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Re: Lateral Stability of a Box Beam ?

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I think that we need to return to the original description of 2 independent
beams with the possibility of adding plywood top and bottom to increase
efficiency of the structure. I did not respond to this since Bill appeared
to have the required approach well considered but was only looking for the
analytical tools to qualify the sheathing.

You raise the special case of single/point symmetric members which require
additional care. I have stated in the past that, in my opinion, purlin
stabilization bracing is an overlooked aspect of building design. I do not
get the impression that this applies to the original post.

I am prepared to discuss L(x,y,z) bracing of cold formed members at great
length but time prevents a rediscovery of published standards.

Per the description of minimizing stable members, the bridging only reduces
the buckle length if there is some added shear resistance mechanism
(stiffness) between the apparent brace points along the length of the
member. That shear path can take the form of sheathing (e.g. plywood, floor
deck, etc., essentially a diagonal link) or a link to anchor or other means
of adding lateral stiffness (re Winter).

Paul Ransom, P.Eng.
ph 905 639-9628
fax 905 639-3866

> From: "Conrad Harrison" <sch.tectonic(--nospam--at)>

> What you say makes sense. But I have to think about it, something seems odd.
> Coldformed C/Z are braced by bridging, normally a channel section fastened
> to the web above and below the neutral axis. The bridging provides both
> lateral and torsional restraint: that is Ly, and Lz are reduced.

> Not really about two tied beams. The bridging reduces the half-wavelength of
> the buckle in the thin plate.

> -----Original Message-----
> From: Paul Ransom [mailto:ad026(--nospam--at)]
> Sent: Sunday, 18 January 2009 06:54
> To: seaint(--nospam--at)
> Subject: Re: Lateral Stability of a Box Beam ?
> If the 2 beams are identical with identical loading, there is no benefit to
> simple ties. Unless there is some shear resistance mechanism between the
> brace points, the unbraced length for lateral buckling is not reduced and
> the ladder simply behaves like 2 tied beams (e.g. Iy1 + Iy2) and they just
> displace in unison.
> Regards
> Paul
> -- 
> Paul Ransom, P.Eng.
> ph 905 639-9628
> fax 905 639-3866
> ad026(--nospam--at)

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