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
| Jun 25, 2014
Best of Britain: 56 buildings make it to the RIBA Stirling Prize's longlist
The longlist for the 2014 prize includes Foster + Partners' Marseille and London's now-famous Shard, designed by Renzo Piano.
| Jun 25, 2014
Green Building Initiative Announces New Appointments to Board
Glumac consulting engineer CEO Steve Straus and Plum Creek director of real estate Doug Cole join GBI's board of directors.
| Jun 25, 2014
Taking a page from Lean manufacturing for improved design review processes
SPONSORED CONTENT As more building project teams look for ways to collaborate better, technology continues to provide solutions. Yet, as I learned from the experience of one of my customers, choosing the wrong technology can have an underwhelming effect, causing a team to simply swap out old challenges for new ones.
| Jun 24, 2014
From Babylon to Sydney: The evolution of the modern workspace [infographic]
This infographic, made by Sunica de Klerk and originally posted by ArchDaily, shows the evolution of the office from 2400 B.C. to the present day.
| Jun 24, 2014
Mayor Rahm Emanuel announces plans for a Chicago Architecture Biennial
Chicago's mayor Rahm Emanuel announces plan to hold the Chicago Architecture Biennial in late 2015, intended to rival Venice's Biennale.
| Jun 24, 2014
Intuit begins work on LEED Platinum campus addition
Demolition will begin this week as a precursor to construction of Intuit's new addition to its Mountain View, Calif., campus. The first of two additions, a 185,000-sf building on Marine Way, is expected to begin construction in August.
| Jun 23, 2014
5 new designs unveiled for Make It Right homes at Fort Peck, Mont.
Make It Right, Brad Pitt's foundation that builds homes for people in need, has just revealed five new designs for the Fort Peck (Mont.) Indian Reservation.
| Jun 23, 2014
Gehry's 'glass sail' cultural center for Foundation Louis Vuitton set to open in October
Comissioned by Bernard Arnault, American legendary architect Frank Gehry's newest structure in Paris for Foundation Louis Vuitton will house eleven galleries and an auditorium for performing arts.
| Jun 23, 2014
Power of IPD: Is integrated project delivery truly a transformative delivery model?
Now that many of the first-generation IPD projects have been completed, CBRE Healthcare's Tim McCurley and Stephen Powell ponder the lessons learned and pros and cons of the industry's newest delivery model.
| Jun 23, 2014
Lilker Associates launches Lighting Group; David Cyr announced as Director
New division rounds out building systems services offerings for the Manhattan-based consulting firm.