[StevenWaterman]

Safety factors account for uncertainty. Uncertainty the quality of materials, of workmanship, of unaccounted-for sources of error. Uncertainty in whether the maximum load in the spec will actually be followed.

Without a safety factor, that uncertainty means that, some of the time, some of your bridge will fall down

[irishcoffee]

A safety factor of 1.0 means “the structural integrity of this construct will meet the expectations of intended use with no issues.”

A safety factor of 1.7 means “if this construct is used in a way that is 70% more abusive than anticipated, the structural integrity should remain in tact.”

You’re hand-waving enough here that you have the luxury of agreeing or disagreeing with me, well-played. Your initial response was glib and not terribly productive.

[StevenWaterman]

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.

[irishcoffee]

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

[StevenWaterman]

Yes, because it would fall down (sometimes, often enough that regulatory bodies forbid it)