huangyhg

truss modeling
i am modeling a truss in staad. when modeling the top and bottom chords where the truss web members frame into them, is it appropriate to make the top and bottom chords continuous at the nodes (without releasing the moments) and only pin the top and bottom chord at the support/column ends? or, should i release the moments at each node? the top and bottom chords will be fabricated as continuous double angles. if i make the chords continuous, there is quite a bit of moment in them.
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today, i model it as i intend for the fabricator to build it.
years ago when we calc'd everything by hand, we assumed all the chord intersections were idealized pins. the moments were refered to as "secondary moments" and were generally ignored in the design if the truss was loaded at panel points only. we would include bending moments however, if the chords were loaded between panel points.
i was always told to model the chords as continuous and the web members as truss
remember at the truss nodes that several members meet (some maybe continuous) so that there is a bunch of load redistribution happening at the node (presumably you've got a gusset connecting the members together). your model welds all the elements together at a node, which isn't the real geometry, so you need to check that the loads can shuffle round the way the fem wants them to go.
if you wanted to model an ideal truss, you could model each span as a single rod element (rather than a beam) ... this'll produce slightly higher loads in the
i vote for modeling exactly as it is going to be constructed. show pins only where the chords are going to be spliced. i am all for modeling as close and as tight as you can, then add conservatism so you can measure how much conservatism you actully have.
why are you getting a lot of moment in the chords? if the configuration looks like a truss (ie all triangles) with no eccentric connections, the only moment should only come from the loads you have applied to the
i believe either method is adequate and permissable as both methods produce statically admissable structures as long as the members are designed for the forces that are assumed. it is really up to the designer.
method 1: chord continuous with pinned web members - will give you moments in the chords and slightly less force in the web members. chords must be designed to handle the moments. a reduced effective unbraced length factor could also be used for axial capacity of the chords (say around k=0.8) considering the joint fixity.
method 2: everything pinned. still statically determinant. no chord moments - higher web forces. in order to get to this condition the
i design heavy timber trusses for a fabricator. if a
just a little pic at willisv's post ...
a "true" truss (with endload members) can be redundant

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