i need a procedure to do it ?
what you mean in the femap?..
FEMAP is related with Finite Element Analysis, is a powerful pre&postprocessor for any FEA solver, but mainly bundled with NX NASTRAN.
Regarding your question, in FEMAP you can model a spring using the following types of elements:
• A simply CROD: yes, you must play with rod cross section area A, rod length L and modulues of elasticity to get K = AE/L
• A genuine CELAS2 spring element: here you must choose DOF SPRING element type where numerically you can enter the stiffness in any direction.
• And finally a CBUSH element, this is a powerful element on the NX NASTRAN library, in FEMAP select SPRING/DAMPER. The generalized spring-damper element CBUSH is a structural scalar element connecting two noncoincident grid points, or two coincident grid points, or one grid point with an associated PBUSH entry. This combination is valid for any structural solution sequence.
Please note there are some advantages of using the CBUSH element over CELAS2 elements. For example, if you use CELAS2 elements and the geometry isn’t aligned properly, internal constraints may be induced. The CBUSH element contains all the features of the CELAS2 elements plus it avoids the internal constraint problem.
Here you are an example tutorial: http://www.iberisa.com/soporte/femap/nolineal/celas2_nolineal.htm