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 12, 2013
5 building projects that put the 'team' in teamwork
The winners of the 2013 Building Team Awards show that great buildings cannot be built without the successful collaboration of the Building Team.
| Jun 12, 2013
Sacred synergy achieves goals for religious education [2013 Building Team Award winner]
A renovation/addition project at Columbia Theological Seminary unites a historic residence hall with a modern classroom facility.
| Jun 12, 2013
‘Talking’ Braille maps help the visual impaired
Talking pen technology, combined with tactile maps, allows blind people to more easily make their way around BART stations in the Bay Area.
| Jun 11, 2013
Music/dance building supports sweet harmony [2013 Building Team Award winner]
A LEED Gold project enhances a busy Chicago neighborhood, meeting ambitious criteria for acoustical design and adaptability.
| Jun 11, 2013
Vertical urban campus fills a tall order [2013 Building Team Award winner]
Roosevelt University builds a 32-story tower to satisfy students’ needs for housing, instruction, and recreation.
| Jun 11, 2013
Building a better box: High-bay lab aims for net-zero [2013 Building Team Award winner]
Building Team cooperation and expertise help Georgia Tech create a LEED Platinum building for energy science.
| Jun 11, 2013
Finnish elevator technology could facilitate supertall building design
KONE Corporation has announced a new elevator technology that could make it possible for supertall buildings to reach new heights by eliminating several problems of existing elevator technology. The firm's new UltraRope hoisting system uses a rope with a carbon-fiber core and high-friction coating, rather than conventional steel rope.
| Jun 10, 2013
Lake Washington STEM school combines modular and site-built construction to meet ambitious schedule
When the Lake Washington School District outside Seattle needed a new high school built on an ambitious permitting and construction schedule of seven months, modular construction proved to be an ideal solution.
| Jun 7, 2013
Must see: Building façade made of massive concrete drain pipes
Looking to create a unique atmosphere using natural materials for the Prahran Hotel pub near Melbourne, local architect Techné Architects cleverly incorporated a series of concrete sewer pipes into the building's main façade.
| Jun 7, 2013
40 Under 40 retrospective: Where are they now?
Every month we’ll be catching up with past 40 Under 40 honorees to see what they’ve been up to since winning the award. This month we focus on a construction manager and a healthcare designer.