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The most expensive number in engineering

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Re: The most expensive number in engineering

#71

Post by londons_explore »

The real number engineers should be considering is not the factor of safety, but the probability of failure.

The probably of failure should be calculated considering material defects, forces larger than predicted, simulation errors, and all other causes the factor of safety is designed to protect against.

Then the engineering process can allocate those probabilities in the most efficient way.

For example, in a rocket it might make sense to make the engine bells stronger (decreasing probably of failure) while making the fuel tanks weaker (increasing probability of failure). The overall probability of failure remains the same, but perhaps the craft ends up lighter/cheaper/better than it would be if all components just built in a fixed factor of safety.

Re: The most expensive number in engineering

#72

Post by londons_explore »

ghaff wrote: post #59
wiredfool wrote: post #56

Earlier quoted context omitted.

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.

But the question is, if you used a probabilistic approach, and tried to model, even very roughly, those things (probability contractor bodges the job in a way inspection doesn't notice: 10%), then would you end up with a safer bridge for the same money spent?

Re: The most expensive number in engineering

#73

Post by im3w1l »

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

I think we need to distinguish between must-have and nice-to-have safety. To give an example, a car must not sponaneously disintegrate during normal high-way driving. That's what safety factor covers. If you floor it during heavy rain, and start skidding, crash into a tree, then that's kind of on you. Nevertheless the car will try to save you with abs breaks, crumple zones and airbags and what not.

So the future you speak of is already kind of a reality with cars. But maybe expressing it as a pure factor is the wrong way to think about it.

Re: The most expensive number in engineering

#74

Post by ghaff »

londons_explore wrote: post #72
ghaff wrote: post #59

Earlier quoted context omitted.

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

But the question is, if you used a probabilistic approach, and tried to model, even very roughly, those things (probability contractor bodges the job in a way inspection doesn't notice: 10%), then would you end up with a safer bridge for the same money spent?

It feels like you're picking numbers out of the air (or basing them on historical experience) either way. Unless you actually have historical data on certain types of problems--but then it seems like you're pretty much back to a safety/fudge factor.

Re: The most expensive number in engineering

#75

Post by firebaze »

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.

The funny thing is a safety factor is a factor after all. Just one needs to be small enough, and the whole construction (pun intended) may collapse in the worst imaginable manner.

This happened recently in Italy: https://www.bbc.com/news/world-europe-57219737. "Engineers" didn't consider the safety brake essential (i mean, why do you even need it?), and Murphy took his chance.

Re: The most expensive number in engineering

#77

Post by wiredfool »

abduhl wrote: post #69
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…

Most modern bridges you drive over are designed the probabilistic way that is suggested in the article. Bridge design followed vertical construction as material science and manufacturing got better for steel and concrete. The probabilistic approaches haven’t been adopted much in engineering fields that deal with too many unknowns. I’m actually incredibly surprised that there was no mention of the technical terms for…

And they're all used, to some extent. If your Wizzy design based on the latest everything doesn't pass the grumpy old partner's YTLRD review, you're going to redo it till you do.

(In my case, that was Bill. He was one of those guys who knew where to put the $50k mark)

Re: The most expensive number in engineering

#78

Post by londons_explore »

ghaff wrote: post #74
londons_explore wrote: post #72

Earlier quoted context omitted.

But the question is, if you used a probabilistic approach, and tried to model, even very roughly, those things (probability contractor bodges the job in a way inspection doesn't notice: 10%), then would you end up with a safer bridge for the same money spent?

It feels like you're picking numbers out of the air (or basing them on historical experience) either way. Unless you actually have historical data on certain types of problems--but then it seems like you're pretty much back to a safety/fudge factor.

But a large number of guestimated fudge factors all added up will approach the true value as long as there is no bias.

The same does not apply for factors of safety - a 1.5 FoS is always between 0 and 50% too much.

Re: The most expensive number in engineering

#79

Post by marmaduke »

charcircuit wrote: post #65

Are we sure this is the most expensive? I would guess the number which represents the bit pattern of a Windows 10 iso cost Microsoft more than $1.5 billion to find. I am sure you can find other examples of numbers that were expensive to find.

Is your username a play on charcuterie?

Re: The most expensive number in engineering

#80

Post by djoldman »

snowwrestler wrote: post #34

Grossly increasing the factor of safety is a subtle way that science fiction stories connote a feeling of very advanced technology. For example in the JJ Abrams movie Star Trek Into Darkness we see the Enterprise operating at depth in an ocean, then dramatically zooming away into space. Then later another ship falls from orbital height and plows through San Francisco without losing its hull shape. In Star Wars the Mi…

I wonder if the strength of the monolith isn't targeting durability on the order of eons?
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