Heat blamed for summer glass showers
The study of fractology rarely makes headlines, but when glass breaks, falling onto people, everyone wants to know why. Two examples:
- Shards of glass fell Aug. 1 from the 36th story of a Shanghai office tower onto pedestrians below, injuring two, according to an Aug. 2 article in the Shanghai Daily.
- In July, three glass roof panels shattered at a Newcastle, United Kingdom, bus station, injuring a passenger, according to a July 18 article in the Evening Chronicle. Twenty similar failures have occurred since the roof was installed six years ago, according to the article.
In both instances, experts blamed the breakages on high temperatures.
“It could be a thermal stress break,” explains Christopher Barry, director of technical services, building products, for Pilkington North America Inc. in Toledo, Ohio. “Heat expands glass, guaranteed, whether it’s properly installed or not. The glass exposed absorbs the heat and expands and the part [of the glass] in the frame stays cool. … The edge, the weakest part of the glass, is in tension and susceptible to breaking.”
Rigid frame materials, such as steel and concrete, can also lead to glass breakage in hot weather if the opening is too small for the lite, Barry says.
“If there is no room for glass to expand, it’s going to come up to the frame edge—and something has to give. So, the glass crunches,” he says.
Impurities in the glass batch also make glass vulnerable to breakage during hot weather, says Patrick Loughran, associate partner for Goettsch Partners Inc. in Chicago.
“Sand can have minerals in it like nickel. [Manufacturers] try to get all the rogue materials out, but rarely an impurity will get into a batch,” Loughran says. “If a nickel-sulfide stone finds itself in a tempered lite of glass that is exposed to temperature changes, the glass will spontaneously explode when it expands."
Batch impurities can often be blamed for multiple instances of spontaneous breakage on a building, he says. "It’s not uncommon that you’ll have several breaks on one project, because the lites come from the same batch,” he says.
Loughran says he has not investigated the Newcastle bus station to know whether batch problems caused the failures.
No matter what the cause of glass breakage, Loughran says it’s “an inherent part of architecture. Architects and engineers have to anticipate it and account for it in their designs.”
Barry agrees: “We can reduce the probability of glass breaking, but we always have to ask the question ‘What happens if it does?.’”
To protect people from injury caused by these sometimes unavoidable breaks, he says glass should be tempered so that if it does fail, it won’t send large shards of glass onto passersby.
In overhead applications, the bottom lites should be laminated, so if the top lite breaks, the laminated glass will catch broken pieces. U.S. building codes require laminated glass in all such applications more than 10 feet from the floor, Barry says.
Nets can also be used to catch falling glass, Loughran says.