This thread started because of "the cheapest bridge that just barely won't fail"

My point was that safety factors are a part of this. A safety factor of 1.0, designing bridges so that they can perfectly withstand the expectations of intended use, means that some unacceptable % of those bridges will fall down in practice.

In other words, it's true that you can explain safety factors as:

> Assuming perfect construction, and no defects, under designed maximum load, make sure that this bridge really stays up by a wide margin

But that misses the point of why we use safety factors. Nobody is paying for a bridge to really stay up by a wide margin. Because there's no material difference between a bridge that stays up, and a bridge that really stays up, right up until the point that the weaker one falls down due to inevitable over-loading or defects in construction / materials.

Nobody in the US builds anything (permitted) with a SF factor of 1.0. Doesn't happen.