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@shaw.ca>
To: seaint@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 WilsonSent: Tuesday, April 21, 2009 6:45 AMSubject: Light gage boxed header limitationsI 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, PEStroudsburg, PA