Thin-walled composite structures are increasingly employed in engineering applications due to their stiffness-to-weight and strength-to-weight ratios, load-carrying capacity, cost-effectiveness and corrosion resistance. However, slender thin-walled composite load-bearing structures are highly susceptible to instability, with an increased risk of buckling and excessive deformations under external loads, leading to complex structural and dynamic response [1– 3]. In previous work [4], a geometrically nonlinear beam model is developed for buckling analysis of shear-deformable cross-ply laminated beam-type structures. An improved model for angle-ply composite beams is introduced in [5], where each cross-section is modeled as a thin, symmetric and balanced angle-ply laminate. The formulation incorporates Hooke’s law and large-displacement nonlinear analysis to account for restrained warping and large rotation effects. Shear deformation is included using Timoshenko beam theory for non-uniform bending and a modified Vlasov theory for non-uniform (warping) torsion. Coupling between shear forces and warping torsional moments arising from cross-sectional asymmetry is modeled. The present study extends the previously developed finite element beam model to perform free vibration analysis of laminated thin-walled composite beam-type structures. Based on the Hamilton’s principle, a consistent mass matrix is formulated and the corresponding eigenvalue problem is solved. Numerical examples show that the model accurately predicts the natural frequencies of thin-walled composite beam and frame structures subjected to initial load. REFERENCES [1] L.P. Kollár and G.S. Springer, Mechanics of Composite Structures. New York: Cambridge University Press, 2003. [2] V.H. Cortínez and M.T. Piovan. Vibration and buckling of composite thin-walled beams with shear deformability. Journal of Sounds and Vibrations, Vol. 258, Issue 4, 2002. [3] T.P. Vo, J. Lee and K. Lee. On Triply Coupled Vibrations of Axially Loaded Thin-Walled Composite Beams. Computers and Structures, Vol. 88, 144–153, 2010. [4] D. Banić, G. Turkalj and D. Lanc. Stability Analysis of Shear Deformable Cross-Ply Laminated Composite Beam-Type Structures. Composite Structures, Vol. 303, 116270, 2023. [5] D. Banić, G. Turkalj and D. Lanc. A geometrically nonlinear finite element formulation for buckling analysis of shear deformable angle-ply composite beam-type structures. Computers and Structures, Vol. 301, 107427, 2024.