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Technical Resources Design Considerations Design professionals should be aware of the following considerations when selecting and specifying heat-treated glass products. · Architectural glass fabricators should be consulted to confirm the ability of the specified glass construction to meet the design parameters. Thermal and mechanical stresses in glass, as well as glass deflection, must be reviewed to ensure a successful application. While a heat-treated lite of glass may meet the design windload, the application may yield a glass deflection that would be psychologically discomforting to persons near the glass. Typically, it is recommended that glass deflection of 1 inch (25 mm) or more be called to the attention of design professionals and building owners for consideration of occupant comfort levels. · The stiffness of annealed, heat-strengthened, and fully tempered glass is the same. Deflection under a given uniform windload will be identical for glass of the same size and thickness. Some glazing applications require thicker glass in order to limit deflection. · Heat-strengthened and fully tempered glass cannot be cut, drilled, or edged after being heat-treated. Sandblasting, etching, or V-grooving should be executed before the heat-treating process. Sandblasting, etching, or V-grooving, if done after the heat-treating process, will reduce the thickness of the compression layer and thus reduce the strength of the lite of glass. Some deep patterns of rolled glass cannot be heat-treated. · When viewing heat-treated glass in certain conditions, a pattern of iridescent spots or darkish shadows may become visible. This is called the strain pattern of the glass and is related to the stresses introduced in the cooling process. Sharp angles, polarized light, thicker glass, and applied coatings increase the visibility of the pattern. · Bow, warp, roll distortion, and strain pattern are inherent characteristics of heat-treated glass. While fabricators take steps to minimize these conditions, they cannot be eliminated. Consult ASTM C 1048 for additional information. · All of these inherent characteristics are accentuated by the application of coatings to the glass. The visibility of distortion in solar-control or "reflective" coated glass is greatly affected by surrounding conditions. If the reflected image is of a uniform blue sky, the glass will appear extremely flat. If the same lite of glass is reflecting the multiple gridlines of an adjacent building, the reflection may appear to be distorted. Surrounding buildings and the level of glass reflectance should be reviewed as a stage of the design process. Viewing full size mock-ups, under typical job conditions and surrounding landscape, is highly recommended for user evaluation and expectation of reflective distortion. The mock-up glass should be retained for future reference. This material has been excerpted from the Glass Association of North America (GANA) Glazing Manual. Information about purchasing the Glazing Manual is available by calling GANA at 785/271-0208, or at its Web site www.glasswebsite.com/gana. The manual provides extensive industry reference information including sections on primary glass products, fabricated architectural glass, reference standards, glass design considerations and extensive glazing guidelines. Source: The Glass Guide for Architects and Specifiers, Glass Magazine, June 2001. |
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