The present paper is concerned with the free vibration analysis of free-form stiffened shells through the isogeometric method. To this end, non-uniform rational basis spline functions are utilized for the modeling of complicated geometries. In this approach, the shell and the stiffener are modeled separately in which the degrees of freedom associated with the stiffener are projected onto those of the shell via interpolation functions. This approach provides users with flexibility in placing stiffeners at any location on the shell. Additionally, the shell is modeled using a degenerated element based on Reissner–Mindlin theory, which neglects torsional warping—a significant consideration in stiffeners. To address this, the stiffener is modeled using a degenerated element based on higher-order shear theory. The proposed method allows for the modeling of stiffeners with any geometric shape and any number on the shell. Numerical examples validate the effec- tiveness of the method by comparing results with state-of-the-art studies and ABAQUS simulations

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