Space frame

[Alex786]

Hi , For Space Frame ( 3D) analysis , what should be the member releases ( Release Mx , My , Mz) , i tried defining truss(3D) for members but it causes a error in output , " **WARNING - THERE WERE MORE THAN 12 DOF WITH ZERO STIFFNESS "
I think the Space frame members should be release at the starting node and end node of the member .
kindly advice ,
Thanks !!!

 

[SethGuthrie]

The problem with space frames where every member is pinned is one of nodal rotational stability. Essentially, at each node, at least one member connected to the node should not be released so that the node does not spin. You won't develop any moments at that point unless two or more members connected to the node are fixed (or an external point moment is applied). So, if you can find a system to keep one member at each joint not released, I think it will work well.

 

[StructuralJay]

Hello Alex786,

Sorry this post is a little late. Usually when model in STAAD I release MZ and MY on each end of the member. This will tell STAAD that it is not a moment connection but a simple shear connection. When you release these two moments you will have to supply horzontal and vertical bracing; if not than you will still get the same warning as you mentioned above and perhaps large deflections.

I really don't agree with SethGuthrie when he says that "at least one member connected to the node should not be released so that the node does not spin". It all depends what you release. Example, if you have a column and have a beam on each side of the column you can release MZ and MY to make the connections a simple shear connection. Again, you will have to supply bracing.

Hope this helps.

 

[SethGuthrie]

Just to clarify, in the case of a column and a beam only, you certainly can and should release the ends of the beam to create a pin connection, just don't also pin the top of the column. Doing so is redundant and leads to rotational instability for that node.

 

[Cherokeedesign]

In regards to this thread and the thread I posted about "Composite Decks".
StructuralJay's reply ends with "Again you will have to supply bracing." As we know, in the real world, a omposite deck consist of a concrete slab (diaphram) connected to floor beams by the shear lugs welded to the top of the beam. The combination is a rigid horizonal member which transfer horizintal shear force to the columns which are braced to transfer the force to the foundation. Columns not braced do not transfer this horozontal load.
My question is how to provide "bracing" for a space frame to get rid of the 12 DOF warning without puting bracing in each of the four corners of my space frame with composite deck and bracing in a "core bay?