[burzum]

There's no kind of existing bridge defect that could be aggravated by a spell of 90F temperatures?

That seems like a bold statement to me.

For structural steel I would be more worried about colder temperatures and brittle fracture than higher temperatures. The yield strength for structural steel does vary for temperature, and I'd need to pull out a table to tell you how much. But the change up to 90 F will almost certainly be insignificant until you get up to temperatures about 800 F.

[Poser]

“But the change up to 90 F will almost certainly be insignificant until you get up to temperatures about 800 F.”

My 1948 Harley has steel pistons. They are regularly subjected to the temperatures associated with burning gasoline. (above 90) They are still running after 59 years.

Steel isn’t human. It doesn’t mind hot weather.

[Petronski]

Again, though, I was not speaking about a bridge failing because of a 90+F heatwave. I was talking about that kind of heatwave combining with other defects to precipitate a failure.

I thought complex systems fail because of a combination of smaller factors which individually could not cause catastrophe alone.