It was so hot last week, even steel bridges were sweating. The DuSable Bridge in Chicago overheated enough to stop working. While it wasn’t dangerous for drivers, the bridge couldn’t open for boats, and had to be closed for a half hour while firefighters hosed it down with cold water.

The 98-year-old double-decker bridge shuttles two levels of traffic back and forth over the Chicago River, and opens its decks to the heavens to let sailboats pass through. Baking under the sun for days on end, the joints of the steel bridge swelled, and were stuck in place. Luis Benitez, chief bridge engineer for the Chicago Department of Transportation, says the surface temperature of the bridge had climbed above 100°F that week. The steel finger joints—long, finger-like parts that accommodate expansion as cars travel over the bridge—interlock leaving a bit of space between each part, but the heat caused them to expand enough to rub together. You can think of it like a bridge with arthritis; the joints had become too inflamed to move properly.

“The temperature was getting too hot, and the bridge isn’t just expanding longitudinally, it was expanding sideways,” says Benitez. “Those two pieces of metal started rubbing together causing friction. Everything is so finely tuned, if you tried to force them apart, those joints can come loose or the expansion could get damaged.”

For the finger joints to touch, Benitez says the steel would have had to expand around an eighth of an inch. That doesn’t seem like much, but in the world of sweaty steel, it’s enough to temporarily shut down a bridge.

Although heat waves like the one that suffocated most of the eastern U.S. recently won’t cause a bridge to suddenly collapse, spells of hot, humid weather can cause a lot of problems for infrastructure. Materials like steel, cement, and asphalt—basically the building blocks of every city—eagerly absorb heat, and can reach surface temperatures of 140 degrees on a scorching day.

“The problem is in high heat waves, steel can expand if its above its tolerance limit,” says Rae Zimmerman, director of the Institute for Civil Infrastructure Systems at NYU’s Robert F. Wagner Graduate School of Public Service.

“When it expands the only way it can go is to the side, so that’s why we see wavy rail lines. Wheels also become deformed in heat,” she says. “When trains run over the rail lines, they also increase the temperatures of the lines themselves and make the problem worse.”

Anna Brooks