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Comment by whatshisface | Hacker News Reader

>It makes sure we calculate the loads correctly and use appropriate materials.

No, it make sure that nobody dies when you calculate the loads incorrectly and use inappropriate materials.

I think the idea is that, if you calculate the load incorrectly enough, or use inappropriate enough materials, the safety factor will not save you. But, if you have done those things correctly, then the safety factor should be enough to save you from normal unknowns, unexpecteds, etc.

Yeah four things going on.

  Design errors
  Probabilistic nature of the loads applied.
  Material defects
  Fatigue
  Deterioration

All structures have a service life and it's the service lifetime an experienced engineer is trying to hit.

For the impeller in a rocket turbo pump the service life is like 5 minutes. For the impeller in a hydro electric dam it's 50 years.

The other thing that one of my professors pointed out was that 80% of engineers end up designing once off designs. Where the NRE cost is a lot more than the material costs. Shaving the safety factor is false economy.

Fabrication errors. Off by one errors.

In what discipline? In Electrical Engineering a fuse circuit breaker or receptacle may be used thousands of times and in dozens of redesigns.

Much of civil engineering is one-off designs.

Also helps when management decides to defer maintenance for a decade or two and someone drives a truck over it that's just a bit over the weight limit what could it hurt?

Perhaps implicit institutional knowledge of large safety factors is why management feels safe deferring maintenance.

Can’t tell if this is sarcasm.

Because of goal of civil engineering is building man made objects with public safety in mind.

Another way to look at factor of safety is margin for error. Implementation variance, material variance, etc can all go wrong if something is designed to be exactly safe.

You need to know that something is redundantly safe, and which parts.

Material variances are included where the capacity is calculated. materials with hIgher variance like concrete (implementation variance) and wood (material variance) get their capacity lowered more than more consistent materials like steel.

The factor of safety is above and beyond material variance. You calculate the worst case load combinations for that component, then you check your factor of safety. Civil engineering is relatively conservative in its estimations for everyone's safety.

In my experience serviceability requirements (like reducing uncomfortable deflections that dont threaten the safety of the structure) often govern, rather than the ultimate capacity.

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