Window Shattered On Its Own – By Michael L. Rupert has witnessed several highly publicized incidents over the past few years, including spontaneous windows and balcony glass falls and falls from tall buildings in Toronto, Chicago, Las Vegas and Austin, Texas. Although such episodes are rare, the danger they pose has forced building code writers, architects, government officials, and related industry professionals to consider the type of glass that should be used for glass applications where strength and protection of It is important for the audience to reconsider.
It is important for architects and architects to have an overview of the possible causes of spontaneous glass breakage, including some common misconceptions about its true spontaneity. The term “safety glazing” generally refers to any type of glass designed to reduce the possibility of serious injury upon human contact. In addition to balcony glass, safety glazing is often required for:
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The most common type of safety glass is toughened glass, which is made by heating pre-cut panels of glass to 650°C (1200°F) and rapidly cooling them through a process called “quenching”. By cooling the outer surfaces of the panel faster than the center, the quench compresses the surfaces and edges of the glass and places the center of the glass in tension.
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With incidents of spontaneous glass breakage on the rise, the glass industry is looking for new ways to make collections safer.
In addition to toughened glass being four to five times stronger than conventional annealed glass, rapid reheating and quenching dramatically change the fracture characteristics of the glass. Therefore, when tempered glass is broken, it shatters into thousands of tiny shards – this virtually eliminates the risk of human injury from sharp edges and flying shards.
Another type of safety glazing, laminated glass, is made by placing a layer of vinyl (usually polyvinyl butyral [PVB]) between two layers of glass to hold the panel together if it breaks. Although laminated glass is commonly associated with automotive windshield glass, it is increasingly being specified for storefronts, curtain walls, and windows to meet hurricane-resistant glass codes.
The third option, heat resistant glass, is not technically a safety glass. This is because when it breaks, it can create larger sharp shards that can cause serious injury. However, heat-resistant glass still complies with Consumer Product Safety Commission (CPSC) 16 Code of Federal Regulations (CFR) Part 1201 and Class A American National Standards Institute (ANSI) Z97.1, Safety Glazing Materials Used in Building I see? and test methods, for many safety glass applications when combined with an interlayer that holds the glass together in the event of breakage.
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Similar to tempering, heat strengthening involves placing pre-cut glass panels at 650°C, but with a slower cooling process. Tempered glass is not as strong as tempered glass because it has a lower compressive strength—about 24,130 to 51,710 kPa (3,500 to 7,500 psi) compared to 68,950 kPa (10,000 psi) or higher. . However, it is twice as strong as annealed glass. For this reason, heat-resistant glass is often specified for applications that require resistance to thermal stress and snow and wind loads.
Incidents of spontaneous glass breakage in Chicago, Las Vegas, Austin, Texas, and Toronto have been exclusively with tempered glass. Despite this material’s high level of strength and capacity to meet safety glazing requirements, it is uniquely vulnerable to this type of failure. Ironically, the central stress zone engineered into toughened glass through the quenching process is also what makes it vulnerable to catastrophic breakage.
Safety glazing is commonly required for sliding glass doors, shower doors and patio furniture. “Safety glazing” generally refers to any type of glass designed to reduce the possibility of serious injury.
There are many potential reasons for tempered glass breaking on its own. The most common damage is to the edges of the glass because it is already cut into panels, or the edges are chipped or nicked during packaging, shipping, or installation of the glass on site.
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Although such damage may not be visible, stress concentrations around defects can occur due to expansion and contraction of the glass in response to changes in temperature in service, wind load, building movement, and other environmental factors. Finally, when stresses cause the glass to break, when in fact the conditions for failure have been present months or even years in advance, the action may take place spontaneously.
Fracture related to the expansion and contraction of the glass frame members can also lead to frame breakage – another common form of apparently spontaneous failure. Such incidents occur when the gaskets, adjustment blocks, or edge blocks in a metal window or curtain wall frame are worn out or do not adequately protect the glass from glass-to-metal contact caused by temperature or wind-induced movement. This can cause damage to the edge and surface of the glass in contact with the metal frame environment and create stresses that eventually lead to its failure for no apparent reason.
