Physic interaction betweem sphere and polyhedra

hello
     i know that Ig2_Sphere_Polyhedra_ScGeom can calculate the ig between polyhedra and sphere, if the material of polyhedra is polyhedraMat and sphere is frictMat, the ip between them can be calculate by Ip2_FrictMat_PolyhedraMat_FrictPhys, and then use cundall law, the interaction between polyhedra and sphere can be solved.
     Ip2_FrictMat_PolyhedraMat_FrictPhys describe in Polyhedra.cpp is as below:
void Ip2_FrictMat_PolyhedraMat_FrictPhys::go(const shared_ptr<Material>& pp1, const shared_ptr<Material>& pp2, const shared_ptr<Interaction>& interaction){
 const shared_ptr<FrictMat>& mat1 = YADE_PTR_CAST<FrictMat>(pp1);
 const shared_ptr<PolyhedraMat>& mat2 = YADE_PTR_CAST<PolyhedraMat>(pp2);
 Ip2_FrictMat_FrictMat_FrictPhys().go(mat1,mat2,interaction);
}

and stiffness culculate in Ip2_FrictMat_FrictMat_FrictPhys() is like that:
 Real Kn = 2*Ea*Ra*Eb*Rb/(Ea*Ra+Eb*Rb);
 Real Ks = 2*Ea*Ra*Va*Eb*Rb*Vb/(Ea*Ra*Va+Eb*Rb*Vb);

What puzzles me is that polyhedra do not have radius r, then how to calculate normal stiffness between polyhedra and sphere use Ip2_FrictMat_PolyhedraMat_FrictPhys?
lookforward for your answer.