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Re: flexible vs. rigid single plate shear connections

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Thanks for the reply Charlie.  Your extreme case for a flexible example is understood.  At the other end of the spectrum would be something like a W10 framing into an HSS 12x12.  Unfortuantely, what we usually have is somewhere in between.  Most of the ones I am looking at right now are W14's framing into HSS6x6.  I would like to just be conservative and say that it is flexible, but I need that extra capacity that I would get if I could say that they were rigid.  The values are so different (for example 27.8 kips vs. 41.7 kips), that I would like to have a little more to back up my assumption.  Tell me it is rigid and I won't bother you any more today. :)
 
Kenneth S. Peoples, P. E.
Lehigh Valley Technical Associates, Inc.
1584 Weaversville Road
Northampton, PA 18067-9039
Phone: (610) 262-6345
Fax: (610) 262-8188
e-mail: kpeoples(--nospam--at)lvta.net
----- Original Message -----
Sent: Thursday, September 25, 2003 3:16 PM
Subject: RE: flexible vs. rigid single plate shear connections

>I have some wide flange beam to HSS column moment connections
>that require a shear connection design.  The connections are at
>the roof level and some are not backed up by a beam connected
>on the opposite side of the column.  I would have thought that this
>would be considered a flexible connection, but page 4-97 of the
>HSS manual leads me to question this thinking.  Can someone
>clarify what is meant by "Tabulated values for flexible supports
>should be used only when the column itself is considered flexible
>at the connection". 
 
Testing by Sherman at the University of Wisconsin was the basis upon which shear tab design procedures were adapted for use with hollow structural sections. Although it was initially thought that the flexible support values would be appropriate, the membrane action in the HSS wall causes the test results to be very close to those for a rigid support. That is, we thought the wall would deform out of plane more under load, but the rotation still comes mostly at the bolt line when shear tabs frame to HSS columns. In crafting the language to present this idea, the Committee was concerned with one potential exception: a flexible column, which in my mind is one for which the flexural stiffness is low enough that it will deform more easily than the HSS wall or at the bolt line.
 
As an extreme example, how about a W24 attached to an HSS 4x4? That's crazy, but I've seen it. The wall is narrow, stocky and stiff. And the depth of the shear tab also made it stiff, so flexure of the column in this case drove the appropriate design assumption to flexible, rather than rigid.
 
For completeness, here is how I keep myself straight between rigid and flexible supports. A rigid support is one for which I expect most of the simple shear rotation to be accomodated within the connection (at the bolt line). A flexible support is one for which I expect most of the rotation to be accomodated by the supporting member.

Charlie