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Re: R-Factors Coldform hat sections

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> From: "Roy Harrison" <ro35984(--nospam--at)bigpond.net.au>

> The hat sections are trapezoidal in profile and are fixed by the lips or
> lower flanges (rim of hat) being screw fixed to the face of the portal
> frame.
> 
> I agree about the L-T buckling sensitivity.  In essence they perform =
> similar to a C profile bending about its weaker axis, so don't
> particularly experience a lateral instability problem.
> 
> I have analysed the section using the CFS package of RSG Software (Bob
> Glauz) which indicates one of the buckling modes involves both webs and =
> lips of the section buckling sideways when placed in compression subject
> to = an outward wind load. This I believe is the flexural-torsional
> buckling = mode that occurs at long wavelengths, 1 or 2 metres.  However
> this mode = involves rotation of the top of the hat. =20
> 
> But similar to the C/Z profiles I believe torsional restraint is =
> provided by the decking or sheeting attached to the top flange.
> 
> To get a solution from the software a suitable R-factor would be =
> necessary. Currently the default is zero and the capacity for the member
> is very = low.
> 
> I believe the R-factor would be between 0.7 and 0.9 but have no =
> eveidence (other than my gut!).
> 
> How does this strike you??

As I said before, the R values were developed by test. You would have to
do the same thing IF you want to use the simplified approach (even in
CFS).

Alternately, you could do the (not so) rigorous analysis on a couple
cases and make a comparison. MatchCAD or Excel are useful.

I suspect that for the purposes of R, you are very close to 1 but for
different reasons. The whole list of caveats that go along with this
clause may not be appropriate for this case. You cannot claim compliance
with the standard by applying this clause to your hat section.

I have little feel for CFS and don't use it often. I prefer my own
analysis templates. However, I believe that you can define the restraint
conditions, explicitly, and not inlcude R. 

You will find that manufacturing variances from ideal forming will have
greater influence than is apparent. Even the grain of the sheet may
affect response in test.

Note that NACFS always defaults to a test to confirm your analysis.
Software and standards can only consider what we have had the foresight
to include but a test considers all the (good/bad) variables that we
don't explicilty control. This is small cost for production components
that will be used in many designs. I love to see wind tests on cladding
panels - the result has no bearing on the analysis concepts based on
small deformation.

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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