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# Re: Precision in engineering calculations

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Precision in engineering calculations
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Wed, 25 Jan 2006 12:47:54 -0500 (EST)

```They did when I was in school, but they did not really "force it".  While
I don't don't really do sig figs in my calcs, I would not keep the decimal
portion in your example below.  So, I kind of do sig figs.

And to me, slide rules help people understand orders of magnitude well
(which is kind of what sig figs are about).

Let's not muddy the issue by talking desing in nuclear facility
world...that entails all kind of atypical things (even though there is
still a certain level of uncertainly in material strength in nuclear
facility design...i.e. your 0.4% overstress example would still be well
within uncertainly limits, but a layperson on a jury is gonna be MUCH less
able to understand that and just see "overstressed" if something goes
wrong in a nuclear facility).

Regards,

Scott

On Wed, 25 Jan 2006, Sherman, William wrote:

> Do engineering schools still teach the concept of "significant digits"
> in design calculations?  I generally like to show results to 3
> significant digits (e.g., 12,300 lbs vs 12,319.82 lbs); although I
> consider the real accuracy probably closer to 2 digits for most design
> work.  But I find that young engineers often show results such as 123.47
> ft-kips.  There are occasions when more significant digits are
> warranted, such as when subtracting two large numbers, e.g. 12,319 lbs
> minus 11,898 lbs.  But most of the time such accuracy in numbers is not
> warranted.  For manual calculations, I feel that it takes extra time to
> enter and write those extra digits, and it implies a greater accuracy
> than is reality.
>
> When I started college in 1970, I used that ancient device called a
> slide rule.  By the time I graduated, I could afford one of those
> new-fangled calculators.  I think that having to use a slide-rule gave
> me a better understanding of significant digits, whereas calculators and
> computers give people a false sense of design accuracy.
>
> Nevertheless, I once did some design work for nuclear power projects and
> was told that if my results showed a calculated stress of 24.1 ksi vs an
> allowable stress of 24 ksi, the member size would need to be increased.
> But that was less about accuracy and more about fear that someone would
>
>
> William C. Sherman, PE
> (Bill Sherman)
> CDM, Denver, CO
> Phone: 303-298-1311
> Fax: 303-293-8236
> email: shermanwc(--nospam--at)cdm.com
>
> Scott Maxwell wrote:
>
> "And I agree with you on the whole knat's rear arguement...it is not
> needed or even warranted frequently in the  "structural world".  And it
> is many times not even useful in repeative situations...in steel design,
> one usually saves  more money by having the same beam over and over and
> over again rather than getting the absolute lightest members possible.
> Not to mention that the concrete actually used likely is not going to
> have the exact f'c that you used in design nor the steel to have the
> exact fy that you used.  So, doing precise calculations to some 5th
> decimal place (although it does somewhat depend on the units used) is
> typically a waste of time.  I learned that one back in school when my
> concrete professor "yelled" at me for using two or more decimal places
> in my homeworks and pointed out that if my capacity was within roughly
> 5% or less of demand, I was likely "good to go"."
>
>
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