden from view.

The approach to curtain wall design that quickly evolved was to make the joints as weathertight as possible, then provide positive means for conducting any water leakage out of the wall. Thus an interior drainage system was provided to collect water that leaks through the cladding and direct it back to the exterior.

The development of new curtain wall materials occurred in the post-war years: thin stone veneers, precast concrete, brick veneers, and structural silicone glazed facades. To bring things full circle, the aesthetic development of "Post-Modernism" led to a return to earlier architectonic forms, but not a return to earlier methods of construction.

Metal and glass curtain walls that we care for today consist of factory-fabricated and preassembled metal units that are connected to the structural frame. Glass has gone through radical technical developments and is typically no longer a monolithic material. In a curtain wall it can appear as an insulated glass unit or a sandwich of materials developed to strengthen it. It may have clear, colored, or reflective coatings installed on one or more of its surfaces or may have transparent or translucent colorants integral with the glass. The assemblies may also include panels of aluminum, ceramics, precast concrete, or stone.

Though not the vision of 1950s designers, metal and glass curtain walls are wholly reliant on sealants to perform adequately. High performance sealants include newly developed families of elastomeric sealants such as polysulfides, solvent acrylics, urethanes, and sili-cones. Numerous sealant products are now available and have been developed to be used in either curtain wall glazing, installation, or repair.

Modern structural frames are more flexible because they are designed to tighter limits with less material and are more exposed to temperature extremes than the frames of a masonry-clad building. Prefabricated curtain wall units are detailed to accommodate these increased movements. However, lateral movements of the frame and differential movement between the frame and the cladding can lead to distress in the glass and metal curtain wall.

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