Substructuring-based section characterization of thin-walled beams from shell finite element models

Accurate beam sectional stiffness and inertia properties are essential for reliable one-dimensional modeling of thin-walled structures, particularly in the presence of material anisotropy and coupling effects. A recently proposed substructuring-based framework extracts these properties directly from a short three-dimensional finite element beam slice model by (i) statically condensing internal degrees of freedom, (ii) enforcing equilibrated generalized load states through reduced interface handles, and (iii) matching the slice strain and kinetic energies with those of an equivalent beam model. While the original formulation was designed for solid-element discretizations, many thin-walled structural components of practical use are more efficiently modeled with shell elements, whose nodal kinematics include rotational degrees of freedom. This paper extends the framework to Kirchhoff–Love and Reissner–Mindlin shell formulations by redefining the rigid interface-handle mapping to constrain both translational and rotational degrees of freedom consistently. The method is validated on: (i) a thin-walled circular cylinder with analytical reference properties; (ii) a flat plate over a range of thicknesses, from thin to moderately thick, to assess transverse shear effects; (iii) a steel I-section (IPE100), to evaluate shell versus three-dimensional accuracy and flange-web junction effects; and (iv) a laminated orthotropic rectangular box modeled using equivalent single-layer shells based on classical lamination theory, with comparisons against ply-resolved three-dimensional references. The study provides practical guidelines for choosing shell formulations and discretization requirements for extracting sectional properties in thin-walled applications.