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Re: Lateral Stability of a Box Beam ?

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Mr. Boene and Conrad
My response interspersed below.

> From: "T. Boene" <t_boene(--nospam--at)>

> What are simple ties by the way? Did you mean pin-pin connection for the
> bracing members?

My assumption was that the ties proposed by Conrad were simple pin-pin ties
perpendicular to two main members. I commend Conrad for offering alternate

> I am not sure what point you are trying to make here.

Such perpendicular, pin-pin ties do not improve the capacity of the
individual principal members if each principal member is identical in form
and load.

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

> Whilst Paul may be correct about the two beams displacing in unison:
> implying a half sine wave buckle for the full span. The battened column
> analogy indicates that such will be at a higher axial load, than for the
> beams acting individually.

Unless the battens, perpendicular to each main member, have designed moment
connections at each main member (e.g. Vierendeel truss), the capacity of a 2
member assembly is only 2x the capacity of each member individually. In the
case of semi-rigid batten connections the design standards are clear about
limitations and conditions and the benefits can be inconsequential.

> Galambos discusses lean on effects for columns and beams

Lean-on effects acknowledged, explicitly, w.r.t. beams with pin-pin
perpendicular ties.

I believe that we have a common understanding of these principles.

> More over then goes on to design bracing, which is similar to the bridging we
> use in Australia for coldformed girts/purlins.

> Which is where I think the issue is. Most of the photos I have seen of
> bridging of purlins in the US, show bracing which appears little better than
> sag rods. So my alternative proposal of providing bridging to the beams
> wasn't a good one. Divided by a common language.

There may be language issues but I don't believe that it is related to
bridging/bracing. The use of channels, or similar, as bracing between cold
formed purlins is also common in North America.

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

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