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Re: FW: Precision in engineering calculations

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We were taught significant figures (and how to use them) in school.  A couple teachers would go so far as to deduct a point or two for using too many sig figs.  In practice, I ignore the rules and take it out to a "clean" number - your example of 12319.82 vs. 12300 would be 12320 to me.

I do agree with your thought that calculators give a false sense of accuracy.  If you take something simple like "25000 - 12000", you're still entering five digits regardless of there possibly being as few as two significant figures, so writing the result with five significant figures is kind of a natural reaction.
-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)]
Sent: Wednesday, January 25, 2006 10:53 AM
To: seaint(--nospam--at)
Subject: Precision in engineering calculations

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
report that the design indicated that some members were "overstressed".

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

Scott Maxwell wrote:

"And I agree with you on the whole knat's rear is not
needed or even warranted frequently in the "structural world". And it
is many times not even useful in repeative 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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