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
| Aug 26, 2014
Ranked: Top industrial sector AEC firms [2014 Giants 300 Report]
Stantec, Jacobs, and Turner top BD+C's rankings of the nation's largest industrial sector design and construction firms, as reported in the 2014 Giants 300 Report.
| Aug 26, 2014
High-rise concept uses 'sky street' to link towers [slideshow]
The design for a new complex in Shenzhen’s bay area consists of highly reflective glass towers, expansive garden space, and a horizontal glass structure that connects the buildings.
| Aug 25, 2014
Restoration of quake-ravaged Atascadero City Hall affirms city’s strength [2014 Reconstruction Awards]
The landmark city hall was severely damaged by the San Simeon earthquake in 2003. Reconstruction renewed the building’s stability, restored its exterior, and improved the functionality of the interior.
| Aug 25, 2014
Ranked: Top cultural facility sector AEC firms [2014 Giants 300 Report]
Arup, Gensler, and Turner head BD+C's rankings of design and construction firms with the most revenue from cultural facility projects, as reported in the 2014 Giants 300 Report.
| Aug 25, 2014
Tall wood buildings: Surveying the early innovators
Timber has been largely abandoned as a structural solution in taller buildings during the last century, in favor of concrete and steel. Perkins+Will's Rebecca Holt writes about the firm's work in surveying the burgeoning tall wood buildings sector.
| Aug 25, 2014
'Vanity space' makes up large percentage of world's tallest buildings [infographic]
Large portions of some skyscrapers are useless space used to artificially enhance their height, according to the Council on Tall Buildings and Urban Habitat.
| Aug 25, 2014
Photographer creates time-lapse video of 1 WTC using 30,000 photos
Choosing from 30,000 photos he took from the day construction began in 2006 to the day when construction was finished in 2012, Brooklyn-based photographer Benjamin Rosamund compressed 1,100 photos to create the two-minute video.
| Aug 25, 2014
Glazing plays key role in reinventing stairway design
Within the architectural community, a movement called "active design" seeks to convert barren and unappealing stairwells originally conceived as emergency contingencies into well-designed architectural focal points. SPONSORED CONTENT
| Aug 25, 2014
An easy trick for minimizing construction delays
About one out of every three construction projects is behind schedule or over budget, according to the Construction Industry Institute's Assessment of Owner Project Management Practices and Performance survey. SPONSORED CONTENT
| Aug 22, 2014
Before & After: Hospital upgrade shows shifting needs in healthcare construction
Community Hospice of Northeast Florida took an outdated 10-bed inpatient hospice unit and created a space that would meet the needs of patients receiving end of life care by creating a place that felt like home.
