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Re: FW: Precision in engineering calculations
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- Subject: Re: FW: Precision in engineering calculations
- From: "Timothy Allison" <t_allison(--nospam--at)illinoisalumni.org>
- Date: Wed, 25 Jan 2006 09:08:16 -0800
- Auth-sender: t_allison(--nospam--at)illinoisalumni.org
- Priority: normal
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)cdm.com] Sent: Wednesday, January 25, 2006 10:53 AM To: seaint(--nospam--at)seaint.org 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)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"." ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ******** . |
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