You Bet Your Glass: June 2012
Raindrops keep falling
June 25, 2012 By Frank Fulton
If you live in a major centre in Canada, watch the TV news or read the
papers, you are no doubt familiar with the urban panic and exaggerated
media frenzy surrounding shattered glass falling from balcony railings
onto the heads of unsuspecting pedestrians.
If you live in a major centre in Canada, watch the TV news or read the papers, you are no doubt familiar with the urban panic and exaggerated media frenzy surrounding shattered glass falling from balcony railings onto the heads of unsuspecting pedestrians. Speculation and rumour have suggested the cause of the spontaneous breakage may be anything from faulty installations to railing fabrication errors, from flying stones from adjacent buildings to windblown balcony furniture, but the proven cause is the majority of instances is now known to be the presence of nickel sulphide inclusions within the glass.
So, what is a nickel sulphide inclusion?
In a batch of glass, contaminants such as stainless steel that contain nickel might be present, and these can combine with sulphur to form nickel sulphide inclusions. The process of tempering float glass can cause an inclusion to change from its normal low-temperature structure to a smaller high-temperature, crystalline structure. When the glass is cooled quickly, as part of the tempering process, the NiS particle is unable to change back to its original form. Over a period of time the NiS inclusion will slowly convert to its original low-temperature structure, increasing in size by two to four per cent. If the inclusion is located at a critical area within the glass, the growth can result in the shattering of the tempered pane for no apparent reason, hence spontaneous glass breakage. Interestingly, the presence of an NiS impurity does not necessarily mean that a lite will ultimately break spontaneously. This only happens when the NiS inclusion is located in a specific area within the lite.
One of the people involved in determining the cause of the spontaneous breakage and playing a significant role in reaching a solution to minimize the potential for injury is Dave Prohaska of Prohaska Engineering. “While issues with spontaneous breakage have existed for several decades, the most publicized issue occurred with 12-millimetre tempered glass installed in the John Hancock Building in Boston. Glass manufacturers are hesitant to publish their research for fear of inciting the masses over an issue which is rare,” says Prohaska.
What is known about NiS is that the concentration of the impurity is batch-related and therefore there is a higher probability of failure of tempered glass from a batch of primary glass with more impurities. According to Prohaska, “To date, the failure rate has been higher in eight-, 10- and 12-millimetre tempered glass than in six-millimetre. Much of the heavier glass has been imported from offshore, primarily Chinese, sources; however, some of the breaks have occurred in glass produced by North American suppliers of primary glass.”
Prohaska is unaware of the exact number of lites that have failed but estimates the number to be between 30 and 50 during the past year. “Statistically, this might represent one lite out of every thousand currently installed in balcony guards in the GTA.” Of note, this rate of failure is much lower than the eight-per-1,000 probability used in the design of glass for typical installations.
During the spring of 2012, to address the problem, the Ontario Ministry of Housing held four meetings, which included representation from many involved parties, to help identify issues and to provide recommendations that may provide the basis for changes to the Ontario Building Code.
The process is ongoing, but what is certain is that the design of balcony guards has changed as a consequence of the spontaneous breakage experienced recently, and it is likely that more changes will be coming to the glass products permitted for use in balcony railings. In fact, the CSA is currently in the process of developing a new standard for balcony glass.
Frank Fulton is president of Fultech Fenestration Consulting. He has
been in the industry for 30 years and can be reached via e-mail at
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