Plate Modeling

[sigma1]

I have a composite tube column (fab'd from plates) that I would like to clamp down with B-B channels during the concrete placement. The clamps (ties) are not welded to the plate column. I have modeled the plate but I am having dificulty with the clamp (B-B Channels)because it is only capable of ressiting the horizontal force due to concrete pressure. If I release all forces and moments except the horizontal force the structure becomes unstable.

Is there a way to model this? Any recommendations will be appreciated