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 2, 2012
Trimble acquires SketchUp 3D modeling platform
The transaction is expected to close in the second quarter of 2012.
| May 2, 2012
Building Team completes two additions at UCLA
New student housing buildings are part of UCLA’s Northwest Campus Student Housing In-Fill Project.
| May 2, 2012
Public housing can incorporate sustainable design
Sustainable design achievable without having to add significant cost; owner and residents reap benefits
| May 2, 2012
SMPS Foundation accepting applications for Garikes Scholarship
One outstanding scholar will be selected this year to receive a $1,500 scholarship award, to be used toward academic expenses, such as tuition and fees, books, supplies, and other similar expenses.
| May 2, 2012
Sasaki selected for 2012 National Planning Firm Award
The award recognizes a firm for its body of distinguished work influencing the planning profession.
| May 1, 2012
White paper discusses benefits of diaphragm and piston flushometer valves
The white paper highlights considerations that impact which type of technology is most appropriate for various restroom environments.
| May 1, 2012
Time-lapse video: World Trade Center, New York
One World Trade Center, being built at the site of the fallen twin towers, surpassed the Empire State Building on Monday as the tallest building in New York.
| May 1, 2012
Bruce E. Brooks Associates announces new commissioning subsidiary
Brooks + Wright Commissioning to be led by Will Wright.