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
Multifamily Housing | Jan 27, 2015
Multifamily construction, focused on rentals, expected to slow in the coming years
New-home purchases, which recovered strongly in 2014, indicate that homeownership might finally be making a comeback.
Office Buildings | Jan 27, 2015
London plans to build Foggo Associates' 'can of ham' building
The much delayed high-rise development at London’s 60-70 St. Mary Axe resembles a can of ham, and the project's architects are embracing the playful sobriquet.
Multifamily Housing | Jan 22, 2015
Sales of apartment buildings hit record high in 2014
Investors bet big time on demand for rental properties over homeownership in 2014, when sales of apartment buildings hit a record $110.1 billion, or nearly 15% higher than the previous year.
| Jan 22, 2015
Architecture Billings Index rebounds at end of 2014
The American Institute of Architects reported the December ABI score was 52.2, up from a mark of 50.9 in November. This score reflects an increase in design activity.
| Jan 21, 2015
From technician to rainmaker: Making the leap in your career
Many AEC firms focus on training for the hard skills of the profession, not so much for business prowess, writes BD+C's David Barista.
Modular Building | Jan 21, 2015
Chinese company 3D prints six-story multifamily building
The building components were prefabricated piece by piece using a printer that is 7 meters tall, 10 meters wide, and 40 meters long.
| Jan 21, 2015
Tesla Motors starts construction on $5 billion battery plant in Nevada
Tesla Motors’ “gigafactory,” a $5 billion project on 980 acres in Sparks, Nev., could annually produce enough power for 500,000 electric cars.
| Jan 20, 2015
Daring hotel design scheme takes the shape of cut amethyst stone
The Dutch practice NL Architects designed a proposal for a chain of hotels shaped like a rock cut in half to reveal a gemstone inside.
| Jan 20, 2015
Avery Associates unveils plans for London's second-tallest tower
The 270-meter tower, dubbed the No. 1 Undershaft, will stand next to the city's "Cheesegrater" building.
| Jan 20, 2015
AIA course: Building with brick, stone, and masonry
Earn 1.0 AIA/CES learning units by studying this article and successfully completing the online exam.