BY PETER KENTER - A well-engineered window or "light" in the right environment can transform a building project.
However, David De Rose, executive vice president of restoration practice at Halsall Associates notes that it isn’t a question of whether some windows will break—it’s about reducing the incidence of breakage and improving window materials, technology, installation procedures and monitoring to ensure they perform to specifications.
Glass can break for any number of reasons, including embedded material impurities and inclusions, edge damage during installation, insufficient accommodation for movement, misunderstood load paths and “direct bearing,” in which loads are placed on glass.
“It’s generally accepted in the industry that a couple of lights per thousand will have impurities, such as nickel sulfide,” said De Rose.
“If sunlight striking a window agitates the impurity, it can spontaneously break. This is a phenomenon associated only with tempered glass and tempered glass is safety glass, so when it fails it breaks into little pieces and that’s the intent.”
De Rose noted that during Toronto’s condo boom, as many as 100,000 windows are being installed each year.
With two breakages per thousand, 200 condominium windows might break annually, of which perhaps 10 or 15 break in such a way that glass fragments reach the ground.
“It’s blown way out of proportion,” he said.
“However, there was a lot of pressure from regulators to do something about it.”
The result was Interim OBC SB-13 “Glass in Guards,” a regulation specifying where heat-strengthened laminated glass, heat-soaked tempered glass, and tempered glass of not more than 6 mm thick could be used in relation to the edges of floor slabs.
However, the interim standard created difficulties in interpreting how glass should be processed, secured and tested.
For example, only a few ovens exist in Canada to process heat-soaked tempered glass, under direction of a borrowed German standard.
“It’s kind of a no-man’s land,” said De Rose.
“And, glass could still get nicked during transport and handling. There’s still impact and there are still lots of reasons why glass breaks.”
Windows can fail for reasons other than breakage.
In one Halsall project, a building designer specified a design that would provide multiple patterns on different levels of glass, changing its appearance depending on the angle from which the patterns were viewed.
“It’s a good thing they took lots of samples,” said De Rose.
“We discovered that the unintended consequence could have been an unsightly moiré pattern that could make the windows look like a bull’s eye. We did samples and full scale mockups so everyone would know what it was going to look like.”
On a building designed with an architectural metal screen in front of glass, Halsall realized that the design would interfere with replacing glass panels.
The screen structure was redesigned with a trolley that could convey replacement glass along the face of the building.
On First Canadian Place, Halsall added a little insurance to the project by creating an additional run of full metal frames containing glass panels, which were stored at the top of the building as “attic stock.”
“You can now just remove the broken unit and lower the new one into place,” said De Rose.
As panels become larger and schedules become tighter, De Rose stresses that it’s even more important to ensure that joints, seals, membranes, drains and air systems work properly before installation.
He also noted that even when design and installation is handled properly, ill fortune can still rear its ugly head — in the form of window washers.
“The Glass Association of North America guidelines say never use scrapers on glass,” he said.
“Dirt and grit can get caught under the blade and you won’t even know you’re scratching the glass. In your written specifications, make sure the building owner knows how to clean glass surfaces properly.”
De Rose spoke at the Construct Canada conference in Toronto.