Risk Management Advisories

GLASS BREAKAGE INJURIES / GLASS INSTALLATION AND REPLACEMENT

1. What is Safety Glass?
2. Wired Glass can cause Horrible Injuries
3. Installation & Replacement/Improvement
4. Risk Management Issues to Consider
5. Wired Glass: Wired is for Fire, not for Strength
6. The Properties of Glass
   

WHAT IS SAFETY GLASS?

Different types of glass provide different safety features but no type of glass is all encompassing. Contrary to public belief, wired glass (also known as Georgian Polish) is not impact safety glass. It is a type of fire safety glass required by fire code to be installed in fire doors or windows. It is a fire safety glass because it shatters rather than explodes under heat and offers protection to firefighters and others against flying glass chards. Under exposure from intense heat during a fire, the glass will be held in place by the wire, thus preventing the fire from spreading to adjacent areas. However, wired glass shatters under impact more readily than plate glass. For this reason, wired glass is especially dangerous in areas where people might easily make contact with it. Not only does wired glass shatter more easily, once it shatters, the mesh wire inside is exposed and can cause horrible injuries to any part of the human body that penetrates it. This is what makes wired glass extremely hazardous in impact situations. While wired glass protects against fire, it can be extremely dangerous upon impact.

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WIRED GLASS CAN CAUSE HORRIBLE INJURIES

OSBIE receives many reports of incidents and notices of claims with respect to injuries received by students seriously cut through impact with glass. Many of these injuries are cause by wired safety glass in schools. From 1987 to 2000 there have been 107 claims against schools for glass injuries. Over this period, costs related to glass injuries amounted to $3,154,202.00. More important than the dollars is the pain and suffering, permanent reduced mobility and scarring caused by these impacts with glass.

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INSTALLATION & REPLACEMENT / IMPROVEMENT

Inspections of many school sites have shown that many schools are installing new wired glass or replacing broken glass in doors and windows with wired or "Georgian Polish" glass. The decision to use wired glass is often made with the misconception that wired glass is overall safety glass. This is not the case. Wired glass or "Georgian Polish" glass should only be used in fire separations and fire doors in accordance with the Canadian Building Code. It can be strengthened against impact by applying a suitable laminate to the wired glass without reducing the fire safety features of the glass. Glass installation or replacement requires selection of the best type of glass for each location. Different choices of glass are appropriate for different areas in a school. For example, Plexi-glass or Lexan might be best for a gymnasium door. This would lower the risk of injury from broken glass. Wired doors may not be necessary in places where they have been used traditionally. The following chart can help in the process in choosing the safest type of glass for each location in a school building.

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Risk Management Issues to consider:

Setting and implementing the following rules may help to reduce the number of injuries related to glass:

1. Avoid running in the hallways
2. Avoid holding track and field practice indoors in locations where there is a chance to impact with glass
3. Ban skateboarding in hallways
4. Be sure to have signage if floors are wet or slippery
5. Make sure through regular inspections that all door glass and windows are in good repair
6. warn students when doors will be locked after hours, and that doors can only be opened with the panic bar
7. Install mats over glass in gymnasiums

(REVISED 2010)

Attached are two articles to assist in gaining a better understanding of the properties of glass: National Research Council of Canada's IRC Structures News, "Wired Glass: Wire is for Fire, Not for Strength", and; Southward Consultants Limited, "The Properties of Glass".

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WIRED GLASS: WIRE IS FOR FIRE, NOT FOR STRENGTH

Wired glass, frequently used mistakenly as safety glass has no special added strength. Indeed, wired glass breaks at only half the force required to shatter regular annealed glass.

Although the wire may seem to reinforce the glass, this special product is designed only to oppose the spread of fire, not resist impact. Yet, wired glass is often used in areas where people might accidentally run into it-school hallways, for instance. And people have put their arms right through both the glass and the wire.

When glass cracks due to fire, the wire mesh holds the cracked segments together. Consequently, using wired glass prevents the spread of hot fire gases and gives the door or wall a good fire-separation rating. However, the wire in the glass compromises the strength of the glass. Sand-witching wire mesh between two layers of glass creates many stress-raising flaws. Unlike ductile materials, glass does not deform plastically to relieve the stress concentrations at these flaws. In fact, the flaws multiply the effect of tensile stresses several hundred times. So if someone or something strikes the glass hard enough, cracks originating from one of these stress points propagate quickly-almost instantaneously.

Some procedures can reduce the risk of injury from broken glass and wire. For instance, applying a tough, trans-parent film to one surface of the glass can make both new and existing installations of wired glass more secure. Products now on the market are specially designed to prevent broken glass from leaving its frame.

Alternatively, laminated glass (used in car windshields) has plastic sand-witched between glass layers. This arrangement reduces the risk of injury, by holding the glass fragments together and, for short-term loading, is about as strong as a single layer of glass of the same total thickness (CGSB-12.20-M89). In a new development another glass product incorporates the wire mesh used for fire safety within a plastic interlayer.

