Everyone loves glass — until it rains down from the sky. Glass is sleek, sexy and it sells. Architects love it, builders adore it and consumers demand it. But engineers can ruin the love-in.

The battle between architect and engineer is a time-honoured tradition dating back to the invention of chalk and tools. Architects, engineers say, are superficial, concerned with nothing but looks. Engineers, architects say, are smart, but unsophisticated droogs obsessed with rules and ruining masterpieces.

Usually the two sides are copacetic. But glass leaves the two symbiotic professions at odds. And there’s tons of glass in Toronto. Some of it is now exploding from balconies 30 storeys high and crashing to the ground in the downtown core.

When Daniel Libeskind sketched out the new addition to the ROM on a napkin, he envisioned a crystal made of nothing but glass. The initial drawings were stunning.

Once engineers at Halsall Associates Ltd. examined the plans, they were incredulous.

Neil Hoult, a civil engineering professor at Queen’s University who used to work at Halsall, had many concerns at the time. Could a giant glass structure hanging over a crowded sidewalk be safe? How would the constant stream of sunshine affect the museum’s artifacts? Is cost no option?

Those concerns morphed Libeskind’s sleek vision into a bird’s nest, composed of just 25 per cent glass and 75 per cent aluminum. The compromise seemingly left everyone unhappy.

“Glass is popular because the market demands it, and from an architectural perspective, it’s beautiful,” said Vincent Hui, an architecture professor at Ryerson University.

But glass isn’t perfect. It remains relatively brittle despite advances in tempered glass technology. It’s horrendously leaky from an energy conservation standpoint, especially when compared with old-school materials such as brick and concrete.

“And how many people want to live in a bunker 30 storeys up?” Hui asks. “In a dense environment, people want views. Glass gives them that.”

And glass explodes. At least 10 panes of glass from two buildings have spontaneously shattered and plummeted to the ground since December. At least eight exploded at a condominium near Bay and College Sts., while two panes shattered and fell from the Festival Tower above the TIFF Lightbox on King St. W. The city’s building inspectors continue to investigate the cause.

But exploding glass “happens all the time,” according to Doug Perovic, an engineering professor at the University of Toronto.

“It is known as a delayed spontaneous fracture,” Perovic said.

Glass can spontaneously explode for two main reasons.

One, imperfections make their way into the glass during the manufacturing process. These small defects can expand with fluctuating temperatures and other stressors such as wind, Perovic said. As the imperfection expands, the pressure within the glass builds.

“At this point, the glass is like a loaded gun,” Perovic said. “And it’s only a matter of time before it goes off, explodes into a million pieces and the sky is raining glass.”

The other reason glass could randomly shatter is related to the installation process. Pressure from the metal railings, wherever it is attached to the glass, is a constant stress that tempered glass can withstand. But weather and wind can alter the stress on any given point, Perovic said, which can build and shatter from the glass from the outer edges in.

Perovic is often brought in to consult on the “post-mortem” of glass and structure failures. But without analyzing the glass himself, he can only speculate given the details he’s read about and pictures he’s seen.

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Exploding glass isn’t limited to Toronto. There’s been a recent spate of “glass bombs” in Shanghai that have ruined vehicles and terrified pedestrians.

FALLING GLASS