Welded Connection Fixity
Welded Connection Fixity
(OP)
We are currently load rating a truss where the connections are welded using gusset plates. When we input the truss into STAAD we assumed fully fixed connections at the ends. The resultant moments in the members were relatively small, but since the members bend in the weak axis, the load rating is very low. Is there any information out there that discusses partial fixity of such connections?






RE: Welded Connection Fixity
Do you want to get information of study or experiment about the connection regidity, or how to model semi-rigid connections?
If it is a truss, the connection rigidity (either it is 100% rigid or simple a pin) shouldn't be a significent factor to affect the load rating.
RE: Welded Connection Fixity
RE: Welded Connection Fixity
I don’t have any document or article about rigidity of steel connection handy. I once wrote a program of concrete flat-plate structure analysis with non-linear “springs” to model the column to plate connections. For steel structures, because of the variety, it is harder to have analytical expression of connection rigidity. I know somebody did this kind of study, heavily based on experimental results. Maybe more study results are available now.
I don’t know whether your truss is 2D or 3D. But if the loads are applied to the joints the bending moment anywhere should be pretty small, and even with the beam-column criteria the strength should be dictated mainly by the compression or tension of the critical member. Or you just use pin connection approach. With gusset plates the connections are a "pin" in one direction already.
RE: Welded Connection Fixity
If there is fixity in the joints, then analyze it so. I'm surprised that your load capacity goes down as compared to pinned joints. Are you sure your model is correct? I have seen the opposite in similar analyses. STAAD is pretty quirky so be careful with your model. Do an animation and see how that looks. That will usually tell you if something is amiss in your model.