WINDOW TECHNOLOGIES: Assembly
Frame Types: Curtain Walls and Storefronts
One basic method of designing the glazing and frame connection retaining glazing infill in a curtain wall is to use pressure plates (see figure to the right). The glass is held in place by a metal pressure plate on the outside which is attached to the metal framing members on the interior. An exterior snap-on cap is used to conceal pressure plate fasteners. Pressure plate fasteners really have little impact on u-factor or condensation.
Synthetic rubber profiles can also be used to hold glazing units in place (see figure to the right).
As a means of minimizing the exterior portion of the frame for aesthetic reasons, structural sealant glazing was developed (see figure below). Silicone adhesives permit the glass to be structurally adhered directly to the interior frame. The silicone carries some of the weight of the glass itself and transfers wind pressure to the frame. All that appears on the outside are the glazing panes with narrow, sealed joints in between.
Another innovation in curtain wall construction is the use of suspended glazing with patch fittings. Point-fixing is an evolution of this curtain wall concept, with various systems developed. Typically, holes are drilled through the glass and bolts or screws attach the glass to the interior frame structure (see figure below). Unlike patch fittings, which project beyond the glazing plane and clamp adjacent panes together, point fittings are held within the plane of the glazing itself. To stiffen glass curtain walls while maintaining the light appearance of the structure, glass fins or networks of truss cables are used. Cable net structures can be used to cover atrium spaces with minimal structure within the glazed opening, instead relying on connections to a substantial building structure at the opening's periphery.
The metal frames in a curtain wall design represent a potential path for heat transfer that can result in increased energy use, discomfort and condensation. Thermal breaks in the aluminum frames are designed to address these problems. However, it is still possible to defeat at least marginally effect the thermal break with bolts and other connections. In addition to careful design of details to prevent thermal bridges, minimizing or eliminating metal frames on the exterior of the glazing is another solution, if the warm edge spacers employed in insulating glass construction are suitable for use in such structurally-glazed applications.