Another type of safety glazing, laminated glass, is made by sandwiching an interlayer of vinyl typically polyvinyl butyral [PVB] between two layers of glass to hold the panel together if it is broken. Although laminated glass is most commonly associated with windshield glass for automobiles, it is increasingly being specified for storefronts, curtain walls, and windows to meet codes for hurricane-resistant glass. A third option, heat-strengthened glass, is not technically a safety glazing.
This is because when it breaks, it may form larger sharp shards that can cause serious injury. Safety Performance Specifications and Methods of Test , for many safety glass applications when it is combined with a laminated interlayer that holds the glass together if it breaks. As with tempering, heat-strengthening involves exposing pre-cut glass panels to temperatures of up to C, but with a slower cooling process. Heat-strengthened glass is not as strong as tempered glass because the compression strength is lower—about 24, to 51, kPa to psi compared to 68, kPa 10, psi or greater.
However, it is about twice as strong as annealed glass. For this reason, heat-strengthened glass is often specified for applications demanding resistance to thermal stress and snow- and wind-loads. Causes of glass breakage The incidents of spontaneous glass breakage in Chicago, Las Vegas, Austin, Texas, and Toronto occurred exclusively with tempered glass. Ironically, the center tension zone engineered into tempered glass through the quenching process is also what makes it so vulnerable to catastrophic breakage.
Poor edge quality There are many potential causes for spontaneous breakage of tempered glass. The most common is damage to the edges of glass as it is being pre-cut into panels, or nicks or chips to the edges that occur when the glass is being packaged, shipped, or installed onsite. While such damage may not be readily apparent, stress concentrations around these imperfections can occur as the glass expands and contracts in response to in-service temperature changes, wind load, building movement, and other environmental factors.
Ultimately, when those stresses cause the glass to break, the action may appear to have been spontaneous when, in fact, the circumstances for failure had been put in place months or even years earlier.
Frame-related breakage Expansion and contraction of glass framing members may also lead to frame-related breakage—another common form of seemingly spontaneous failure. Such incidents occur when the gaskets, setting blocks, or edge blocks in a metal window or curtain wall frame are missing or do not sufficiently cushion the glass against glass-to-metal contact caused by temperature or wind-related movement. Thermal stress Another potential cause of spontaneous glass breakage is thermal stress.
Thermally induced stresses in glass are caused by a positive temperature difference between the center and edge of the glass lite, meaning the former is hotter than the latter. The expansion of the heated glass center results in tensile stress at the edge of the glass. If the thermally induced stress exceeds the edge strength of the glass, breakage occurs. Accounting for thermal stress is especially critical today, as current design trends and the desire for daylighting are driving the industry toward the specification of larger insulating glass units IGUs with high-performance solar control coatings.
Large IGUs have inherently greater glass surface and edge areas. Nickel-sulfide inclusions A far less common—but often cited—cause of spontaneous glass breakage is nickel-sulfide NiS inclusions in tempered glass. Small nickel-sulfide stones can form randomly in the production of float glass. They are typically benign, even when occurring in tempered glass.
North American glass manufacturers do not use nickel in batch formulations for primary glass and go to great lengths to avoid nickel-bearing components in their glass-melting processes. Nickel-sulfide stones are quite small and their occurrence in the final glass product is covered under ASTM C, Standard Specification for Flat Glass , which permits blemishes including nickel-sulfide particles of between 0.
While nickel-sulfide inclusions may be present in annealed or heat-strengthened glass, the problems they cause are specific to tempered glass because of the tempering process. Breakage is due to a volumetric growth in the size of the stone.
As detailed earlier, during the annealing and heat-strengthening processes, glass is cooled at slower, controlled temperatures that enable nickel-sulfide particles that are present to complete a phase transformation known as the? In the tempering process, this phase transformation is arrested during rapid quenching, which causes any nickel-sulfide particles present to remain confined to their shrunken, pre-transformation states.
Then, when the tempered glass is exposed to higher in-service temperatures caused by solar heat gain or other high-temperature influences, nickel-sulfide particles have the potential to resume their volumetric growth. If the expansion is large enough—and the particle is located in the center tension zone of the tempered glass panel—the resulting stress may be enough to shatter the glass. Is heat-soaking a solution? As indicated, nickel-sulfide particles are tiny, extremely rare, and only found randomly in float glass.
