Glazing Systems
Framed Systems
Framed systems support the glass continuously along two or four sides. There are many variations of framed systems, most of which fall into two general categories. Conventional unitized curtain wall systems are seldom used with structural glass facades.
Veneer
Truss systems can be designed with an outer chord of square or rectangular tubing, and may include transom components of similar material, presenting a uniform flat grid installed to high tolerances. Such a system can provide continuous support to the simplest and most minimal off-the-shelf glazing system, thus combining relatively high transparency with excellent economy. A veneer glazing system is essentially a stick-built curtainwall system designed for continuous support and representing a higher level of system integration with resulting efficiencies. Variations can include 4-sided capture, 2-sided capture, structurally glazed and unitized systems.
Orange County Performing Arts Center, Eskind Biomedical Library
Point-fixed Bolted
The most popular glass system — and frequently the most expensive — for application with structural glass facades is the bolted version. The glass panel requires perforations to accommodate specialized bolting hardware. Specially designed off-the-shelf hardware systems are readily available, or custom components can be designed. Cast stainless steel spider fittings are most commonly used to tie the glass to the supporting structure, although custom fittings are often developed for larger facade projects. The glass must be designed to accommodate bending loads and deflections resulting from the fixing method. An insulated-laminated glass panel as required for overhead applications will require the fabrication of 12 holes per panel, which can represent a cost constraint on some projects.
Cathedral of Christ the Light,
Lloyd D. George Federal Courthouse, Newseum,
Suvarnabhumi Airport Bangkok
Stick-built glass facades
Stick-built glass facades are a method of curtainwall construction where much of the fabrication and assembly takes place in the field. Mullions of extruded aluminum may be prefabricated, but are delivered as unassembled “sticks” to the building site. The mullions are then installed onto the building face to create a frame for the glass, which is installed subsequently. Economical off-the-shelf stick curtain wall products are available from various manufacturers that may be suitable for application in structural glass facades, primarily on truss systems.
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Panel/Cassette
Panel systems are typically constructed of a framed glass lite. The framed panel can then be point-supported by a supporting structural system, while the glass remains continuously supported on two or four sides. This also allows the panel to be stepped away from the support system — a practice that tends to visually lighten the facade. Panel systems can be prefabricated, benefiting from assembly under factory-controlled conditions.Cassette systems combine properties of stick, veneer and panel systems. While variations exist, the predominant makeup of a cassette system is comprised of a primary structural mullion system, which is stick built. These provide the support and facilitate the attachment of the glass panels. The glass lites are factory assembled into minimal frames, which form an integral connection with the primary mullion system. A cassette system can be designed to be fully shop-glazed, requiring no application of sealant during field installation.
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Frameless Systems
Frameless systems utilize glass panes that are fixed to a structural system at discrete points, usually near the corners of the glass panel (point-fixed). The glass is directly supported without the use of perimeter framing elements. The glass used in point-fixed applications is typically heat-treated.
Point-fixed Clamped
Point-fixed clamped systems accomplish the point fixing without the requirement for perforations in the glass. In the case of a spider type fitting, the spider is rotated 45 degrees from the bolted position so that its arms align with the glass seams. A thin blade penetrates through the seam between adjacent pieces of glass. An exterior plate attaches to the blade and clamps the glass in place. The bolted systems present an uninterrupted glass surface, while the clamped systems expose the small exterior clamp plate. Some facade designers prefer the exposed hardware aesthetic. While the clamped systems have the potential for greater economy by eliminating the need for glass perforations, the cost of the clamping hardware may offset at least some of this savings depending upon the efficiency of the design.
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Weather Seal
The weather seal in most structural glass facade systems is provided by a field applied butt-glazed silicone joint. This technique provides a reliable and durable weather seal if simple procedures are followed during installation. An advantage of this sealing strategy is that any leaks, usually caused by installation errors, are easily detected and repaired. The joint design is critical, and is largely a function of the glass makeup and thickness. Compatibility between the field-applied silicone and the interlayer, if using laminated glass, or the edge seal in the case of an IGU, must be confirmed with the silicone material provider. The provider should also be consulted about the joint design. Craftsmanship is critical for the field application of the sealant to assure a visually satisfactory result.
