111–120 of 176 posts
Because in many applications, the full cost of failure can be unimaginable.
Also fudge factors have a tangible benefit: time. It permits declaring a design "good enough" sooner.
It’s kind of mentioned in the article, but to be more explicit: reducing safety factors has asymmetric risk vs reward. Reducing the factors “just” lowers cost or improves performance. But if your field is padding by 50%, then you need to tradeoff an “up to 50%” cost reduction (or similar) versus “had a catastrophic failure”. So, reducing the padding from 5x to 1.5x was already most of the benefit. If you were at 1.2,…
I feel like that's what was so unconvincing to me in this article. The only argument they gave against over-engineering was cost. At that point you have to decide how much risk is worth how much in savings. Is a 1% increase in the likelihood of the bridge failing in extreme conditions and killing 10 people worth a savings of $100k? What's a human life worth? What's a low increase in risk to a human's life worth? How…
Earlier quoted context omitted.
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…
In what discipline? In Electrical Engineering a fuse circuit breaker or receptacle may be used thousands of times and in dozens of redesigns.
Reading 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 w…
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 can sort of buy this. In Gravity things are probably a lot more robust than they actually should be, but collisions with debris or other objects tend to be immediately fatal for the vehicles involved. Debris at high velocity goes straight through stuff. Similarly, in For All Mankind, most stuff is pretty fragile and volatile, as you’d expect for Apollo-era tech. It’s only the later stuff that the show makes up whic…
Similarly, in the prime timeline, Enterprise-D ends its life with is engine section exploding, and the saucer section crash-landing into a forest. Externally, it looked pretty salvageable, but the insides were completely smashed. The crew survived, possibly because inertial dampeners still worked somewhat (I don't remember now), or maybe just because it flew on almost flat trajectory and used the aforementioned forest to slow itself down.
"""A non-empirical alternative to the factor of safety has been around since the 1940s, but still doesn’t have widespread adoption. I think the image below describes the concept, called probabilistic design, best. """ This is _exactly_ LRFD (Load Factor Resistance Design) which has been in the Civil Engineering building codes since the mid 80's, and became common in use the 90's when I was an Engineer (in training).…
It might be a good idea to lie to the contractors for a project about the margin of safety so they take better care to make it right. But that may not be possible.
I asked whether it gave different answers than the standard 1.5 safety factor. As I expected, the answer was yes. It turns out that in a conventional skyscraper, there is a tiny proportion of the structure that needs to be done right. This is good news as you can x-ray those beams, and check and double check that all the bolts are installed correctly. The cost to do this is tiny. The rest of the building can be built with an effectively smaller safety factor, and it will be fine. This leads to overall cost reductions.