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
| Nov 6, 2012
Honeywell donates first responder products for Hurricane Sandy relief and recovery efforts
Honeywell Humanitarian Relief Fund to provide aid to local employees affected by Hurricane Sandy.
| Nov 6, 2012
Uponor files patent infringement suit against Sioux Chief Manufacturing
Uponor is seeking damages and an injunction to prevent Sioux Chief from selling the PowerPEX F1960 Ring with Stop, which it believes violates Uponor’s patent.
| Nov 6, 2012
Simpson Strong-Tie donates $25K to hurricane support
The company will also be matching employees’ personal contributions to the Red Cross and other nonprofit disaster relief organizations.
| Nov 6, 2012
Goettsch Partners designs new tower in Shunde, China
200-meter-tall building will be located between Guangzhou and Hong Kong.
| Nov 5, 2012
Brasfield & Gorrie awarded new steel processing facility for Kloeckner Metals
The construction will take place on a 16-acre greenfield site at ThyssenKrupp Industrial Park in Calvert.
| Nov 5, 2012
Trimble acquires Vico assets, extends design-build-operate capabilities
Software to add 5D management to Trimble’s Solutions for vertical construction contractors.
| Oct 30, 2012
Lord, Aeck & Sargent announces four student life facility wins
Projects recognize the architecture firm’s expertise on a nationwide basis.
| Oct 30, 2012
The Beck Group announces CEO succession plan
Current Eastern Division Director and company veteran, Fred Perpall, named CEO.
| Oct 30, 2012
Two-hour fire rated curtain wall passes CDC tests
CDC (Curtain Wall Design and Consulting) is an independent firm providing a wide range of building envelope design, engineering, and consulting services to design professionals nationwide.
| Oct 30, 2012
Wight & Company announces three hires
Expands team and adds staff to work on the UNO project.