[burzum]

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.

In engineering systems (my experience is with nuclear reactor pressure vessels), there is usually a design factor of 2 to 3. This is different with aircraft due to weight concerns, but in other areas (like bridges) it would be very surprising if a low design factor was used. It is much more likely that fatigue failures, corrosion induced failures, or damage to a structural member due to construction caused this bridge collapse than that the ultimate strength was exceeded for any component.

[editor-surveyor]

"but in other areas (like bridges) it would be very surprising if a low design factor was used"

But wasn't the bridge widened? That might have resulted in a deep reduction in safety factor (to 0.9?).