Information: W.A. Dalgliesh

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THE PROPERTIES OF GLASS

The glass we see around us every day, in most applications, is plate, or soda-lime glass. The chemicals which comprise the glass are melted together, then floated on molten tin. When the float of glass is the appropriate thickness, it is drawn off the tin and fed into an oven that allows it to cool in a controlled manner called annealing. As the glass is being drawn off the tin a wire mesh may be inserted, to make wired glass.

Glass is a brittle material and does not deform plastically. It fails to tension regardless of the nature of loading. The theoretical tensile strength of glass is 1,000,000 psi but failure occurs at stresses much lower than that because of surface imperfections. Typically, an untreated, plain annealed plate glass window fails at tensile stresses around 10,000 psi to 20,000 psi. Because the actual strength of glass is dependant upon the extent, type and depth of surface imperfections, there is considerable variability in the stress at which the failure of any individual piece of glass will occur.

To improve the strength of glass it is usually heat-treated. The heat treatment processes cause the outer surface of the glass to go into compression, thus increasing the resistance to a tensile failure. Heat strengthening glass involves cutting a piece of annealed glass to size, then heating the glass to near its softening point, around 1300 F, and allowing it to cool. Heat strengthening the outer surface may increase the glass strength over that of the annealed glass, by a factor of two. Tempering involves cutting a piece of annealed glass to size, raising the temperature of the entire glass sheet uniformly to 1300 F then rapidly cooling it in air. Fully tempering glass generally increases its strength three to five times.

Glass can be further strengthened through lamination. Laminated glass is constructed of two or more sheets of glass permanently bonded together under heat and pressure, typically with a plastic interlayer. The sheets to be laminated together may be annealed, heat strengthened, tempered or wired glass, in any combination.

In an attempt to reduce the likelihood of cutting and piercing personal injuries from glass, broken as a result of human impact, the concept of safety glass evolved.

The American National Standards Institute published ANSI Z97.1 a safety performance specification outlining methods of tests for safety glazing materials used in buildings. The Standard prescribed an impact test to determine if a glass could be considered safety glass. A similar Canadian government Standard, Canadian General Standards Board CGSB-12.1 exists.

Through the Consumer Products Safety Commission, the United States Federal government also introduced mandatory regulations in the United States in 16 CFR Part 1201-Safety Standards for Architectural Glazing Materials.

Any of the previous described types of glasses, annealed, tempered, laminated, wired etc., could be classified as safety glass providing it did not disintegrate when subjected to specified impact loads.

In Canada, Tempered or Laminated Glass is tested to CAN/CGSB-12.1 Tempered or Laminated Safety Glass. The test involves swinging a bag, filled with lead shot to weigh 100 pounds, against a sheet of glass, from a height of 18" or 48". The test creates 150 - 400 foot pounds of energy impact. The lower energy level is intended for glass with an area less than 9 square feet, the higher energy level is for larger pieces of glass.

The scope of tempered and laminated glass Standard advises it applies to the use of glass intended to reduce the likelihood of injury to persons by objects projected from an exterior source or by glass fragments when the glass is cracked or broken. The Standard is intended to be used primarily for interior/exterior doors, and their adjacent panels, and shower and bathtub enclosures.

In Canada, Wired glass is tested to CANB/CGSB-12.11 Wired Safety Glass. Contrary to popular belief, the introduction of wire into annealed glass actually increases its risk of fracture under load, effectively decreasing its strength. Wired glass is actually weaker than comparable plain glass, because of the flaws inherent at the edges of the glass as each wire is cut. Internal stresses are also inherent in wired glass because of the differential cooling rates between glass and wire during the manufacturing process and because pf corrosion. According to CGSB-12.20 Structural Design of Glass for Buildings the strength of wired glass is considered to be half the strength of plain annealed glass.

Under the CANB/CGSB-12.11 Standard, cracking can be initiated in a test on wired glass by as little as 100 ft-lbs of impact energy (100 pounds dropped from 12"). The glass is accepted as a Safety Glass however if, once cracked, it does not allow a 3" diameter space to pass freely through an opening.

The scope of the wired glass Standard advises it deals with glass intended for use on skylights, and general glazing in building construction where fire-retardation, security and safety are a consideration.

Wire was introduced into glass to hold the glass pieces together after cracking had occurred. This feature is particularly relevant in fire separation applications. If a window is required in a fire separation, so that people can see through the separation such as in an internal fire door, the only acceptable glass presently recognized by our Canadian Building Code is wired glass.

Glass fractures quickly in the heat of a fire. Plain, heat treated or tempered glass would disintegrate and fall out of its frame. An opening is thus created in the fire separation which would allow the fire to spread. Even though the wired glass cracks under heat, will be held together by the wires, preventing the creation of an opening and the risk of the fire spreading.

While its traditional role was for protection of openings in fire separations, wired glass has often been used incorrectly, as if were expected to resist injury from human impact. While the wire serves to hold the glass together to maintain a fire separation, it also holds any jagged pieces in place, if the glass and wire are penetrated. Serious injuries typically occur, when a hand is drawn back through the hole in a broken wired glass window.

* reprinted with the permission of Southward Consultants Limited.

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