This combination makes visual inspection for such inclusions highly impractical, if not impossible. For that reason, some glass fabricators and glazing contractors offer heat-soaking of tempered glass as a potential solution for minimizing the risk of spontaneous glass breakage. In this procedure, the glass supplier exposes an entire lot or statistical sampling of tempered glass panels to temperatures of to C to F for two to four hours.
The goal is to initiate or accelerate the phase change of any nickel-sulfide inclusions that may be present and to cause the glass to break before it is shipped to the end customer. Even advocates of heat-soaking are careful to state the procedure can only reduce or minimize the risk for spontaneous breakage of tempered glass. Risks of heat-soaking There are risks associated with the heat-soak procedure that may outweigh any perceived benefits.
For instance, small, stable inclusions could undergo the beginning of a phase change during the heat-soak. While the phase change may not be sufficient to cause breakage during the procedure, the transformation could potentially continue after the glass is installed, causing it to break in-service.
Re-exposing tempered glass to the increased temperatures of heat-soaking also has the potential to reduce its surface compression, which is the source of its strength. Solutions for safety In recent months, two organizations made major announcements prompted largely in response to the incidents of falling glass in Toronto, Chicago, Las Vegas, and elsewhere.
Both shared a common assessment—namely, that using laminated tempered glass or heat-strengthened glass is the most viable solution to making balcony and other types of overhead glass safer.
In Canada, an Expert Panel on Glass Panels in Balcony Guards established by the Ontario Ministry of Municipal Affairs and Housing MAH recommended local building codes be amended to mandate the use of heat-strengthened laminated glass for any outboard guard or glazing located beyond the edge of a floor, or within 50 mm 2 in.
For outboard glazings located more than 50 mm inward from the edge, the panel recommended heat-soaked tempered glass or heat-strengthened laminated glass. Conclusion Given the developments and recommendations outlined in this article, it is clear a laminated glass interlayer in combination with tempered or heat-strengthened glass may offer the optimal blend of characteristics for applications where the risk of injury from glass fallout is a primary concern.
For non-safety glass applications, where strength and resistance to spontaneous breakage is desired, non-laminated heat-strengthened glass should be considered due to its lower costs. Michael L. He can be reached at mrupert ppg. Glass breakage especially on high rise buildings can frequently be caused by bird impacts.
Not necessarily by defects or inclusions in the glass. We also have self-adhesive films that can be applied to existing glass. We offer Technical Service and Support. The reason nickel sulfide causes shattering of glass is that the metal will expand and contract at a different rate than the surrounding glass. This leads to violent, spontaneous shattering of tempered glass without any warning.
Put differently, 1 in every panes of glass in your building could just randomly shatter. First, buy your windows from a quality window company. The glass and its installation will be of higher quality. The real guarantee here is in preventing the dangers caused by broken glass. Install exterior window safety film to hold your glass together if it ever breaks. This film encapsulates your glass so if it does shatter, the pieces stay put.
Commonly used to prevent burglaries, break-ins, and bomb blast shrapnel; security window films also help with spontaneous breakage.
The week of writing this post, we are working with two clients who have spontaneous glass breakage and are installing exterior safety film as a solution. And when toughened glass breaks, it does it in style, shattering in its entirety into small pebbles.
In many respects these pebbles are far safer than the razor-sharp shards of regular annealed glass, which is why toughened glass is classed as a safety glass and is specified in so many areas where safety is a concern. However, in certain circumstances the small broken chips can still pose a danger. For example, a glass balustrade, spandrel or overhead glazing that shatters and falls from its frame could cause significant injury if anyone happened to be unfortunate enough to be standing underneath it.
Even without the issue of safety, replacing toughened glass in inaccessible areas can be a major not to mention expensive inconvenience. This is known as spontaneous glass breakage and is, in actual fact, generally triggered by one of four factors:. Minor damage during installation such as nicked or chipped edges that later develop into larger breaks.
Overly tight binding of the glass in the frame, which causes stresses to develop as the glass expands and contracts due to thermal changes or deflects due to wind. While factors such as installation damage and incorrect fitting can be avoided to some degree by experienced glass technicians, avoiding glass that contains nickel sulphide is somewhat more difficult.
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