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
| Jul 8, 2013
RSMeans cost comparisons: hotels, motels, and apartment buildings
Construction market analysts from RSMeans offer construction costs per square foot for hotels, motels, and apartment buildings.
| Jul 8, 2013
Learn from the world’s fastest animal: speed isn’t everything
Every schoolchild knows that the cheetah is the world’s fastest land animal—70 mph, no sweat. And, as we all know from National Geographic television documentaries, it is speed that makes Acinonyx jubatus such a fearsome hunter, right?
| Jul 3, 2013
Mid-year special: Top 13 stories for 2013 (so far)
Every six months, we like to take a look back at the stories that generated the most interest from our readers. Here's a roundup of the top 13 articles on BDCnetwork.com for the first six months of 2013.
| Jul 3, 2013
World's biggest freestanding building opens in China
Measuring a stout 100 meters high, 500 meters long, and 400 meters wide, the New Century Global Centre in the Tianfu New District of Chengdu, China, is officially the world's largest freestanding building.
| Jul 3, 2013
Mall of America will double in size after $2.5 billion expansion
The nation's largest indoor mall will undergo a $2.5 billion, 10-year expansion project that will add attractions like an NHL-sized skating rink and an indoor water park.
| Jul 3, 2013
Architects team with HUD to promote 'Rebuild By Design' competition for Hurricane Sandy recovery effort
The American Institute of Architects (AIA) today announced a communications campaign urging its membership to enter the “Rebuild by Design” multi-stage regional design competition announced by Department of Housing and Urban Development (HUD) Secretary Shaun Donovan on June 20.
| Jul 2, 2013
LEED v4 gets green light, will launch this fall
The U.S. Green Building Council membership has voted to adopt LEED v4, the next update to the world’s premier green building rating system.
| Jul 1, 2013
Firestone Building Products Company LLC Announces Sustainability Report
Firestone Building Products Company, LLC, the leading manufacturer of superior “Roots to Rooftops” products for commercial building performance solutions, today announced the release of its 2012 Sustainability Report. The report, the first for Firestone Building Products Company, covers the fiscal 2012 year and is available for download at firestonebpco.com.
| Jul 1, 2013
LEGOLAND builds 12-foot replica of One World Trade Center
The LEGOLAND Discovery Center Westchester in Yonkers, N.Y., celebrated the completion of a LEGO replica of One World Trade Center by lighting the 12-foot-tall, 100-pound model.
| Jul 1, 2013
Report: Global construction market to reach $15 trillion by 2025
A new report released today forecasts the volume of construction output will grow by more than 70% to $15 trillion worldwide by 2025.