] [Thread] [Date] [Author]

[James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)]

For the relatively small 125 ft truss you describe (we're not talking sports 
dome trusses) and assuming the pipe web members are reasonably smaller diameter 
than the pipe chord members, as is usually the case, it is acceptable to take 
the connection of the web members to the chords as pinned in the analysis. 
However, since the chord members are in fact continuous, it makes no sense to 
assume that they too are pin connected at the panel points. The chords should be
modeled as continuous members to determine the moments (in addition to axial 
forces) that will actually exist in the chords. The only question here is with 
the continuity assumption at the ends of the web members. I do not believe, for 
the size truss you describe, that either web members or their welds will be 
underdesigned for forces resulting by assuming they are pinned in the analysis.

Now, if you are uncomfortable with making a judgement as the the "smallness" of 
the pipe web members compared to the pipe chord members, then do a second 
analysis in which the web members are taken as rigidly connected with the chord 
at the panel points, and design for the controlling results from the two 
analysis. I don't see any reason why a partially rigid restraint has to be used 
in the analysis. Actually, I'm not sure a program like STAAD can model such a 
condition.