The most expensive number in engineering • Hacker News Board

Re: The most expensive number in engineering

#51

Post by hypertele-Xii » Mon, May 31, 2021, 6:52 PM UTC

unethical_ban wrote: post #47

iscrewyou wrote: post #12

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.…

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.

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.

Re: The most expensive number in engineering

#52

Post by iab » Mon, May 31, 2021, 6:56 PM UTC

amelius wrote: post #37

Clickbait title, not what I expect from an engineer.

This article is definitely written from the perspective of a novice without real-world experience. Empiricism is not a dirty word!

Re: The most expensive number in engineering

#53

Post by iab » Mon, May 31, 2021, 6:57 PM UTC

jvanderbot wrote: post #6

The factor of safety probably doesn't have a specific definition because it's application and part specific. Its an axiom like the 5-sigma rule not a property of the system.

Exactly, it buffers against modeling inaccuracies

Re: The most expensive number in engineering

#54

Post by chrisgp » Mon, May 31, 2021, 7:00 PM UTC

In finance and gambling, the kelly criterion is used to evaluate maximum bet sizing while keeping risk-of-ruin near 0. Using it correctly requires understanding your own expectation and variance to a high degree of confidence. Everyone in these industries uses kelly to figure out the maximum size they can bet based on these careful expectation and variance calculations, then just divides by 2.

Re: The most expensive number in engineering

#55

Post by wiredfool » Mon, May 31, 2021, 7:01 PM UTC

Gibbon1 wrote: post #45

rossdavidh wrote: post #33

Earlier quoted context omitted.

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…

Fabrication errors. Off by one errors.

Re: The most expensive number in engineering

#56

Post by wiredfool » Mon, May 31, 2021, 7:08 PM UTC

codeflo wrote: post #35

The article suggests using a probabilistic failure model instead of a large a safety factor, and explains how a safety factor established in the 1930s affected the cost of the Space Shuttle. But spacecraft might be a special case, where any additional weight is so expensive, and you also expect the models to be especially accurate and manufacturing to be extremely precise. For more everyday civil engineering, I think…

I'll go out on a pretty small limb and say that the vast majority of Civil Engineering failings are not a matter of an incorrect safety factor, but are things that are explicitly not part of it.

1) Blunders. (Many places. You do the math wrong, or approve the wrong shop drawing, and no factor of safety is going to save you). (See the Hyatt Regency Walkway Failure) 2) Inadequate Geotech Info. (Basically every dam failure ever) 3) Genuinely new behavior. (Tacoma Narrows) 4) Contractors. (I-90 Bridge Sinking) 5) Deferred Maintenance. (Fatigue on bridges, Minneapolis)

Re: The most expensive number in engineering

#57

Post by lamontcg » Mon, May 31, 2021, 7:10 PM UTC

codeflo wrote: post #35

The article suggests using a probabilistic failure model instead of a large a safety factor, and explains how a safety factor established in the 1930s affected the cost of the Space Shuttle. But spacecraft might be a special case, where any additional weight is so expensive, and you also expect the models to be especially accurate and manufacturing to be extremely precise. For more everyday civil engineering, I think…

Or more simply: planes that get hit with massive unexpected turbulence shouldn't just drop out of the sky and kill everyone on board.

And to paraphrase the great philosopher Donald Rumsfeld, it is all about the unknown-unknowns.

Re: The most expensive number in engineering

#58

Post by ben_w » Mon, May 31, 2021, 7:16 PM UTC

glitchc wrote: post #49

UncleMeat wrote: post #41

Earlier quoted context omitted.

I don’t think this is science fiction doing this to connote the future in many of these cases. The cars in the “fast” franchise also survive ludicrous damage. The bodies of action heroes survive falls from ridiculous heights and blows and stab wounds that would kill somebody with ease. This is more a property of action movies (and adventure movies, to a lesser degree).

Seems disingenuous to lump a science fiction movie where the focus is future technologies to an action movie with exaggerated physics and lack of real damage. In action movies, bad guys die with one bullet while the hero finds his way to safety (and survives!) despite having 10 or more lethal bullet wounds. For the record, Star Wars is not science fiction nor has it ever been portrayed as such. It's very much an acti…

> For the record, Star Wars is not science fiction nor has it ever been portrayed as such.

FWIW this is literally the first time I’ve encountered someone claiming that it isn’t science fiction.

(Soft SciFi, to be sure, but still: https://tvtropes.org/pmwiki/pmwiki.php/Main/MohsScaleOfScien...)

Re: The most expensive number in engineering

#59

Post by ghaff » Mon, May 31, 2021, 7:19 PM UTC

wiredfool wrote: post #56

codeflo wrote: post #35

The article suggests using a probabilistic failure model instead of a large a safety factor, and explains how a safety factor established in the 1930s affected the cost of the Space Shuttle. But spacecraft might be a special case, where any additional weight is so expensive, and you also expect the models to be especially accurate and manufacturing to be extremely precise. For more everyday civil engineering, I think…

I'll go out on a pretty small limb and say that the vast majority of Civil Engineering failings are not a matter of an incorrect safety factor, but are things that are explicitly not part of it. 1) Blunders. (Many places. You do the math wrong, or approve the wrong shop drawing, and no factor of safety is going to save you). (See the Hyatt Regency Walkway Failure) 2) Inadequate Geotech Info. (Basically every dam fail…

Agreed. Although I'd probably also argue that a safety factor probably papers over those kind of problems in many many cases.

Re: The most expensive number in engineering

#60

Post by Galxeagle » Mon, May 31, 2021, 7:20 PM UTC

bretpiatt wrote: post #14

I'm not a civil or aerospace engineer so this could be built into the safety factor models already. Reading the post had me wondering: If safety factor adds mass and additional mass requires additional force to accelerate is a lower safety factor safer since you'll lower the amount of force required thus increasing the structural safety? Calculating safety factor for a given scenario feels like a complex multivariabl…

Also not an engineer but watching a real-world example of that thought process was fascinating during NASA and SpaceX's design process for Dragon Capsule, that contained a requirement that the capsule needed to have a statistical probability of loss-of-crew less than 1:270 flights, which is the alternative design measure in TFA.

One challenge was NASA's modelling of in-orbit micrometeorite strikes was complex, and there were concerns that extra complexity to provide redundancy and armor would make an overall less-safe vehicle.

“Blindly striving to achieve a statistical loss of crew number may drive you to design a system that is less safe" -Bill Gerstenmaier, NASA associate administrator for human exploration and operation [0]

[0] https://spacenews.com/commercial-crew-vehicles-may-fall-shor...