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RE: R-Factors Coldform hat sections

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G'Day Paul

Thanks for your input on this issue, I appreciate your comments.

How would you allow for the torsional restraint provided by the sheeting
attached to the tension face, in a rigorous analysis using Excel?

I am aware the values are derived from tests, as were the C/Z values in the
North American codes :)

The Australian code AS4600 "Cold-formed Steel Structures" is based on the
extensive work that has gone into the development of the AISI LRFD
Specification.  

The R-factors specified in AS4600 were derived from Australian testing in
lieu of adopting the AISI values:

Apparently this was done 

A) to confirm the AISI values and

B) to explore the influence of the Australian construction practice of using
side cleat connections welded to the portals, whereby the rails are attached
to the structure through their webs. This was shown to be beneficial as
torsional restraint is provided at the end connection.

As a consequence the values specified in AS4600 are slightly higher than
those published in the AISI, which we are told are for the North American
practice of using rail flange to portal flange fixings.

Locally in the "domestic" and "light industrial" market, the shed frames
with spans ranging from 6m to 22m have developed using cold-formed members.
The portal frames are usually spaced not more than 5m apart and hence the
North American practice of the flange/flange fixing has been adopted.

A development in recent years has been to use the "top-hat" section
discussed, and two of our major cold-forming manufacturers have published
"Safe Load" and "Design Capacity" tables (SLT/DCTs) for their particular
variant, based on programs of testing. From these tables I am able to deduce
that the R-Factor is around 0.85.

On the other hand there are a number of manufacturers supplying a similar
product with published selection tables, purportedly based on AS4600.

In view of the number of years this form of member has been in the
marketplace, both here and overseas, I am trying to track down any
non-proprietary research data that has been published.

Regards

Roy
 
Roy Harrison & Associates
Consulting Structural Engineers
Adelaide - South Australia
 
mailto:ro35984(--nospam--at)bigpond.net.au
http://users.senet.com.au/~tectonic/

	-----Original Message-----
	

 


-----Original Message-----
From: Paul Ransom [mailto:ad026(--nospam--at)hwcn.org] 
Sent: Saturday, 1 April 2006 1:32 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: R-Factors Coldform hat sections


> 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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