A curtain wall is defined as thin, usually aluminum-framed wall, containing in-fills of glass, metal panels, or thin stone. The framing is attached to the building structure and does not carry the floor or roof loads of the building. The wind and gravity loads of the curtain wall are transferred to the building structure, typically at the floor line. Aluminum framed wall systems date back to the 1930′s, and developed rapidly after World War II when the supply of aluminum became available for non-military use.

Curtain wall systems range from manufacturer’s standard catalog systems to specialized custom walls. Custom walls become cost competitive with standard systems as the wall area increases. This section incorporates comments about standard and custom systems. It is recommended that consultants be hired with an expertise in custom curtain wall design for projects that incorporate these systems.

DESCRIPTION

The following are brief descriptions of commonly used curtain wall framing methods and components.

Curtain walls can be classified by their method of fabrication and installation into the following general categories: stick systemsand unitized (also known as modular) systems. In the stick system, the curtain wall frame (mullions) and glass or opaque panels are installed and connected together piece by piece. In the unitized system, the curtain wall is composed of large units that are assembled and glazed in the factory, shipped to the site and erected on the building. Vertical and horizontal mullions of the modules mate together with the adjoining modules. Modules are generally constructed one story tall and one module wide but may incorporate multiple modules. Typical units are five to six feet wide.

Curtain walls can also be classified as water managed or pressure-equalized systems. See Moisture Protection below.

Both the unitized and stick-built systems are designed to be either interior or exterior glazed systems. Interior and exterior glazed systems offer different advantages and disadvantages. Interior glazed systems allow for glass or opaque panel installation into the curtain wall openings from the interior of the building. Details are not provided for interior glazed systems because air infiltration is a concern with interior glazed systems. Interior glazed systems are typically specified for applications with limited interior obstructions to allow adequate access to the interior of the curtain wall. For low rise construction with easy access to the building, outside glazing is typically specified. For high-rise construction interior glazing is sometimes used due to access and logistics of replacing glass from a swing stage.

In exterior glazed systems, glass and opaque panels are installed from the exterior of the curtain wall. Exterior glazed systems require swing stage or scaffolding access to the exterior of the curtain wall for repair or replacement. Some curtain wall systems can be glazed from either the interior or exterior.

Typical opaque panels include opacified spandrel glass, metal panels, thin stone, and other materials, such as terra cotta or FRP (fiber-reinforced plastic).

Vision glass is predominantly insulating glass and may have one or both lites laminated (see Glazing), usually fixed but sometimes glazed into operable window frames that are incorporated into the curtain wall framing.

Spandrel glass can be monolithic, laminated, or insulating glass. The spandrel glass can be made opaque through the use of opacifiers (film/paint or ceramic frit) applied on an unexposed surface or through “shadow box” construction, i.e., providing an enclosed space behind clear spandrel glass. Shadow box construction creates a perception of depth behind the spandrel glass that is sometimes desired.

Metal panels can take various forms including aluminum plate, stainless steel or other non-corrosive metal, thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer, or panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel.

Thin stone panels are most commonly granite. White marble should not be used due to its susceptibility to deformation due to hysteresis (thin stone is not covered in this chapter).

The curtain wall often comprises one part of a building’s wall system. Careful integration with adjacent elements such as other wall claddings, roofs, and base of wall details is required for a successful installation.