Instability in shell model

[MRCH94D9W]

I'm experiencing some stability issues which I can't figure out.

I have some shell elements supported by two columns with fixed supports in the bottom, (See Figure 1).

As the model is right now, there are no instability issues. However, I want to put in an element on which I can apply some load between the two columns. This element is connected by 4 points to the columns and 1 point to the roof.

When I attach a beam element to the column I get instability in the shell elements, (See Figure 2).

My mesh geometry is healthy and no incoherence is reported, so I'm at a loss here.

I have attached the model and I hope you will take a look at it .

Thank you very much.

Bonus question:

I have tried to use the "Reduction of forces above columns and wall" feature under the "Maps > Parameters", but it would seem this feature only works with vertical frame elements connected perfectly perpendicular to the shell element. Is this really true?

Initially I tried to use this feature, but I ended up connecting the frame and shell elements by rigid links, which I suspect is the root of my instability problem.

[Rafacascudo · Accepted solution]

No need to worry .This instability type 3 warning can be ignored as your node displacements are normal for that type/size of structure. Search the forum for similar posts and check webinar 16 about this on Build your Robot Structural Analysis IQ - YouTube 

For the " reduction of forces" use rigid links around the columm section perimeter .

And yes the Rlinks are the root of the instability warning type 3 , which can be caused by a large difference in stiffness between members