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# Re: Precision in engineering calculations

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Precision in engineering calculations
• From: "Jordan Truesdell, PE" <seaint1(--nospam--at)truesdellengineering.com>
• Date: Wed, 25 Jan 2006 13:27:17 -0500

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
```
```
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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