Falling glass in Condo buildings – why does it happen?

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Falling glass in Condo buildings – why does it happen?


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August 25, 2011

By Michael VanDusen, P.Eng., Halsall Associates

Glass falling from downtown Toronto buildings has become a major safety hazard. Here is a primer on why this is happening, and what to do to manage your risk.

What is tempered glass?

Tempered glass is used for balcony guards, doors and windows that extend close to the floor because it is considered a “safety glass”. It is strengthened through a process that locks in stress. If something causes it to break, this stress causes the glass to shatter into small fragments rather than large shards as happens with standard glass. Compared to standard glass shards, tempered glass fragments pose less risk for seriously cutting or injuring people.

Why does it break?

Other than excessive impact, conditions that can cause tempered glass to break unexpectedly relate to material properties and system assembly.

Localized stress concentrations can result when “inclusions” - natural imperfections that are infrequently incorporated into some panes by the manufacturing process - expand. Localized stress concentrations can also result from expected deviations in workmanship such as accidental minor damage to glass edges.

These conditions typically lead to glass failure within the first two or three years after installation. When this happens, the solution is to replace the panes.

The exception – more serious problems

The exception is where general problems with design, manufacturing and/or installation exist. Examples could be manufactured glass that does not meet standards, details that do not freely allow for normal movements associated with change in temperature, or excessive deflections arising from a weak design.

How to manage risk

Risk management when using tempered glass is achieved primarily through a project’s quality assurance processes. These include:

Specifying the appropriate glass type to minimize inclusions/imperfections;

Confirming manufacturing quality control processes as part of the procurement process;

Confirming that stress concentrations will not occur at the glass support details with expected movements related to temperature and structural deflection; and

Obtaining the commitment of the project team to ensure that the construction complies with the protocols developed through these processes.

What about laminated glass?

Where breakage presents increased risk of injury or damage, “laminated” glass can be used.

Laminated glass uses a layer of plastic to laminate two (or more) glass panes together. In this way, if the glass does break, the shattered pieces are held in place. This would help restrain the glass from falling when it breaks, even when subjected to abuse from accidental impact or severe weather. (For reference, a common use of laminated glass is in car front windshields. Here, it also reduces the risk of objects or occupants penetrating the windshield. On the other car windows, the glass is only tempered, not laminated, as the risk of penetration is much lower.)

The building code could adopt a similar approach for buildings. This would involve evaluating the risk of injury or damage to, say, pedestrians, if the glass were to break.

Next steps

As the use of tempered glass over sidewalks in our urban cores becomes more prevalent, the discussions related to by-laws and/or building codes will invariably consider whether changes are appropriate to reduce risk.

In the interim, designers, developers and owners should ensure that they have the appropriate quality assurance processes in place and could consider glass upgrades at higher risk areas.

Halsall is the Canadian operating company of Parsons Brinckerhoff, a global professional services firm specializing in infrastructure planning, design, program/construction management and operations and maintenance. www.pbworld.com Reproduced with permission of Halsall Associates

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