Steel Frame with shearwall - instabilites

[rn14]

Hello all,
I am trying to model a steel frame with a wood shear wall which seems like an easy enough task. The columns are pinned at the base and the beam is simply supported. When the shearwall fills the entire frame everything is dandy. When the shearwall only fills a portion of the frame Risa reports instabilities and locks two nodes. Any suggestions? Note that all steel elements are "physical members".

The frame to the left results in the top rght and lower left nodes getting locked. The one on the right is fine.

 

[JoshPlumSE]

Which joints is it locking and for which degree of freedom?

 

[rn14]

Joints N1 and N5 which are the bottom joint at the left steel column and the top joint at the right steel column. The locked DOF is MY (moment about vertical).

 

[jayrod12]

Provide My reaction at the base (since your anchor layout would likely result in the column not being allowed to twist significantly) and that should solve your issue.

 

[rn14]

Thanks Jay. That makes sense I guess I was more curious as to why this became unstable due to the shearwall placement.

 

[jayrod12]

Once you connect the wall to the node at the column connection it was performing the same thing. the wall had out of plane strength to resist the rotation. At least that's what I envision was happening.

 

[JoshPlumSE]

If the beam ends were BenPinned to the columns then this is a common instability. The column is torsionally unstable in the mathematical model unless you have an MY restraint at the base.