Truss Bending

[cazkoop]

We are currently load rating a truss where the connections are welded with gusset plates. When we input the truss into STAAD we assumed fully fixed connections at the ends. The resultant moments in the members were 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?

 

[wiktor]

Some of the textbooks on the subject discuss this approach i.e. treating the truss (or modeling the truss) as a frame.
This approach is sometimes called "secondary stress level" analysis. Some of the codes and authors calls for the increase in the allowable stresses (approx.25% range) in such case, with yours definitely qualifying. I would strongly advise to re-analyze the truss using true truss model, likely in 2-D, as the presence of portal frames tends to complicate the 3-D model and could lead to instability. One should also remember that self-weight generates moments too, but at the nodes they should be zeroed.

 

[Qshake]

Keep in mind that the "pin" doesn't have to occur right at the intersection of all the elements of a joint. If the members framing into the joint are relatively weaker in bending than the joint itself, the joint will behave as a "pin" overall. This would occur as each member will yield just outside of the gusset plate, so that no matter how the members are connected to the gusset (weld, bolt) it is still seeing very little moment or rotation as the members will rotation before transfering the moment to the joint.

Regards,
Qshake

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