Over the past several years, there have been many instances of glass balcony guards breaking.
All glass breaks, but different types of glass break more or less easily. The type of glass typically used in windows, for example, is annealed glass, which is the weakest type of glass. When annealed glass breaks, it breaks into long sharp shards. This can create a dangerous situation if, for example, such glass is used in a door that someone might push their hand through.
Glass can be made stronger through a process known as heat treating. In this process, annealed glass is heated to about 700 degrees Celsius, then rapidly cooled, or “quenched”. The glass surfaces cool at a faster rate than the interior of the glass, which creates a compression zone on the face of the glass and a tension zone inside the glass. This makes the glass more resistant to breakage.
There are two types of heat-treated glass: heat strengthened and tempered. Heat-strengthened glass is about twice as strong as annealed glass, whereas tempered glass is four to six times as strong as annealed glass. The difference between heat-strengthened and tempered glass is the rate at which the glass is cooled. The faster cooling of tempered glass results in a stronger compressive surface, improving its strength, but it also results in more tension in the glass.
In situations where injury due to breakage is a concern, tempered glass is typically used. Because of the stresses in the glass, when tempered glass breaks, it breaks into tiny crystals with relatively blunt edges that are far less likely to cause injury. For this reason, and because of its improved strength, tempered glass is used for balcony guards.
Over the past 20 years or so, glass balcony guards have become more common. Glass is architecturally appealing and allows an unimpeded view from a balcony, unlike panel or picket balcony guards. Glass is also relatively inexpensive, it doesn’t corrode and it requires little maintenance, unlike metal guards that require regular painting.
As balcony lengths have increased, the size of the glass panels used on balconies has also increased. Not only has the length and height of glass balcony guards increased; the thickness of the glass has also increased to enable the glass guards to resist the increased loads from wind and potential impacts.
So why have glass balcony guards been breaking?
Nickel sulphide inclusions
One of the possible causes of such glass breakage is a nickel sulphide inclusion. Sulphur is used in the glass-manufacturing process to help eliminate bubbles. Glass can be contaminated by nickel being introduced into the glass-manufacturing process in the raw materials, during handling and storage, or from the fire brick in the furnaces. A single gram of nickel can contaminate thousands of tonnes of glass.
At high temperatures, such as those encountered during heat treating, the nickel sulphide particle is smaller than it is at lower temperatures. Due to the rapid cooling required to manufacture tempered glass, the particle can be trapped at the smaller size. Over a period of anywhere from a few months to a few years, the nickel sulphide converts to the larger size and that expansion of the nickel sulphide creates stresses in the glass.
If the nickel sulphide inclusion is in the perimeter compression zone of the glass, the glass may survive the expansion forces. However, if the nickel sulphide inclusion is in the tension zone in the interior of the glass, the expansion forces can cause spontaneous breakage.
The problem of nickel sulphide inclusions is not new. However, as the size of glass balcony panels has increased, so has the probability of having a nickel sulphide inclusion in the tension zone of a particular panel.
There is a telltale sign that a nickel sulphide inclusion has caused a panel to break, and that is a “Double D” or butterfly pattern in the broken glass at the point of the inclusion. When a breakage has occurred, examine the broken glass for this sign before it is cleaned up to determine whether a nickel sulphide inclusion was the cause of the breakage, as opposed to a projectile. The likelihood of a piece of glass breaking due to a nickel sulphide inclusion will tend to decrease over time.
There are varied opinions as to what is an “acceptable” breakage rate. While building owners would like to see zero breakage, it is this author’s opinion that two breaks in 1,000 panels should not be considered unusual. If a building experiences more breaks than this, seek professional expertise to determine an appropriate course of action to minimize further breakage.
Glass breakage can also be caused by glass-to-metal contact or by glass-to-glass contact. The simple act of the glass edge rubbing against a mounting plate, a guard post or the edge of an adjacent piece of glass can cause the glass to break. Breakage can also be caused if there is a temperature difference between glass and metal if they are in contact.
To prevent such contact, appropriate clearances must be maintained, typically by mounting the glass on setting blocks (pieces of neoprene, EPDM, silicone, or other elastomeric material) and using gaskets or sealant to separate glass from metal supports. One of the problems that has occurred with the larger sizes of glass panels now being used as balcony guards is that, due to their weight, they are difficult to handle and assure that the appropriate clearances are maintained when they are installed.
