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RE: Precision in engineering calculations

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I had the concept of 3 significant digits hammered into me by my concrete design and steel design professors, but I got my BSCE in the late 80's, when we were just getting the first IBM PCs in the Civil Engineering lab. How does 256KB of RAM sound to you these days???

However, the concept of precision vs. accuracy was not fully grasped in my head until I grudgingly took a required 2nd year survey class in my senior year (yeah I was avoiding that one). The professor had us do various field surveys, including on that involved a horizontal and vertical survey from one end of the campus to the other and back home to the engineering monument mounted in front of the Engineering School building. We came back after a hot summer afternoon traversing the campus a distance of about 2 miles (we looked like serious geeks in our safety orange vests that our professor insisted that we wear at risk of dropping a grade point) and found that we were 1.2 feet too high and about 3 feet off horizontally to close the traverse. He said, "do it again."

Needless to say we read the rod several times through the transit, each member of the party taking their best read, and then averaging the results for field book on the second time around (we did it the next week). I think everyone in the survey party had a better understanding of true accuracy vs. precision after our little exercise.

I think that the ubiquitous presence of computer-based engineering computations in current engineering practice give younger engineers ( namely, engineering students) the impression that very accurate work is being done, when computer calculation output typically produces something that looks like "22.1877 kip-ft", regardless of the fact that a Load Factor of only 2 significant digits was used in the original load assumption.

Best regards,

Thomas Honles, SE, PE
Los Angeles, CA

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com]
Sent: Wednesday, January 25, 2006 10:27 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Precision in engineering calculations


I don't know whether they still teach it, though I hope they do (I'll 
consider myself "young," having gotten my BS in the 90s, though I 
actually had a pencil-and-paper drafting course in my freshman year at 
VT - on the quarter system, no less). To believe that anything we 
analyze in buildings and bridges is going to come within 10% in the 
field is foolishness.  I usually keep two-and-a-half to three digits, 
and occasionally will write three-and-a-half or four in my notes.  Since 
I use a HP calculator with a stack, I generally keep the results in the 
stack so that there's no loss of precision, and I can do a single round 
at the end. 

I think it would be a good exercise to take a class of Junior engineers 
and have them calculate the deflection of a 200lb load on a floor joist 
in a residence, then take them out to a finished house with a couple 
bags of sand and have them measure the actual deflection.  I'd be 
surprised if they were within 15%.

Jordan



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
>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"." 
>
>
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