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FW: Steel HSS Compression Analysis

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Dave,

It looks to me like your 4" sidewall is slender and your 2" flange is
compact.  Depending on your stresses, the 4" wall may buckle and may be
partially ineffective.  Calculate your effective b.  The ineffective part is
the difference between actual and effective.  Multiply ineffective width by
wall thickness and subtract this from gross area (for HSS there are 2 areas,
one for each 4" width).  With this effective area you can then figure your Qa
and Q.  This discussion is assuming uniform compression.  If this is a
bending problem you'll need to do some additional sorting out.

Curt La Count
Jacobs Engineering
Portland, OR

-----Original Message-----
From: David Maynard [mailto:davemaynard(--nospam--at)ceincorp.com]
Sent: Wednesday, August 17, 2005 12:39 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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