Rope net in Kangaroo

Hi,

I've got a problem with my geometry in Kangaroo. 

I'm using a vertical force in the center to lift up my geometry of springs, while the periphery remains on the ground. It's something like: lifting a rope net from the ground. The springs are working fine, but my aim is to keep always the same radius of the central circle. In other words, does anyone know, how to make a rigid geometry connected to springs? 

Thanks a lot.