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Re: Simple connection eccentricity on columns

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I have picked out of the tables many times myself. I was recently reviewing some HSS column sizes that RAM had generated and it was making more of a difference than I had expected, particularly at the typical perimeter of the floor (only one beam framing into the column in one direction). I need to check to make sure it is sizing them correctly with the 4.5" or so eccentricity it is estimating for the connection with 35kips of load.  What I had figured by the table could easily be an HSS 4x4x1/4, it was generating HSS 6x6x1/4's.  (It is a small access level floor, hence the small columns). 
Will Haynes
On 7/25/06, Jordan Truesdell, PE <seaint1(--nospam--at)> wrote:
I'll be honest - I usually don't.  The amount of eccentricity in an
unbalanced column is usually very small compared to the allowable moment
capacity, and tends to make a very small difference in the design. I
usually leave a bit of headroom on any column picked out of a table,
because hand calcs are just not that accurate. Leaving 10-15% capacity
"on the table" also puts in a little future capacity should the usage
change.  It rarely increases the weight of the column appreciably, and
I'd rather have the stability.  Also, the amount of time to hand calc
the full solution on a small job rarely justifies the potential savings
in steel.

This is, of course, subject to engineering judgment.  My background is
such that I feel I should be able to calc most common things in my head,
on the fly, to within 20-25%.  It comes from reviewing presentations in
meetings.  Knowing simple mass properties, CTEs, common emissivities,
etc. can quickly identify if someone has slipped a decimal place, goofed
an in-ft dimension (factor of 3.5 or 2.3 off)  or fouled up their units
completely (frequencies are often off by 386.4^0.5, or about a factor of
20 by young engineers). If I get out of a tight range of shear offsets,
I'll check it just to make sure, though it might just be a scribble in
the margin of a calc sheet or a number run in the memory of my HP48.

Oh, and I'm not an "old guy" - not even 40 yet -  though my hair is a
bit grayer than I'd like. I've just done a lot of calcs by hand, and
have gotten to work with some pretty darned smart folks over the years
and was able to pick up some of their tricks.


Will Haynes wrote:

> How many of you (that do not use RAM or other software that calculates
> it automatically) actually figure the eccentricity of a simple beam to
> column connection and add that moment to the column design? I know
> when I first started, the older guys would always pick a column out of
> the AISC concentric load tables without regard to the moment from the
> actual connection eccentricity.
> Will Haynes

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