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Re: Increasing Kl/r

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I don't think you're missing anything; it sounds like a good idea.
 
My initial reaction was that the bulkheads (ribs) are labor intensive, therefore costly.  Depending upon the number of trusses you are reinforcing, the solution may or may not be worth trying to economize.  It is at least worth giving some thought to other methods of accomplishing the same thing.
 
Contact in three places (120 degrees apart) at each brace location (24" oc) is the minimum requirement.
 
Some shots from the hip:
 
-  Use an encapsulation pipe whose ID is slightly larger than the diagonal's OD, epoxy grouting the annular space.
 
-  Grout fill the diagonal itself, like a lally column, small port holes drilled each end.
 
-  Use a different "K", I'll bet you used 1.0.
 
_________________
John P. Riley, PE, SE
Riley Engineering
20 Oakwood Drive, Blue Grass, Iowa 52726
Tel & Fax:  319-381-3949
jpriley485(--nospam--at)peoplepc.com
I would like to bounce some thoughts by you.
  
I am looking at some existing trusses (wide flange chords, pipe webs) with a 130 foot span.  For the new loading condition, everything checks out OK except one diagonal compression element at the third panel from each end (section changes at two end panels).
 
The sections have sufficient area to carry the required forces if the Kl/r could be reduced.  The area is sensitive to welding, and we would like to create a bolted solution if possible.
 
The concept is to split a larger diameter section, add interior ribs at approx. 24" like a plane fuselage and bolting flanges, and encapsulate the existing member for the full length minus a foot ea. end.  The two halves would be attached with high strength bolts pulling the ribs uptight against the existing member (design would allow sufficient gap to ensure the interior member is clamped).
 
The resulting section would then be viewed as the original section only for stresses, but as a combined section for buckling resistance. An r value would be calculated for the combined section with both the existing and new parts contributing.
 
What opinions do you have?  Do you see something I am missing?
 
Paul Feather