Nickel sulfide particles are very small and very rare and are only found randomly in flute glass. This combination makes visual inspection for such components highly impractical – if not downright impossible.
Another potential cause of spontaneous glass breakage is thermal stress. Thermal stresses in the glass are caused by a positive temperature difference between the center and the edge of the light glass, meaning that the former is hotter than the latter. The expansion of the center of the heated glass leads to tensile stress at the edge of the glass. If the pressure caused by the heat exceeds the resistance of the edge of the glass, breakage occurs.
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Calculation of thermal stress is important today, as current design trends and the desire for daylighting drive the industry to specify larger insulating glass units (IGUS) with high performance coatings for solar control. Large IGUs inherently have larger glass surface and edge areas. When they are combined with coatings designed to direct solar energy, a more detailed thermal stress analysis is required.
There is no known technology that completely eliminates nickel sulfide scale formation in flute glass. Furthermore, because nickel sulfide rocks are so small, there is no practical way to confirm their presence in flute glass.
Nickel-Sulfide Inclusions One of the less common—but often cited—causes of spontaneous glass breakage is nickel-sulfide (Nis) inclusions in tempered glass. Small nickel sulfide stones can form accidentally in the production of flute glass. They are usually good, even when seen in tempered glass.
North American glass manufacturers do not use nickel in their primary glass batch formulations and go to great lengths to avoid nickel-containing components in their glass melting processes. Despite quality controls and rigorous procedures aimed at reducing the likelihood of nickel sulfide scale, no technology exists to completely eliminate their formation in today’s float glass.
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Nickel sulfide stones are very small, and their presence in the finished glass product is covered under ASTM C1036, Standard Specification for Flat Coated Glass, which allows spots (including nickel-sulfide particles) between 0.5 and 2.5 mm ( 1/50 to 1/1) to be created. ) 10 inches) in fluted glass depending on the size and quality of the glass.
While nickel-sulfide inclusions may be present in annealed or heat-resistant glass, the problems they cause are due to the tempering process specific to tempered glass. Fracture is due to volumetric growth in stone size. As explained earlier, during the annealing and heat strengthening processes, the glass is cooled at slower, more controlled temperatures that allow the nickel-sulfide particles present to complete a phase transformation (known as the ?to? phase change) during which It expands completely. They reach their final size and then remain constant.
In the tempering process, this phase transformation is stopped during rapid quenching, which causes the nickel-sulfide particles to remain in their contracted, pre-transformation states. Then, when the tempered glass is exposed to higher temperatures during operation due to increased solar heating or other high temperature effects, the nickel-sulfide particles have the potential to resume their volumetric growth. If the expansion is large enough—and the particle is located in the central stress zone of the tempered glass panel—the resulting pressure can be enough to shatter the glass.
When tempered glass is broken (as shown above), it shatters into thousands of tiny shards, virtually eliminating the risk of human injury from sharp edges and flying glass shards.
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The surface compression of heat-resistant glass makes it almost twice as strong as annealed glass. Heat-resistant glass is usually used when the glass is required to respond to thermal or mechanical loads caused by heat, wind or snow.
As noted, nickel-sulfide particles are very small, very rare, and are only found incidentally in float glass. This combination makes visual inspection of such components impractical, if not impossible. For this reason, some glass manufacturers and glazing contractors offer thermal soaking of toughened glass as a potential solution to minimize the risk of spontaneous glass breakage.
In this method, the glass supplier exposes a complete or statistical sample of tempered glass panels to a temperature of 288 to 316 degrees Celsius (550 to 600 degrees Fahrenheit) for two to four hours. The goal is to initiate or accelerate the phase change of any nickel-sulfide material that may be present and cause the glass to break before shipping to the end customer.
While a “now instead of later” approach can eliminate defects
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