The most expensive number in engineering
surjan.substack.com
The most expensive number in engineering
1–10 of 176 posts
Re: The most expensive number in engineering
#2"""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's the difference between the older green book and the newer (at the time) silver steel design handbook)
It was absolutely drilled into us in school though, that Safety Factors and LRFD factors covered material and other uncertainty, they did not cover blunders.
Re: The most expensive number in engineering
#3Probabalistic failure analysis is certainly something engineers do for determining system risk, else how do you determine how many redundant components to include? I seems like having a higher safety factor just means having a lower probability of failure, and these two concepts are very compatible.
Re: The most expensive number in engineering
#4Ok let's wing the entire thing from cardboard then!
* "poorly representative material test data available" that's 5+
* "extremely challenging environment" 5+ again
* "models are crude aproximation" is another 5+
So we should be able to get a cardboard spaceshuttle, if we only use a safety factor of 125+! Moar cardboard! Great job team!
Re: The most expensive number in engineering
#5Probabalistic failure analysis is certainly something engineers do for determining system risk, else how do you determine how many redundant components to include? I seems like having a higher safety factor just means having a lower probability of failure, and these two concepts are very compatible.
The probabilistic failure analysis (as practiced in LRFD) is essentially a pencil sharpening exercise where the margins can be reduced a bit. For example, some loads are better known than others (e.g., dead load vs live load), some materials have better QC or a more uniform quality than others (think concrete vs steel).
The end result is generally in the ballpark of the old factor of safety, but might be up to 10% less in some cases.
Re: The most expensive number in engineering
#6The 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.
Re: The most expensive number in engineering
#7Ok let's wing the entire thing from cardboard then! * "poorly representative material test data available" that's 5+ * "extremely challenging environment" 5+ again * "models are crude aproximation" is another 5+ So we should be able to get a cardboard spaceshuttle, if we only use a safety factor of 125+! Moar cardboard! Great job team!
You’re joking but in aerospace they use factors of not 10x or 2x but 10%. On a 30m high rocket. Just 10%. Here’s a tour with the CEO of ELA, as a bonus: https://youtu.be/OdPoVi_h0r0
Re: The most expensive number in engineering
#8Ok let's wing the entire thing from cardboard then! * "poorly representative material test data available" that's 5+ * "extremely challenging environment" 5+ again * "models are crude aproximation" is another 5+ So we should be able to get a cardboard spaceshuttle, if we only use a safety factor of 125+! Moar cardboard! Great job team!
Sounds like a few software projects I've been involved in over the years D:
Re: The most expensive number in engineering
#9You could just square your quantity before applying the 1.5 factor. Instead of "Our shuttle is safe up to 150% of the required speed!" design for "safe up to 150% of the required kinetic energy (1/2 m v^2)". Then you only need to design up to sqrt(1.5) = 123% of the required speed.
(My point is that the scaling of the importance of quantities is arbitrary so a single safety factor doesn't make sense to be applied to every quantity.)
Re: The most expensive number in engineering
#10This 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 favor of more safety.