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Re: Light gage boxed header limitations

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You're more likely to get shear buckling / web crippling than LT buckling. LT is an issue when sections have a low torsional rigidity, but for a properly designed/built box beam you should have a surplus of torsional moment capacity unless you have a very deep header.

BTW do you have to design something or can you just call a vendor like Metwood (who makes CFS box headers with rebar added for additional strength) and give them the loading?
Jordan


Jim Wilson wrote:
At some span, I would think that 6" wide light gage box beams would become susceptible to lateral-torsional buckling where there are no lateral brace points along its length.  Light gage framing bending and compression members are typically locked into sheathed wall/floor/roof systems and I just have to question if there is a way to address this in accordance with the AISI Specs for header design.
 
Jim


From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
To: seaint(--nospam--at)seaint.org
Sent: Tuesday, April 21, 2009 12:49:22 PM
Subject: Re: Light gage boxed header limitations

Jim,
 
        Vertical load shouldn't be a problem; you can always make a light truss.
 
        For horizontal you might check deflection; see how it compares with a limit of say half an inch.  That is where I think you might have problems.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
From: Jim Wilson
Sent: Tuesday, April 21, 2009 6:45 AM
Subject: Light gage boxed header limitations

I have been asked by an EOR to design a 24ft long boxed header with light gage framing.  This member will be laterally unbraced in a curtain wall assembly.  Are there any practical limits to the span of such a header in a 6" wide wall?  The loads are only in the 200lb/ft range vertical.  Horizontal loads are less.
 
The "Standard for Cold-Formed Steel Framing - Header Design" by AISI only states length limits for L-shaped headers.  Header calcs in this standard only address bending, shear and web crippling.  I don't believe that the AISI main spec will adequately model a long-span composite assembly.
 
I would like to either find a conclusive way to design this header or prove that it is not practical.
 
Thanks in advance.
 
Jim Wilson, PE
Stroudsburg, PA
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