Sounds like you've been instructed to do so. If that's the case, where did the instruction come from? I can't think of a scenario when it is necessary to model the stiffness of a strut. If one end of the strut is attached to a relatively flimsy structural member, then in CAESAR I may model the strut as a rod with stiffness. And the stiffness would be that of the structural member, not the axial stiffness of the strut.