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 20, 2013
Jones Lang LaSalle: All U.S. real estate sectors to post gains in 2013—even retail
With healthier job growth numbers and construction volumes at near-historic lows, real estate experts at Jones Lang LaSalle see a rosy year for U.S. commercial construction.
| May 17, 2013
First look: HKS' multipurpose stadium for Minnesota Vikings
The Minnesota Sports Facilities Authority (MSFA), the Minnesota Vikings and HKS Sports & Entertainment Group have unveiled the design of the State’s new multi? purpose stadium in Minneapolis, a major milestone in getting the $975 million stadium built on time and on budget.
| May 17, 2013
40 Under 40 winners: Meet the architects
Of the up-and-coming AEC professionals to be named 40 Under 40 winners by the editors of Building Design+Construction, 18 make their living in the architecture profession.
| May 17, 2013
5 things AEC pros need to know about low-e glass
Low-emissivity glasses are critical to making today’s buildings brighter, more energy-efficient, and more sustainable. Here are five tips to help AEC professionals understand the differences among low-e glasses and their impact on building performance.
| May 17, 2013
University labs double as K-12 learning environments
Increasingly, college and university research buildings are doing double duty as homes for K-12 STEM programs. Here’s how to create facilities that captivate budding scientists while keeping faculty happy.
| May 17, 2013
LEED v4 has provision to reduce water use in cooling towers
The next version of the U.S. Green Building Council's LEED rating system will expand water-savings targets to appliances, cooling towers, commercial kitchen equipment, and other areas.
| May 16, 2013
Chicago unveils $1.1 billion plan for DePaul arena, Navy Pier upgrades
Hoping to send a loud message that Chicago is serious about luring tourism and entertainment spending, Mayor Rahm Emanuel has released details of two initiatives that have been developing for more than a year and that it says will mean $1.1 billion in investment in the McCormick Place and Navy Pier areas.
| May 16, 2013
Michael R. Bohn named Executive VP at Gilbane
Gilbane has promoted Michael R. Bohn to executive vice president. With over 28 years of service to the company and leadership roles on such high-profile projects as the University of Michigan Biomedical Science Building and the University of Chicago Medical Center, Bohn will now have responsibility for Gilbane’s New York and Midwest business units.
| May 15, 2013
Schneider Electric announces Global Xperience Efficiency Events for 2013
Schneider Electric’s Xperience Efficiency series will begin with events in the United States, China, Colombia, Brazil and Russia.