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RE: 125' long steel truss, connection question

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We designed several structures like that. Normally, we assume that the Top
and Bottom Chords are frame members. During actual construction they are
specified as continuous members. While the web members are (vertical and
diagonal) are all axial members. Therefore, the connection between the web
members and the Top and Bottom Chords are designed for axial loads only.
The Top and Bottom Chord connections are designed for flexure and axial
force. You don't have to worry now about the amount of fixity for the web
Now, the flexural forces would come when a purlin is connected not at the
joint but anywhere between the joints. This would induce moment along the
Top Chord. Normally, connections between web members (diagonals and
verticals) and top chord and bottom chord are welded to have an amount of
offset (their centroidal axis would not coincide) inducing flexure. This is
done so that 
the produced moment would counteract the dead and live forces but it should
be ensured that downward loading would govern and not wind suction forces
(vertical) otherwise the moment due to member offset would add and not
counteract. A thorough explanation of this procedure with details can be
found i the book "Design of Welded Structures.
For the Top and Bottom chord splices, they are designed for flexure and if
the pipes are thick, full penetration butt weld is done.
I hope this would help.

A. Yango
Toyota Motor Philippines

> ----------
> From: 	Simon Solorio[SMTP:simon(--nospam--at)]
> Reply To: 	seaint(--nospam--at)
> Sent: 	Thursday, June 24, 1999 8:01 AM
> To: 	seaint(--nospam--at)
> Subject: 	125' long steel truss, connection question
> 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.