Structures: Glass Fallout
December 1, 2012
By By Gerald R. Genge, P.Eng. GRG Building Consultants
Over the past two years there has been a lot of press about glass falling from buildings across North America. Balcony guard glass has shattered and been the topic of news, parody, and law suits. There have been allegations about “bad...
Over the past two years there has been a lot of press about glass falling from buildings across North America. Balcony guard glass has shattered and been the topic of news, parody, and law suits. There have been allegations about “bad glass,” “bad design,” “bad construction,” “bad standards.”
After several glass panels fell from balcony guards in Toronto, the Ontario Ministry of Municipal Affairs and Housing assembled an expert panel, whose recommendations resulted in changes in the Ontario Building Code. These amendments for glass guards on balconies in new buildings became effective July 1, 2012. They are the most restrictive rules in Canada, and in North America, as far as I am aware, but we could well see similar rules adopted across Canada.
Larger panels and less framing
Glass is architecturally appealing and relatively inexpensive. It provides an open feeling to the balcony area, and it does not corrode or require painting. From a marketing perspective and for aesthetics it is an ideal material for the purpose.
Not surprisingly then, over the past 20 years the amount and prominence of glazing has increased to the point where the balcony is becoming a continuous perimeter element on condominium developments.
But increasing the area of glass balcony guard panels means individual panels are longer, higher and thicker. The assembly has changed from being smaller panels mounted on the top of the balcony slab, to larger panels mounted on the face of the slab. To sustain specified loads from wind and soft impact, this increase in the glass panel size requires an increase in thickness. The thicker the glass, the more likely it is to include an impurity that can cause breakage.
As well, the framing around the glass panels is being reduced or eliminated in some instances, which requires the glass to have even greater thickness. Many glass guard panels installed recently are retained only at the top and bottom, and then only within aluminum channels or by fasteners through holes in the glass, rather than being fully captured by four-sided framing.
Types of glass
There are various types of glass used in buildings. Normally glass is annealed, which toughens it somewhat, but annealed glass still breaks into shards. Heat strengthened glass is stronger than annealed glass, but it doesn’t have the same degree of tempering as fully tempered glass and it does break into shards.
Fully-tempered glass fragments into small crystals with blunt edges should breakage occur. Heat soaking of tempered glass is recommended because it forces the majority of panels that have inclusions to break before being put into use. There are no “heat soaking” standards in any Canadian building codes, but European standards are readily applicable and designers should specify them.
Laminated glass can provide supplemental safety. It is made by laminating two or more pieces of glass together with an interlayer film, which bonds the fragments together should the glass break. Automobile windshields for example are laminated glass. Lamination can be multi-layer and essentially bullet-proof. For the most part though, one interlayer film is used between two sheets of glass.
Laminated heat-strengthened glass is preferred over laminated tempered glass because the breaking of laminated tempered glass would result in a heavy flexible blanket of shattered glass that may fall.
What causes tempered glass
to spontaneously shatter
Glass is a brittle material and quite different from the metal panel and picket materials that once were the norm for balcony guards. In Canada, designers have been using tempered glass for balcony guards and complying with a standard that has been around for the past 20 years — CAN/CGSB 12.1 M90 — Tempered or Laminated Safety Glass. That standard doesn’t say that tempered glass won’t break. It says that if the glass breaks it will break in a safe manner into little pieces. That’s fine if it is used in a glass door or a sidelight panel because it just falls in place, but if it is a balcony guard panel it could fall from quite a distance.
The preponderance of reasoned opinion on what caused many recent guard panel failures is “spontaneous breakage” of tempered glass caused by impurities in the glass. These come from the raw materials used to manufacture glass and most are not a problem. Nickel is a problem. When combined with sulphur from the sodium sulphate used to help eliminate bubbles it can create a nickel sulphide (NiS) inclusion. A single gram of nickel can contaminate thousands of tonnes of glass, and while it has been recognized as an issue since the 1940s and reasonably well managed, minor amounts of nickel cannot be eliminated.
If we should expect some breakage, how much is normal? Opinions vary but reasonable opinion is that if you have around 2 breaks in 1,000 panels you should not consider that to be unusual.
How do I know if my glass has impurities?
You can’t see a particle of NiS with the naked eye. There are non-destructive methods to look for NiS but they are costly, time consuming, and largely impractical.
In the end though, just because your glass panels include NiS impurities, doesn’t mean that the glass is doomed to break. The impurity size, location, and other stresses from wind load all contribute to the likelihood of the glass suddenly shattering.
Changes to the Ontario Building Code
The Canadian glass standard for tempered and laminated safety glass CAN/CGSB 12.1 M90 referenced in Parts 3 and 9 of the National Building Code of Canada is the material standard. For structural design, Part 4 of the National Building Code of Canada references CAN/CGSB 12.20 M89 – “Structural Design of Glass for Buildings.”
With the amendments to the Ontario Building Code (OBC) in July, the situation has changed and the choice of glass materials and procedures for structural design have become more complicated.
There are now clearer requirements in Ontario for glass guard panels — but also more restrictions.
All glass guard panels mounted over the slab edge or within 50 mm back of the slab edge must be laminated heat strengthened glass.
Glass panels between 50 and 150 mm back from the slab edge can be laminated heat strengthened glass or heat-soaked tempered glass. Glass set back more than 150 mm can be just tempered glass.
[See Digital Edition for photos]
For the structural design of a glass guard, the Ontario code requires load from wind and live load from impact to be considered in combination. For some designers, this is seemingly at variance with CAN/CGSB 12.20 M89, which is referenced in the Ontario code and deals with load combinations differently. Priority rules state that any variance between codes and standards gives preference to the code. But for many people, it’s not that simple. In prior years, some designers may not have considered live loads and wind loads in combination and as a result, the overall design of a guard may have been weaker.
New standard in the works
The recent breakage of glass has opened a long list of other issues on building guards. For instance; it is not clear to all if codified live load requirements in OBC 184.108.40.206 are specified loads to which load factors must be applied.
Applicable load combinations remain a disputed point; and there remains no design criterion to deal with effects of vibration of glass panels under wind loads.
Recognizing that the industry needs a more comprehensive standard on building guards, the Canadian Standards Association has initiated a technical committee. The committee is represented by specialists representing designers, manufacturers, owners, regulators, and installers from across Canada. The standard is just getting under way but promises to be a state-of-the-art design, material, installation, and overall performance document. cce
Gerald R. Genge, P.En
g., C.Eng., BDS, BSSO, C.Arb., Q.Med. is a building science specialist and principal of GRG Building Consultants and ArbiTECH ADR, Newmarket, Ont. He was a member of the Ontario Ministry of Municipal Affairs’ Expert Advisory Panel on Glass Panels in Balcony Guards. He is also past president of the Ontario Building Envelope Council. www.grgbuilding.com