Thermal stress is one of many factors that can cause glass to break, even when it is not impacted by a projectile.
Such failures usually are not a testament to the quality of the glass, but rather the result of temperature imbalance within the glass, which can occur when the center of a window or insulating glass unit (IGU) heats and expands while the edges remain cool.
The resulting thermally induced tensile stress on the glass edges can exceed the strength of the glass itself, causing it to break at approximately a 90-degree angle.
Working closely with the IGU fabricator and glass supplier, and conducting a thermal stress analysis at the beginning of the design phase can help architects specify the proper glass for their projects. Here are five thermal stress related rules-of-thumb to consider when specifying glass:
1. Clear or color? Glass can be clear, ultra-clear or tinted in colors such as gray, blue, green and bronze. Tinted glasses absorb solar radiation more than clear glass, which can cause them to collect heat and make them more susceptible to break from thermal stress.
2. Inner or outer surface? Reflective and low-emissivity (low-e) coatings, which improve the solar performance of glass primarily by reflecting solar radiation, can be placed on any one of four surfaces in a dual-pane IGU. While they are usually placed on the inner glass surfaces of the IGU, the coating orientation and the associated risk of thermal stress has to be taken into account.
3. Sunny or shady? Outdoor shading, including overhangs, adjacent buildings, and trees, is one of the most dynamic elements to consider when analyzing thermal stress. Minimizing locations where non-uniform shading of IGUs can occur will help avoid extreme temperature gradients. Interior shading devices, such as blinds or drapes, can increase glass temperature by reflecting solar radiation back through the glass or by reducing the convection and conduction of heat away from the glass. To minimize thermally induced edge stress, the air space around the window glass should be ventilated. There should be a gap of several inches between the glass and shades, blinds or drapes.
4. Where to place a vent? If heating vents, registers and grilles point directly at glass units, warm air will cause the glass to heat up and, under certain conditions, break. Make sure vents are carefully placed to reduce thermal stress risk and avoid placing them between the glass and interior shading devices.
5. How to frame it? Glass framing systems that have low heat capacity also can minimize the chance of a thermal stress break. Structural gaskets and narrow metal framing are favorable because they have less effect on the glass temperature around the edges. Conversely, massive framing – whether metal, masonry or even wood – can have more of an effect, resulting in thermal imbalance and risk of glass breakage.
To learn more about avoiding thermal breakage of insulated glass units and other glass-related topics, visit the PPG Glass Education Center at www.educationcenter.ppg.com.
Related Stories
| May 7, 2013
First look: Golden State Warriors stadium by Snøhetta, AECOM
Architects Snøhetta and AECOM have revealed their latest renderings of a new stadium for NBA basketball team the Golden State Warriors on the waterfront in San Francisco.
| May 7, 2013
Renovated bridge building will anchor Nashville riverfront master plan
Renovations to the former Nashville Bridge Company building were recently completed, including a newly-built modern wing. The facility has been re-dubbed The Bridge Building and now offers spaces for meetings, parties, weddings, and other events.
| May 7, 2013
First look: Adrian Smith + Gordon Gill skyscraper designed to 'confuse the wind'
The 400-meter-high, 116-story Imperial Tower in Mumbai will feature a slender, rounded form optimized to withstand the area's strong wind currents.
| May 6, 2013
SAFTI FIRST announces 3D Autodesk Revit models for fire rated wall, window, and door systems
SAFTI FIRST, leading USA-manufacturer of fire rated glass and faming systems, is proud to announce that Autodesk Revit models are now available for its fire rated walls, window and door systems via www.safti.com and Autodesk Seek.
| May 6, 2013
7 major multifamily residential projects in the works
A $140 million redevelopment of a landmark, 45-building apartment complex in Los Angeles is among the nation's significant multifamily developments under way.
| May 3, 2013
5 common failures in paints and coatings
As experienced designers, contractors, and owners know, most paint and coating problems are correctable, but some are especially stubborn to address. Here is a partial compendium of typical failure modes and methods for addressing the problem.
| May 3, 2013
'LEED for all GSA buildings,' says GSA Green Building Advisory Committee
The Green Building Advisory Committee established by the General Services Administration, officially recommended to GSA that the LEED green building certification system be used for all GSA buildings as the best measure of building efficiency.
| May 3, 2013
SMPS Foundation accepting applications for Ron Garikes Student Scholarship
The SMPS Foundation is now accepting scholarship applications from eligible students majoring in marketing, communications, or public relations who are planning a career in professional services marketing in the architecture, engineering, and construction industry.