Truss Modeling
Truss Modeling
(OP)
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.






RE: Truss Modeling
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.
RE: Truss Modeling
RE: Truss Modeling
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 members, but it'll miss the higher loadeds on the joints (due to the fixity that occurs in your design (particularly with the continuous elements).
RE: Truss Modeling
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 members them selves. You might want to check your model. I am not sure how STAAD works, but RISA will give you a warning and lock nodes if you have not modeled it properly.
RE: Truss Modeling
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 members of the actual chord at ultimate load will have to go plastic at the web joints - thats fine as long as they are of a ductile material (such as steel). Chord axial strength would be based on K=1.0 of length between panel points.
RE: Truss Modeling
RE: Truss Modeling
a "true" truss (with endload members) can be redundant