Warning in this context means something obvious to the unsuspecting user that collapse is imminent. Like a building creaking or buckling minutes or hours before finally giving way.

I take that to mean there were no signs of imminent collapse, just sudden failure.

In engineering terms thats a mode of collapse, thing, not about people actually warning.

When we do design, we're generally designing things so that if it fails it does so in a way that doesn't suddenly all collapse at once, but first deforms for a while so people can get away.