how to get the value of stiffness of certain structure?

Depends what you mean by 'stiffness'.  My interpretation of stiffness is a measure of the deflection under load therefore in the simplest terms, you can use the displacement result as a measure of stiffness.

Regards

Keith

Hi Werther: We do this frequently, using the unit-displacement method, for example. You can apply a non-zero displacement say, "1" inch or "1" centimeter/meter (depending on the overall size of the structure to stay withing small displacement theory), to a component face in the desired direction. Run static. Then, in the Results folder you can "probe" the face with the applied displacement and it will report the force required to hold the structure at that displacement value. Then of course K = F/1 (in units of force/unit displacement).

Generally to apply a non-zero displacement you can use the "Advanced" Fixture type and enter a value and direction. We made a little YouTube video of that at GoEngineer, at link below. We hope that helps!

SolidWorks Simulation - Non-zero "Fixtures" and Reaction Force

http://youtu.be/Px1sweM_lqw