Telescopic movement in Karamba

Hey,
I am going to analyze a kinetic structure that transforms by sliding movement like telescopic piston. However I failed to define the cirrect joint. In a simple explanation, imagin two beams (A,B) that connect each other with a third beam (C) in a middle and top of them. A and B can close to each other by a translational movement on C. Also A and B joint to columns to connect to the earth.
Thanks for any helps or idea!