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Re: "Big Column and small beam" Rule

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Isn't it a fair statement that strict application of such rules was more 
useful before it became fairly simple to do computer analysis of various 
scenarios to verify things for yourself?

For instance, I did a lift analysis for a prefabricated equipment module 
last week. I don't do things like this very often and so I needed to not 
only do the analysis and design, but come up with a rational method of 
analysis.

And in less than one week, I was able to analyze the module using about 8 
or 9 configurations to come up with one that was optimal. This even though 
for each configuration, I had to run it a number of times to tweak individual 
 members in the frame

In the end, it was interesting that using a moment frame at each end of the 
 module, it sort of proved out the large column/small beam rule. In effect, 
 I stumbled upon the rule without deliberately attempting to apply at. And 
that's the thing I think we have to remember: the rules come as a result of 
 observation.

And by the way, I was satisfied with the way that turned out, but of course 
 down here on the Gulf Coast we don't see a whole lot of dynamic analysis 
like this for onshore, and my boss was appalled by how large the columns 
were. He asked that I redesigned it with temp bracing, to see if that would 
 yield a more "reasonable" (in his way of looking at things) column size.

(Bracing needed to be temporary because of the piping and instrumentation 
connections that would be made once the module was set; the bracing would 
then be in the way).

So, with the column sizes reduced, temporary bracing applied, and a structure 
 that had actually about 2 tons more steel  than the large-column version 
(not to mention the additional labor required to remove the bracing), we 
continued on.

And by the way, he would rather the client have to pay for a little extra 
material and labor, than hear him complaining to us that the contractor is 
complaining to him that "in my 27 years in the 'bidness', I ain't never seen 
 no columns like that."

I of course understand his sentiment, I'm just more inclined to tell the 
client to tell the contractor to go hire his own engineer to give him a 
design he likes better. And that, my friends, is why they don't let me do 
much talking to the client.

William L. Polhemus, Jr. P.E.

6

> On Mar 20, 2015, at 9:25 AM, Alex C. Nacionales <acnacionales(--nospam--at)gmail.com> 
> wrote: 
> Hello List,
> 
> The " Big Column and small beam rule" for SMRF gave me problems when I was 
> designing
> 
> because it made the columns very big for low rise long span structures. I
> was worried when
> 
> my former boss who is a senior engineer increased a beam depth  on a
> structure because
> 
> I initially thought this  violated the rule.
> 
> I realized later the building with RC columns, beams and slabs has a rigid 
> diaphragm so the rule might not apply in that case.
> 
> I have read from other books in Seismic design that this rule may not be 
> a good theory. Your comments please.

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