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Re: HSS Connections - Not Conventional

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David,
 
The 4 ga members are really thin.  Structural welding (thickness, not length) other detailing issues may be a problem.  With continouos chords, you may get significant moments in addition to the axial forces, hence localized buckling and associated issues.  The edges of tubes may need to be capped - extra work.
 
If you need additional info, you may contact me privately.
 
Steve Gordin, SE
Irvine CA
  
----- Original Message -----
Sent: Friday, August 19, 2005 12:40 PM
Subject: HSS Connections - Not Conventional

Sorry for bugging everyone with these problems, but if there is an area of the AISC design manual that isn't extremely clear, I believe the HSS one to be it.  Then again, it could be the lack of clarity results from a VERY long week.  Never the less....
 
My contractor made the brilliant move of estimating a project that he is doing based on, what he thought were, members sizes and geometry that would suit the purpose of a 50 ft mono-slope roof truss.  He's using rectangular tube shapes, 2x4x14 ga., ASTM A588 (got the mill specs on this one to verify).  However, after running several analysis on varied geometries, I can't get a single 2x4 to work for his application, with respect to the top and bottome chords.
 
His next option was to double up the members on the top and bottom, so that two members would share the load.  His connection option was to have the web members "sandwiched" between the chord members, much like a typical steel bar joists (bar ends are connected between the vertical legs of the angles of the chords).
 
I have never done this type of connection and I was hoping to get a little assistance on this.  If I can develope enough weld to make the connection (which may be a feet in and of itself, seeing as how the walls are so thin), is this connection a viable option???  And if so, what sort of local stresses should I be checking for this condition???  It isn't very conventional, but this is something that the contractor is looking into in, what he believes to be, a cost saving option.  (I tend to not argue with contractors about "cost savings" when they come to me with this type of post-proposed problem and ask me to "make it work")  Any assistance would be greatly appreciated.
 
Sincerely,
David Maynard, PE
Gillette, Wyoming