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The most expensive number in engineering
31–40 of 176 posts
Re: The most expensive number in engineering
#32I'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…
Yes, all (essentially all) engineering design ends up being multivariable. For even something as simple as a cantilevered beam supporting a load, if you can change the shape, material, material treatment, length, width/height, all of which affects cost. Usually due to limits of manufacturing and availability of standard parts, the exploration space can be greatly reduced.
Re: The most expensive number in engineering
#33Factor 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.…
> 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.
Re: The most expensive number in engineering
#34Grossly 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 Millenium Falcon is constantly doing things that would seem to be outside a normal design for a spacecraft, and it survives (aside from the radar dish).
Even as far back as the movie 2001, the monolith is made out of a material that humans can’t dent or cut. Why so strong? It’s basically just an automated radio.
The idea is: this advanced civilization has such command over physical technology, that they can effortlessly engineer unnecessary strength without losing any of their designed performance.
Re: The most expensive number in engineering
#35The 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 the safety factor "covers up" a lot of systemic inaccuracies everywhere in the system, from modeling to design to manufacturing to unintended uses. Some of those you might account for in a probabilistic model. But it's very difficult to probabilistically model errors in the model itself, as the financial industry found out the hard way.
When driving over a bridge built the way suggested here, how comfortable can we be that certain stresses aren't correlated in ways that the engineers didn't anticipate? Or that a certain distribution isn't actually as well approximated by a Gaussian as it was assumed to be? Intuitively, it's a lot harder to be wildly wrong with the factor of safety approach.
To put this another way, a more complex way to reason about safety necessarily has more moving parts, and is thus more likely to be wrong. So in effect, adopting more complicated safety models introduces a safety risk all on its own. I think that needs to be considered as well.
Re: The most expensive number in engineering
#36Reading this it seems like something like this could be used for software estimates as well? Bake in a factor of safety into your estimates depending on the type of work, the track record of the team that’s doing the work etc
I have seen attempts at it. One is to multiply your estimates by the number of different pieces you're estimating. So, if you have estimated for three different pieces, multiply those estimates by 3 when deciding how much the whole thing will take. If you have estimated for five different pieces, multiply the sum by 5, etc. The idea is that the more estimates you have made, the more likely that at least one of them will "blow up" and take far longer than expected.
Generally speaking, though, software is far less advanced than civil or aeronautical engineering in this kind of thing.
Re: The most expensive number in engineering
#37Clickbait title, not what I expect from an engineer.
Re: The most expensive number in engineering
#38This analysis is great. It makes for a great blog post, university lecture or similiar. But unfortunately you can't have these kinds of discussions in an engineering meeting, or other shared context because anything that could be perceived as arguing for less safety will attract opposition because there's tons of people who want the cheap virtue points and ass-covering that goes with being the guy who's always in fav…
I think it tends to come up more in fields, such as aeronautical engineering, where there is a safety tradeoff. If you make the plane heavier, it may be safer from material failure, but now there may be less margin for error by the pilot because the plane does not respond the same. You have traded one kind of risk against another. I remember being present when a friend who was a civil engineer heard that they used a safety factor of only 1.5 in aeronautical engineering, and she was kind of shocked it was so low; when you don't have to fly the thing, you can afford to make the factor significantly higher.
Re: The most expensive number in engineering
#39Not just 'stress safety' but all the other things. A NASA project I was related to we logged every single bolt that went on the device, where it came from, the batch number, and had to keep all the old software around in the event we had to reconstruct something. The amount of overhead was pretty amazing. Most of that is for safety.
Overhead that saves lives isn’t really overhead
Re: The most expensive number in engineering
#40Earlier quoted context omitted.
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.