Proper Testing is Critical When Using Tempered Glass in Architecture
When scanning any skyline of a major city from a distance, a person’s eyes almost always gravitate towards the highest buildings. However, once inside the city, some of the most interesting structures that are even more aesthetically pleasing are those that contain large transparent decorative glass panels and unique facades made entirely of glass. Glass architecture tests ones’ common sense on how something so beautiful (and seemingly delicate) can be strong enough to resist/suspend thousands of pounds of force, yet be resilient enough to weather extreme changes in heat, cold, and wind.
If it sounds complicated and highly technical to make a solid structure out of glass – it is. W&W Glass, one of the most highly regarded ornamental metal and glass installers in the country, has been engineering and installing architectural glass masterpieces for over 70 years. They have worked on over a thousand sophisticated, ground-breaking buildings in New York City and throughout North America. The professionals at W&W understand that using only the highest quality fabricated glass will be sufficient for some of the world’s most iconic buildings and skyscrapers. More importantly, the W&W Glass pros know most of the issues and potential problems that can be incurred when it comes to designing with structural glass.
One of the most heated topics of discussion among architects and construction companies when it comes to point-supported glass structures is spontaneous breakage with tempered glass. There was an article about a point-supported glass application recently installed at the The University of Kansas (KU) DeBruce Center that has been attributed to nickel sulfide breakage of tempered glass.
Spontaneous breakage of tempered glass is immediately noticeable because an initial crack exceeding a critical size (depending on its location within the thickness of the glass) causes complete fracture of the plate, typically into very small cube-shaped particles, also known as “dice”. Although the small glass fragments do not present a great threat, this can be a dangerous situation for point-supported applications as there are much higher stresses on the glass than with fully-supported systems like aluminum curtain wall. Driven by designers requiring greater transparency, more applications utilize glass held in at specific points or glass members to support face glass. These are applications where engineers do not want to see a failure from a public safety perspective, even if it will remain structurally safe until temporarily braced or repaired.
So why do these lites break without warning? There are multiple reasons why glass can spontaneously break. One of the reasons can be from cracking incurred from glass to glass contact during storage or transportation. Large changes in temperature on tinted glass substrates, low-e coatings, ceramic frit paints, films, or uneven shading can cause uneven expansion on the glass (thermal shock) causing breakage as well. Other reasons could be anything from a weld splatter to a hard impact to a deep scratch that penetrates the tensile strength zone. However, a rare but increasingly more concerning culprit for spontaneous breakage is the expansion of nickel sulfide (NiS) inclusions. These inclusions are imperceptible by the naked eye and can cause major issues, often being more prevalent with thicker, heavy tempered glass used for point-supported structural glass applications.
(Above photo is a highly magnified close up photo of spontaneous breakage due to a nickel sulfide inclusion. The arrow points to the impurity where the break originated from.) Photo Credit: http://chicagowindowexpert.com/page/2/
W&W Glass relies on the time-tested materials and processes of Pilkington Architectural in St. Helens, United Kingdom when it comes to high quality point-supported structural glass systems and materials. Officials at Pilkington explain the more technical definition of this phenomenon, since they are a float glass manufacturer. They say that the NiS inclusions as very small, spherical shape, opaque solids, typically 0.1 to 0.5 mm diameter (0.005” to 0.020”) and which change their crystalline phase (Alpha to Beta) and size long after the tempering process. This Alpha to Beta change makes them expand slightly. When glass is tempered a NiS inclusion (stone) actually shrinks more than the surrounding glass during the quench cooling. This sudden quench catches the inclusion in its smaller, hot, Alpha phase, not giving it time to make its usual transition to the slightly larger, cold, Beta phase.
You might ask why anyone would consider tempered glass for structural glass projects, especially since NiS inclusions can occur in any batch of float glass from any manufacturer. Spontaneous breakage due to nickel sulfide inclusions can occur without any load being applied to the glass, at any time, even five or ten years after the glass has been tempered. It may not happen every time a nickel sulfide inclusion is present, however. Tempered glass has been the material of choice for many years in the construction industry for safety applications. It allows the glass to be much stronger to carry or resist greater loads, and to break into small pieces in lieu of sharp shards. It is a very important product in the industry. There is a very reliable scientific way to reduce potential for spontaneous breakage due to these inclusions and that is to put the glass through a process called “heat soaking”. This process helps better insure that if there is an inclusion in the glass that could cause the glass to break spontaneously, it occurs destructively in the safety of a test chamber and not on the job site after installation.
Video Describing the Process of Heat Soak Testing provided by
Currently, there is not a United States standard for heat soaked tempered glass. The standard most often referenced in specifications is the European DIN Standard (EN 14179-1). This standard requires a heat soak test of 290 degrees Celsius (+/-10) for a minimum dwell time of two (2) hours in a heat soak chamber on every lite of tempered glass. Pilkington Architectural takes it one step further for their Pilkington Planar™ structural glass systems by requiring an eight (8) hour dwell time on EVERY lite of tempered glass. From their research, this helps them have even greater confidence that they have reduced the probability of spontaneous breakage due to nickel sulfide even further. There is no guarantee from any manufacturer that this will eliminate all risk. Some manufacturers may recommend statistical heat soaking to check one out of so many hundred or thousand lites, but this would not greatly reduce the chances of nickel sulfide inclusions being present, especially for low volume orders like that of structural glass where the fabricator may not know what part of run the issue may be located in. It may be missed entirely with this method.
When it comes to tempered glass for structurally glazed point-supported applications, architects, contractors, and owners have the ability to specify 100 percent heat soak testing with a the European DIN (EN 14179-1) Standard or better to help mitigate risk. A small percentage of the glass should not be expected to fail due to nickel sulfide inclusions. There can be a slight upcharge for this testing from manufacturers, but if this testing is not done, there can be no way of knowing if nickel sulfide inclusions are present until it’s too late.
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