At 07:59 AM 4/11/2005, Doug Renwick wrote:
>David Robbins K1TTT wrote:
>
> > The only 'sensible' conclusions to this are the following:
> >
> > 1. do what the manufacturer says for 'standard' installations
> > 2. do what a qualified mechanical engineer who has performed the
> analysis on
> > the specific design says for non-standard installations
>
>Is a single qualified mechanical engineer equal to God? Did this engineer
>achieve 100% on every one of his/her exams from a respected college? Can
>a lot of faith be placed in a single engineer's calculations. Would it be
>a better idea to compare the answers from say three independent engineers
>and see if they agree, PE stamp or not?
>
>Doug/VA5DX
This goes more to the word "qualified"...
1) acts of god and natural disasters are more in the nature of statistical
outliers. Engineering generally designs for the main chance, and ensures
that if it does fail, it fails in an appropriate way. To paraphrase
Einstein, God may not play dice with the universe, but engineers do(and so
do you), every day. All designs are really designed for a statistican
environment (the XX mi/hr wind rating, is a statistical measure... doesn't
mean that faster winds won't occur, just that if you design for that wind,
the probability of a failure due to faster winds is "acceptably small")
2) Exam scores are not particularly well correlated to quality of
engineering design. If I were hiring a "fresh out of school" engineer, I
might ask what classes they had taken, how they did, and what they got out
of the class. But that's basically qualifying an apprentice. If I am
hiring an engineer to design a structure, I want to know how many other
structures like this one they've designed, what the analysis entailed,
where the problems were, etc. I note that licensure doesn't depend on
grades in your classes, but more on experience (6 yrs in CA), having signed
references from engineers who have supervised you, and whether you "make
the cut" on the PE exam. In fact, one can get a PE license, and
successfully practice, with zero college classes. You might even be able to
do it without a high school diploma. You probably DO need to be over 18.
3) Can a lot of faith be placed in a single engineer's
calculations? Depends. What's your risk posture? What's the potential
downside? If you're flying the shuttle, having someone check the calcs
might be a good idea. If you're putting in sprinklers in the back yard,
probably not. Depends a lot on who that single engineer is. There's a
famous example about the fallacy of "averaging" opinions: You'd rather have
one opinion of someone who really knows the material than the average of 30
guesses.
4) Comparing answers from 3 engineers would certaintly give you more
information about the analysis, if only from looking for commonalities and
differences in the analysis. It might reduce the risk. It might not. If
one of the engineers designs towers for a living, and the other two design
apartment buildings, what you'd get would be an interesting variety, but
little additional information.
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