Also, over time, if a glass panel is not secured on all four sides, it may shift its position due to buffeting from wind or movement of the building structure. So even if correctly installed in the first instance, over time, contact breakage may increase. Unlike a nickel sulphide inclusion problem, shifting of panels can be readily observed and corrected before breakage occurs.
New requirements for new construction
In 2011, following a series of cases of glass balcony guards breaking, the Ministry of Municipal Affairs and Housing struck an expert panel to study the issue. In 2012, the Ontario Building Code was amended to include Supplementary Standard SB-13, which became effective July 1, 2012.
The supplementary standard prescribes requirements for the design and construction of glass in guards that are intended to reduce the probability of breakage or injury; it is recognized that it is not possible to eliminate breakage. One such requirement is to use heat-strengthened laminated glass for guards located beyond the edge of a floor (e.g. on the face of the balcony slab).
SB-13 is applicable only to new construction or when a major renovation is being undertaken, such as the replacement of the balcony guards. The supplementary standard does not require existing balcony guards to meet its requirements.
Can glass breakage be prevented?
Unfortunately, it is impossible to entirely eliminate nickel sulphide inclusions in glass, so it’s difficult to prevent all glass breakage. However, there are steps that can be taken to ensure that the risk of injury is minimized if glass does break.
Some experts recommend that tempered glass be heat soaked to determine if it has, or likely has, nickel sulphide inclusions. In this process, the tempered glass is reheated in a controlled factory environment to force breakage of glass with nickel sulphide inclusions. The problem with this approach is that it adds significantly to the cost of the glass, and some uncontaminated glass may break, further adding to the cost. However, using only heat-soaked glass will certainly minimize, if not eliminate, the use of glass with nickel sulphide inclusions.
A buffer zone, about 20 feet wide, can be created around the perimeter of the building using planters or other means to limit pedestrian access. Should a glass panel break, injury will be minimized as there should be no one in the area where the glass falls. Of course, this approach may not be feasible in dense, urban areas.
Support of the glass panel
In recent years, framing around glass balcony panels has been reduced or even eliminated in some cases. This has resulted in the need for thicker pieces of glass to compensate for the reduced structural contribution from framing. And thicker glass increases the chances of nickel sulphide inclusions in the tension zone of the glass. Designing balcony guards with metal framing allows thinner glass to be used, reducing the probability of a nickel sulphide inclusion.
In some designs, the balcony panels are mounted on the face of the balcony slab rather than on the top of the slab. When the glass breaks, all of the glass will fall to the ground. Some experts believe that keeping the glass back from the edge of the balcony slab will minimize the amount of glass falling to the ground and potentially injuring someone.
On some new buildings, laminated glass is being used for balcony guards. An interlayer film bonds layers of glass together. As a result of the glass being held together, it is less likely that glass pieces will fall to the ground should the glass break.
However, this approach will only work if the laminated glass is held securely in a frame. If the laminated glass is not held on all four edges, it could fall, and a large piece of falling laminated glass is likely to cause more injuries than small pebbles of tempered glass. For this reason, the use of structural silicone sealant on the exposed edges is recommended to adhere laminated glass to adjacent metal posts.
For retrofits, there are films that can be applied to the surface of the glass to hold it together should it break. To be properly applied, the balcony glass should be removed from its supports so that the entire surface of the glass can be covered with the film, not just the area of glass within the inside perimeter of any framing. As with laminated glass, if a film is applied, the glass must be secured in place on all four sides.
It is difficult to install such films outside of a controlled environment, and as the films are on the surface of the glass, they are prone to degradation due to ultraviolet radiation from the sun. It is preferable to use laminated glass rather than apply films, though the use of laminated glass would require complete replacement of the glass, which is an expensive proposition.
As noted earlier, while glass breakage due to nickel sulphide decreases over time, contact breakage will increase. Annual inspections of balcony guards are recommended to identify potential glass-to-metal or glass-to-glass contact and to take corrective action. These inspections can be conducted by the property manager.
Dale D. Kerr, M.Eng., P.Eng., BSSO, ACCI, is a building science specialist with GRG Building Consultants with more than 30 years’ experience in assessing and repairing buildings.