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
| Oct 15, 2012
Silicones from Dow Corning solve high-altitude technical challenges at the world’s tallest tower
The spectacular Burj Khalifa skyscraper soars to 828m above ground level, holding the record for being the world’s tallest building and also for the highest installation of an aluminum and glass façade. To take on its numerous challenges and technical difficulties, it required proven silicone solutions and full support from Dow Corning.
| Oct 15, 2012
Three new members elected to AISC Board of Directors
New members will immediately begin serving on the AISC Board of Directors, assisting with the organization's planning and leadership in the steel construction industry.
| Oct 11, 2012
Hank Adams Named to Lead HDR’s Healthcare Program
With more than 25 years of experience, HDR vice president is tapped to lead firm's healthcare projects.
| Oct 10, 2012
Foster + Partners to Design New 425 Park Avenue Tower
Conceptual designs submitted by Foster, Hadid, Koolhaas and Rogers to be on exhibit during Municipal Art Society’s Annual Symposium
| Oct 9, 2012
AIA billings index sounds a positive note
The so-called new projects index was at a relatively healthy 57.2, up from 56.3 the previous month.
| Oct 9, 2012
Celebrating brick in architecture
The Brick Industry Association’s 2012 Brick in Architecture Awards put the spotlight on new projects that make creative use of one of humankind’s oldest and most beloved building materials.
| Oct 5, 2012
2012 Reconstruction Award Special Recognition: Joplin Interim High School, Joplin, Mo.
At 5:41 p.m. CDT on Sunday, May 22, 2011, an EF5 tornado touched down in Joplin, Mo. In the next 31 minutes, the mile-wide, multiple-vortex tornado, with winds up to 250 mph, destroyed two thousand buildings, including Joplin High and nine other schools.
| Oct 5, 2012
2012 Reconstruction Award Bronze Winner: DPR Construction, Phoenix Regional Office, Phoenix, Ariz.
Working with A/E firm SmithGroupJJR, DPR converted a vacant 16,533-sf one-time “adult-themed boutique” in the city’s reemerging Discovery Triangle into a LEED-NC Platinum office, one that is on target to be the first net-zero commercial office building in Arizona.
| Oct 5, 2012
2012 Reconstruction Award Bronze Winner: Pomeroy Senior Apartments, Chicago, Ill.
The entire interior of the building was renovated, from the first floor lobby and common areas, to the rooftop spaces. The number of living units was reduced from 120 to 104 to allow for more space per unit and comply with current accessibility requirements.
| Oct 5, 2012
2012 Reconstruction Award Bronze Winner: Walsh Group Training and Conference Center, Chicago, Ill.
With its Building Team partners—architect Solomon Cordwell Buenz, structural engineer CS Associates, and M/E engineer McGuire Engineers—Walsh Construction, acting as its own contractor, turned the former automobile showroom and paperboard package facility into a 93,000-sf showcase of sustainable design and construction.