Truss joint fixity

[cazkoop]

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?

 

[havesealwilltravel]

Generally, a truss should be designed with pinned joints. This is in keeping with classical structural concepts.

The AISC code allowables assume "standard" structural assumptions will be used in a design. Pinned ended trusses is a standard assumption.

I had an old boss who believed that there should be 2 allowable stresses. One for hand design and one for computer design. When a computer design is used, every member becomes a biaxial beam column leading to a more conservative design. This exceeds the assumptions of the codes.

 

[BATman2]

Cazkoop,

There are is an entire section on the design of trusses in Design of Welded Structures, by Blodgett (Lincoln Arc Welding Foundation publishes this) which includes consider for moment loads due to eccentric load of the chord sections. Perhaps you can break down your truss into several joints and check it with hand calc.

If the chords are long, perhaps local buckling is the limiting factor.

My 2-cents,
Batman2

 

[feadude]

Most finite element packages have an option called release moment or release end moment for joints. You can release moments for members that will carry axial loads only i.e. diagonal members and let larger x section area members to trnsmit moments i.e. top and bottom chords.

 

[oalfishcivil]

I think you may not be able to end release the moment for a truss. At least it is so in ADINA. We can only end release beam element. However, without end release the truss and just use it. It will automatically assume there is no moment in the memeber. Yet, I am not sure how it did this when the truss and a beam element shar the same node?

 

[feadude]

According to thread owner he used beam elements. Here is his statement
"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..."
The only way you can develope moments in the members is to use beam elements.