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RE: Steel HSS Compression Analysis

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It has been a while since I worked this through, but as I recall you
calculate the section properties based on the effective section area.  If
both your primary dimensions are slender then you change both and
recalculate the section properties.  If only the 4" side is slender you
reduce that wall on that side and recalculate the section properties.  Even
though the section gets smaller, the distance from the NA stays the same so
make sure you use the original dimensions in you're Ix and Sx calculations.

Rich


-----Original Message-----
From: David Maynard [mailto:davemaynard(--nospam--at)ceincorp.com] 
Sent: Wednesday, August 17, 2005 2:39 PM
To: seaint(--nospam--at)seaint.org
Subject: Steel HSS Compression Analysis

Here's what I got.  I have a tube shape, 2" wide by 4" deep by 14 ga.
(0.0747" thick).  I'm using LRFD 3rd Ed. Nov. 10, 2000 HSS Design
Specification found within.  I check my Slenderness Limits as described in
Table 2.2-1.  Assuming, because this is not an AISC shape, that Fy = 33 ksi,
I found Lambda r to be 41.5.  My controlling wall slenderness ration, h/t,
lambda is 50.5, which exceeds the lambda r.

Flipping to section 4.2 of this design spec, I see that if lambda is greater
than lambda r, for rectangular HSS, Q = effective area/Ag.  It then states
that the effective area is equal to the summartion of the effective areas of
the sides using be = <long equation>.

The question that I run into is, what does this mean, "effective area"???

Do I have to use this equation for both wall dimensions, first with b and
then with h???  Or do I just use this equation as a substitute for b and
leave h untouched, even though that is the controlling slenderness limit???

And once this question is answered, I'm assuming I just take these adjusted
dimensions, multiply them by the thickness and combine all four side for an
effective area.

Right now, and I'm not sure if this is my illness/vacation talking but, this
just doesn't make any sense to me.  Can someone shed some light on the
situation???

Sincerely,
David Maynard, PE
Gillette, Wyoming
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