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Re: PEMB question - unbraced length

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> From: "Yi Yang" <YI(--nospam--at)summit-sr.com>

> For those pre-engineered metal building experts out there, what would be
> considered as the unbraced length for both axial and flexural strength
> of the column / rafter in a metal building frame, if there is no flange
> braces attached to the bottom / inside flange of the rafter / column?

You should be able to analyze any component of a metal system building
using the requirements of the applicable design standards. Apply the
same bracing concepts that you would for a conventional steel structure
- except that you may have to do the analysis for specific stress levels
for each load case and at many locations on the structure using the
least conservative alternate analysis approach. Your model may be
different for uplift than for gravity loads. Bracing that is capable due
to gravity loads may not be effective during uplift conditions. The
metal building designer ought to know the limits. 

Common analysis approach for rigid frame sections:
Weak axis unbraced length of the top/outside flange is typically the
space between purlins or girts.
Weak axis unbraced length of the bottom/inside flange is typically the
space between flange braces. In your case, there are no braces to the
inside/bottom flange so the unbraced length is the effective length (KL)
of the member, at minimum, when the inside/bottom flange is in NET
compression.

There are many caveats. This falls apart when flanges get large, webs
get deep, roof systems are not "rigid" or there are high axial loads in
the member.

In general, you can apply the bracing approaches supported by the SSRC
through the Yura/Helwig work.

> Am I correct to assume that only the top / outside flange is considered
> braced for flexural strength, and the weak axis is braced for axial
> capacity?  Thanks for any comment.

Hmmm. Not necessarily. We think in terms of Axial, Bending, Shear,
Torsion as separate components within a rigid cross-section member but
these are just segments of the total effect in flexible sections,
segregated for analytical convenience. Typical PEMB analysis will only
consider unbraced lengths of the inside/bottom flange if there is net
compression there. The member is treated as two separate members
(inside/outside), with the same section properties, for the purpose of
evaluating braced condition capacity. Lots and lots of extra analysis.

 Even if the axial load is small, the inside flange may not be
adequately restrained by purlins/girts on the outside due to
out-of-plane flexibility. Usually, the analysis considers the flange
having net compression and uses the unbraced length of that flange for
the analysis of the member. Typically, one flange will be in tension
when the compression flange is at the controlling compression stress.

The whole approach comes down to squeezing efficiencies where the steel
design standards have included conservativeness for analytical
convenience.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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