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 18, 2012
Alcoa appoints Hunter Architectural Manager
Hunter to operate with the goal of driving specification, new product adoption and overall demand for the Alcoa BCS North America product range.
| Jul 17, 2012
AIA and Architecture for Humanity select Disaster Response Grant recipients
Awards help each group implement their locally driven preparedness project in the second half of the year.
| Jul 17, 2012
KM/Plaza changes name to Plaza Construction
Lands new projects including the Perry South Beach Hotel and Dadeland Mall Kendall Wing Expansion.
| Jul 17, 2012
Dr. Phillips Charities Headquarters Building receives LEED Silver
The building incorporates sustainable design features, environmentally-friendly building products, energy efficient systems, and environmentally sensitive construction practices.
| Jul 16, 2012
BD+C Under 40 Leadership Summit scheduled
Attendee registration for U40 Summit II now open.
| Jul 16, 2012
Construction spending at 2 ½ year peak
Construction economist Ken Simonson says that four private nonresidential categories each posted 12-month spending increases of more than 25%: power and energy construction, 35%; hotels, 29%; educational and manufacturing, 27% apiece.
| Jul 16, 2012
Chen named design director at Heery
Chen comes to Heery from his own firm, Mark Chen Architect, a design and planning consulting firm, based in New York City, whose recent work includes large-scale planning studies for mixed-use projects.
| Jul 16, 2012
Reed Construction hires new project manager
Fread is a LEED AP and received his degree from Purdue University.
| Jul 16, 2012
Business school goes for maximum vision, transparency, and safety with fire rated glass
Architects were able to create a 2-hour exit enclosure/stairwell that provided vision and maximum fire safety using fire rated glazing that seamlessly matched the look of other non-rated glazing systems.
| Jul 12, 2012
New firm RSK Avanti Partners offers market development services
Full-service consulting firm is focused on working with product manufacturers, designers, specifies and contractors in the building construction market.
