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 18, 2011
Raphael Viñoly’s serpentine-shaped building snakes up San Francisco hillside
The hillside location for the Ray and Dagmar Dolby Regeneration Medicine building at the University of California, San Francisco, presented a challenge to the Building Team of Raphael Viñoly, SmithGroup, DPR Construction, and Forell/Elsesser Engineers. The 660-foot-long serpentine-shaped building sits on a structural framework 40 to 70 feet off the ground to accommodate the hillside’s steep 60-degree slope.
| May 18, 2011
New center provides home to medical specialties
Construction has begun on the 150,000-sf Medical Arts Pavilion at the University Medical Center in Princeton, N.J.
| May 18, 2011
Improvements add to Detroit convention center’s appeal
Interior and exterior renovations and updates will make the Detroit Cobo Center more appealing to conventioneers. A new 40,000-sf ballroom will take advantage of the center’s riverfront location, with views of the river and downtown.
| May 18, 2011
One of Delaware’s largest high schools seeks LEED for Schools designation
The $82 million, 280,000-sf Dover (Del.) High School will have capacity for 1,800 students and feature a 900-seat theater, a 2,500-seat gymnasium, and a 5,000-seat football stadium.
| May 18, 2011
Carnegie Hall vaults into the 21st century with a $200 million renovation
Historic Carnegie Hall in New York City is in the midst of a major $200 million renovation that will bring the building up to contemporary standards, increase educational and backstage space, and target LEED Silver.
| May 17, 2011
Redesigning, redefining the grocery shopping experience
The traditional 40,000- to 60,000-sf grocery store is disappearing and much of the change is happening in the city. Urban infill sites and mixed-use projects offer grocers a rare opportunity to repackage themselves into smaller, more efficient, and more convenient retail outlets. And the AEC community will have a hand in developing how these facilities will look and operate.
| May 17, 2011
Architecture billings index fell in April, hurt by tight financing for projects
The architecture billings index, a leading indicator of U.S. construction activity, fell in April, hurt by tight financing for projects. The architecture billings index fell 2.9 points last month to 47.6, a level that indicates declining demand for architecture services, according to the American Institute of Architects.
| May 17, 2011
Sustainability tops the syllabus at net-zero energy school in Texas
Texas-based firm Corgan designed the 152,200-sf Lady Bird Johnson Middle School in Irving, Texas, with the goal of creating the largest net-zero educational facility in the nation, and the first in the state. The facility is expected to use 50% less energy than a standard school.
