Substructuring is a technique in finite element analysis for solving mechanical engineering problems involving large structures which are too enormous for existing computer hardware and software to handle.
This technique involves breaking the system into parts, analyzing the parts, and then re-assembling the total system using selected degrees of freedom from the parts.
Learn how to do sub structure analysis in ansys with this excellent PowerPoint titled The Art Of Matrix Reduction
Well the fun starts only after this, when you need to access the mass and stiffness matrices. I wrote a small code last year just to do that.