Heat likely culprit for shattering glass on Jacksonville house door

JACKSONVILLE, Fla. — This heat wave is dangerous to people and windowpanes. A Jacksonville homeowner says the heat broke the glass in his front door.

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Jason Haraldsen is a physics associate professor at the University of North Florida. He teaches about materials and thermodynamics. He says it’s extremely rare for the window to break the way it did. A lot of factors play into creating a shatter like this one.

“Glass is usually very robust, especially at the hot temperatures of Florida,” Haraldsen said.

He says Michael Hammond’s door fell victim to a rare event.

“Yesterday, I was at work in my office and I heard a pop,” Hammond said.

He added on to say, “Immediately, I thought that someone was cutting grass and hit a rock. And I expected to find a rock outside of my, on my front door. And opening the door was not there.

So, Hammond believes it was the heat. According to the Action News Jax First Alert Weather team the heat index peaked at 112 degrees yesterday afternoon.

“It gets really hot because this door faces west.”

Haraldsen says it was a perfect storm. With the sun shining on this door during peak hours, it created this broken glass. But the glass didn’t break on the outside, it shattered on the inside.

“If he puts tint on it, if it’s a fairly thin piece of glass, it’s on a front door, which means that rattles and gets shaken,” Haraldsen said. “And the makeup of the door could be putting strain and stress on it. All of those things combined with a really hot day. Something can happen.”

Hammond says the door is made up of wood on the inside and steel on the outside. Since it was getting so hot, he added a tint to the inside window.

“The light is traveling through the window and would typically travel through the second window, but is being absorbed by that tint,” Haraldsen said. “And when that is absorbed by the tint, that energy goes somewhere and it typically goes into heat.”

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Heraldsen says this combined with thermal fracturing – which is when glass expands from increasing temperatures – could cause microfractures on the glass.

“And if the steel is actually putting pressure on the wood, then once one of them breaks, that would send a ripple effect through those and it would happen instantaneously,” Haraldsen said.

Should you be worried about your home windows? Haraldsen says no.

When I reached out to glass repair shops in the area, they said they don’t receive a lot of calls about shatters like this one.

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