Understanding and analyzing the complex behavior of a structural member is often achieved by decomposing it into simpler phenomena and interpreting it as a superposition of these simpler phenomena. Accordingly, the deformations of thin-walled members are commonly decomposed into global, distortional, local-plate, and other modes. One well-known practical technique is the constraining of shell models; the idea first appeared in the constrained finite strip method, then later in the constrained finite element method, or cFEM in short. In spite of cFEM’s remarkable ability to handle a wide range of problems, one of its disadvantages is that it relies on a specific rectangular shell finite element. The research presented here seeks to overcome the limits set by this shell element. It is proposed to use two discretizations and two corresponding basis function systems. One is based on virtually arbitrary, ordinary shell finite elements, whereas the other is based on cFEM shell elements. Constrained spaces are defined with the basis functions of the cFEM and then mapped to the ordinary shell model. The modal analyses are finally completed in the ordinary shell model, but by using the constrained spaces obtained from the cFEM basis functions. In this paper we describe the proposed method and demonstrate its application with numerical studies.