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
Office Buildings | Jul 21, 2015
Finally! There's a workplace trend that’s worth embracing
There’s a realization by corporate real estate executives that in order to create a successful workplace, there must be alignment between their people, their place, and the tools they have to do their jobs.
University Buildings | Jul 21, 2015
Maker spaces: Designing places to test, break, and rebuild
Gensler's Kenneth Fisher and Keller Roughton highlight recent maker space projects at MIT and the University of Nebraska that provide just the right mix of equipment, tools, spaces, and disciplines to spark innovation.
Architects | Jul 21, 2015
Architecture Billings Index at highest mark since 2007
This is the first month in 2015 that all regions are reporting positive business conditions, said AIA Chief Economist Kermit Baker.
BIM and Information Technology | Jul 20, 2015
New stylus brings digital sketching to the next level
Without buttons, users can change the weight of the stylus’ stroke.
Architects | Jul 20, 2015
New York design competition looks to shed the sidewalk shed
New York, which has nearly 200 total miles of sidewalk sheds, is seeking a concept that is practical but that also looks good.
Cultural Facilities | Jul 19, 2015
SET Architects wins design competition for Holocaust Memorial
The design for the memorial in Bologna, Italy, is dominated by two large metal monolithic structures that represent the oppressive wooden bunks in concentration camps in Germany during World War II.
Sports and Recreational Facilities | Jul 17, 2015
Japan scraps Zaha Hadid's Tokyo Olympic Stadium project
The rising price tag was one of the downfalls of the 70-meter-tall, 290,000-sm stadium. In 2014, the cost of the project was 163 billion yen, but that rose to 252 billion yen this year.
Cultural Facilities | Jul 16, 2015
Louisville group plans to build world's largest disco ball
The sphere would more than double the size of the current record holder.
Education Facilities | Jul 14, 2015
Chile selects architects for Subantarctic research center
Promoting ecological tourism is one of this facility’s goals
BIM and Information Technology | Jul 14, 2015
New city-modeling software quantifies the movement of urban dwellers
UNA for Rhino 3D helps determine the impact that urban design can have on where pedestrians go.
