Thanks for the welocme and the helpful suggestions guys.
I initially tried to simulate the slingset using springs, but found rigid links prefereable as I could attach them to the padeye hole surfaces. Springs require a point connection and you get high localised stresses when you make the springs really stiff (a wire rope sling has a stiffness about 50kN/mm). Rigid links work fine, I have constrained them to allow movement in y and x (towards each other so that I get compressive stress in the top beam) and rotation about the hook CSYS. The hook constraint is zero DOF as a point in space so the analysis hangs off that, but I have now made it able to rotate about the x axis to allow some swing towards the COG.
So as not to impart unrealistic loads into the padeyes at the connection with the rigid links I have made the link constraints able to rotate about the hook CSYS.
It's constraining the frame to the ground thats the problem. I have had some success constraining it on the bottom surface with movement in the y and z (direction of swing) direction. I don't know why CREO needs this though, g x mass in the y vector is surely all the constraint it should need? CREO ought to allow the load to swign to align the COG directly below the hook constraint. Maybe I am asking too much of it?
I'm finding that different types of lower constraint produce a result, but the result can vary significantly depending on how that constraint is configured. This is what concerns me. The load has to swing to aling the COG or else it puts bending moments in the padeyes that are not there in real life.
I guess another approach would be to reorient the hook constraint y vector to allign with the CSYS vector at the COG, and apply g in that direction so it imposes a concentric load. It's more work though that I hoped CREO would do for me.
This is my apporach, will le tyou know how it goes.