7 ms·

Factor of Safety or F.S. for short was something us civil engineers were taught to never forget. You got grades deducted if you solved the problem correctly but

Factor of Safety or F.S. for short was something us civil engineers were taught to never forget. You got grades deducted if you solved the problem correctly but forgot to include it in the very last line. It makes sure we calculate the loads correctly and use appropriate materials. You can't fix a bad design. The Arkansas bridge that has been in the news lately probably would have collapsed if it wasn't for the F.S. https://www.ardot.gov/divisions/public-information/40-ms-river-bridge/ https://www.ardot.gov/divisions/public-information/40-ms-riv...

whatshisface 5y ago
>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.
- rossdavidh 5y ago
  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.
  - Gibbon1 5y ago
    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.
    - wiredfool 5y ago
      Fabrication errors. Off by one errors.
    - mike50 5y ago
      In what discipline? In Electrical Engineering a fuse circuit breaker or receptacle may be used thousands of times and in dozens of redesigns.
      - jeremyjh 5y ago
        Much of civil engineering is one-off designs.
- wiml 5y ago
  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?
  - whatshisface 5y ago
    Perhaps implicit institutional knowledge of large safety factors is why management feels safe deferring maintenance.
- iscrewyou 5y ago
  Can’t tell if this is sarcasm. Because of goal of civil engineering is building man made objects with public safety in mind.
  - zdragnar 5y ago
    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.
    - LegitShady 5y ago
      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.
unethical_ban 5y ago
Aren't all the major bridges in New York built with ridiculous safety factors? It's why these century old bridges built for carriages and small trucks in a city of a million, can deal with 2021? It's fascinating to me, and I feel like over-spec'ing certain chokepoints in infrastructure makes sense like this.
- hypertele-Xii 5y ago
  Then there's the factor that, in the times such a bridge was built, having it collapse would've been a larger catastrophy than today. Today, we can quickly fix things and build another. In the old times, that bridge might've been the only bridge making trade at all possible and it might've taken years to stack stones. An unnecessarily strong castle takes you more time and resources to build. A slightly too weak castle means you die, your family dies, and you lose all wealth and power.
  - thaumasiotes 5y ago
    > An unnecessarily strong castle takes you more time and resources to build. > A slightly too weak castle means you die, your family dies, and you lose all wealth and power. You're really glossing over a lot of important concerns here. There are many, many ways to lose all your wealth and power, and lots of them might have been avoided if you'd had a few more spare resources.
    - candiodari 5y ago
      Actually one of the reasons big infrastructure projects were undertaken in the middle ages was just the opposite: to provide a reason for an economy to exist. Something for the population to do. Most spectacularly this is seen in Cathedrals, but castles, certainly some castles, definitely show this. I would argue pretty much everything the Romans built is more than a little overspecced, certainly at construction time (do you really need 3 story POOLS, meaning pool on -1, pool on ground floor, pool on first floor, pool on 2nd floor buildings ? Yes the higher ones were apparently rented out to very rich Romans, or more often provided as favors to them, so there was some function, but ... come on. They were constructed to mostly look very convincingly to be 3 story pools without actually being that. Despite that, none of the buildings survived for very long. But "over the top" can be said about many Roman structures, from the Pantheon to the Aya Sofia) So having an unnecessarily big infrastructure project has it's own advantages that actually increase your odds of survival, and that didn't start with the space race.

whatshisface 5y ago

>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.

rossdavidh 5y ago
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.
- Gibbon1 5y ago
  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.
  - wiredfool 5y ago
    Fabrication errors. Off by one errors.
  - mike50 5y ago
    In what discipline? In Electrical Engineering a fuse circuit breaker or receptacle may be used thousands of times and in dozens of redesigns.
    - jeremyjh 5y ago
      Much of civil engineering is one-off designs.
wiml 5y ago
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?
- whatshisface 5y ago
  Perhaps implicit institutional knowledge of large safety factors is why management feels safe deferring maintenance.
iscrewyou 5y ago
Can’t tell if this is sarcasm. Because of goal of civil engineering is building man made objects with public safety in mind.
- zdragnar 5y ago
  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.
  - LegitShady 5y ago
    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.

unethical_ban 5y ago

Aren't all the major bridges in New York built with ridiculous safety factors? It's why these century old bridges built for carriages and small trucks in a city of a million, can deal with 2021? It's fascinating to me, and I feel like over-spec'ing certain chokepoints in infrastructure makes sense like this.

hypertele-Xii 5y ago
Then there's the factor that, in the times such a bridge was built, having it collapse would've been a larger catastrophy than today. Today, we can quickly fix things and build another. In the old times, that bridge might've been the only bridge making trade at all possible and it might've taken years to stack stones. An unnecessarily strong castle takes you more time and resources to build. A slightly too weak castle means you die, your family dies, and you lose all wealth and power.
- thaumasiotes 5y ago
  > An unnecessarily strong castle takes you more time and resources to build. > A slightly too weak castle means you die, your family dies, and you lose all wealth and power. You're really glossing over a lot of important concerns here. There are many, many ways to lose all your wealth and power, and lots of them might have been avoided if you'd had a few more spare resources.
  - candiodari 5y ago
    Actually one of the reasons big infrastructure projects were undertaken in the middle ages was just the opposite: to provide a reason for an economy to exist. Something for the population to do. Most spectacularly this is seen in Cathedrals, but castles, certainly some castles, definitely show this. I would argue pretty much everything the Romans built is more than a little overspecced, certainly at construction time (do you really need 3 story POOLS, meaning pool on -1, pool on ground floor, pool on first floor, pool on 2nd floor buildings ? Yes the higher ones were apparently rented out to very rich Romans, or more often provided as favors to them, so there was some function, but ... come on. They were constructed to mostly look very convincingly to be 3 story pools without actually being that. Despite that, none of the buildings survived for very long. But "over the top" can be said about many Roman structures, from the Pantheon to the Aya Sofia) So having an unnecessarily big infrastructure project has it's own advantages that actually increase your odds of survival, and that didn't start with the space race.