Help on ABAQUS (modelling of rivets on fuselage panels)

Good mourning,

 I'm a student of mechanical engeneering in the Universidade de Aveiro in Portugal and I'm writing my masterthesis on fuselage panels. I'm doing models of skins and stringers and at the moment I'm trying to model the rivets used to assemble these two to make a stiffened panels. I'm making the models and mesh on FEMAP and then I export the .INP to run the analysis. I'm using rigid links to model the rivets with all the degrees of freedom restrained (both 3 translations and 3 rotations) but I'm getting buckling loads higher than the buckling load of the same model (with the same geometry) acting as an ISP (Integrally stiffened panel) which of course makes no sense!

 I've tried several types of elements to models the rivets but or I get errors or I get incorrect results. I've tried rigid line elements such as R2D2, rigid links, RB2D2...now I am trying to use JOINTC elements but I think I cannot make those type of elements in FEMAP so I have to type the commands on the .INP file but I do not know how to. 

My questions are if anyone know a better way to model the rivets and how I use the JOINTC command in the .INP file.

 Sincerely,

              Sérgio Oliveira