By Russell M. Sanders, AIA and Craig A. Hargrove, AIA LEED AP
Glass curtain walls consist of two basic components: the glass and the frame. How these two building structure elements are manipulated, including the proportion, properties, and anchorage of each, constitutes the essential distinction among individual curtain wall systems.
Composed of steel, aluminum, multi-laminate glass, or other resilient material, the frame is the support grid that holds the glass in place.
Stick systems are curtain walls at their most basic, with individual mullions, or framing elements, assembled in the field.
Unitized systems apply the same design principles as stick systems, but sections of the curtain wall are assembled in the shop and installed as a unit.
Unit mullion systems combine the pre-assembled panels of unitized systems with the multi-story vertical mullions of stick systems. Upright mullions are installed first, with horizontal mullions and glazing installed as a unit.
Column cover and spandrel systems articulate the building frame by aligning mullions to structural columns. Pre-assembled or field-assembled infill units of glass or opaque panels are fitted between the column covers.
Point-loaded structural glazing systems eliminate visible metal framework by incorporating tension cables, trusses, glass mullions, or other custom support structures behind the glass panels.Glazing is anchored by brackets or by proprietary hardware embedded in the glass.
Curtain wall glazing ranges in price, durability, impact resistance, safety, and stability, depending upon the manufacturing process.
The most common types are:
Float glass was developed in the 1950s by Alastair Pilkington, whose breakthrough float process enabled production of the large glass sheets that characterize curtain wall construction. Molten glass is fed into a bath of tin, where it flows along the surface, forming smooth glass with even thickness.
Annealed glass undergoes a controlled heating and cooling process that improves its fracture resistance. Despite its improved durability, annealed glass can break into sharp pieces, and many building codes limit its use in construction.
Tempered glass is chemically or thermally treated to provide improved strength and shatter resistance. On impact, tempered glass shatters into tiny pieces that are less likely to cause injury than are larger shards.
Heat-strengthened glass and chemically strengthened glass fall between annealed and tempered glass in terms of strength. Unlike tempered glass, strengthened glass can be sharp when shattered, so it is best suited to areas with limited access. Scratches in strengthened glass have also been shown to compromise its strength.
Laminated glass bonds two or more sheets of glass to an interlayer of plastic, generally polyvinyl butyral (PVB), which holds the glass in place if broken. Laminated glass is often specified for curtain walls in hurricane-prone regions or in areas requiring blast protection.
Insulating glazing units (IGUs) improve thermal efficiency and performance through the use of double or triple panes of glass, separated by a space that is filled with air or with an inert gas.
Spandrel glass, which is darkened or opaque, may be used between the head of one window and the sill of the next. To create the illusion of depth at spandrel areas, transparent glass may be used in a shadow box, with a metal sheet at some distance behind the glass.
For all their variation, glass curtain walls retain the same basic components as they did when Willis Polk first envisioned them in the early 1900s. Understanding the composition of glass and frame, as well as the relationship between them, is vital to maximizing the longevity of a glazed curtain wall system and building structure.
Russell M. Sanders, AIA is Executive Vice President and Senior Director of Technical Services with Hoffmann Architects, Inc., an architecture and engineering firm specializing in the rehabilitation of building exteriors. Sanders is responsible for the technical rigor of project documents produced by the firm. Craig A. Hargrove, AIA LEED AP, Senior Vice President and Director of Architecture with Hoffmann Architects, manages the firm’s New York City office and has extensive experience in the design and rehabilitation of glazed curtain walls.