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Re: 125' long steel truss, connection question

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Simon Solorio wrote:
> 
> I'm designing a 125' long steel truss, a section view of the truss is
> a triangle. The top chords (2 top chords) are pipes and the bottom
> chord (1 bottom chord) is also a pipe. The web members are pipes.
> Questions;
> 1. the top and bottom chords are going to be a continuous pipe, so
> when I analyse the truss should I assume the connection at the node
> intersections (at the chords only) fixed or should I analyse it
> assuming the the chords are pinned between each node of the truss?
> 
> 2. the pipe web members are going to welded to the pipe chord members
> with fillet welds, where can i find information on how much fixity i
> should design this connection.
> 
> The architect does not want to see any plates, so I think if the
> connection of the web to chord is all welded then the connection has
> some fixity.
> I've check the truss with all the members fixed and the truss with all
> the members pinned but i think, with the fillet weld connections, the
> percent of fixity is in-between a fixed and hinged connection. The
> reference books I have show a welded connection like the one I need to
> design and seem to assume the connections are pinned.
> 
> Thank you in advance
> 
> Simon Solorio, P.E.
> 
Better study up on tubular connections (TYK) in the AWS Code and also
"Hollow Structural Section Connections & Trusses - A Design
Guide"available from the Canadian Institute of Steel Construction
($70).  Buckling can definitely be a problem if you're not careful.

If you design according to the above, you can model the joints with some
degree of fixity (not perfectly fixed though).  It is easier to analyze
as pinned and have some reserve strength, however.

I prefer also to use PJP groove welds and overbuild, thus combining the
better joint of the PJP style, with a fillet style exterior resulting in
a larger (E).

Enjoy.

Laurence Oeth, P.E.