A numerical approach to the assessment of the critical stresses for imperfect ribbed shells is developed. Initial deflections occupying a part of the shell surface (they are circumferentially bounded) are considered. The couple stresses and nonlinearity of the precritical state are taken into account. Numerical examples are given Keywords: imperfect ribbed shell, stability, critical loads Introduction. To analyze the precritical state of thin-walled ribbed shells and to determine their critical loads, use is made of one of the four possible methods. One of the methods uses a structurally orthotropic model, another method considers discrete elastic bars joined with the shell [1-4, 6, 14]. The precritical state is assumed to be homogeneous and membrane. The third method is based on the theory of imperfect thin-walled ribbed shells with an inhomogeneous membrane precritical state [6][7][8][9][10][11][12]. The fourth method regards the shell and ribs as a solid, i.e., as a three-dimensional medium [5]. Composite shells are analyzed for stability in [15][16][17][18].This present paper uses the third method because the first two have well-known disadvantages and disregard the couple stresses and the nonlinearity of the precritical state.1. Numerical Technique. We used a special modified software package for critical-load analysis that is capable of analyzing not only general distributions of initial imperfection but also their different types: from local to localized in any direction. Relevant studies for smooth shells are reported in [8,13], while there are just a few examples of ribbed shells with general and longitudinally localized deflections [10].It is necessary to assess the effect of some types of localized (bounded circumferentially but not longitudinally) imperfections on the critical load. This cannot be done in full with analytical approaches. Therefore, use should be made of the finite-element method [8,[10][11][12][13].It is generally known how to determine the minimum critical load. A portion of the shell bounded by its length and a kth of its circumference is covered by a mesh. The force and displacement functions are unknown at the nodes of the mesh. The nonlinear system of equations (their finite-difference analog) is solved with prescribed accuracy by the method of successive approximations. The Dl-method is used, i.e., given load parameter, the determinant of the left-hand side of the equations is calculated to detect sign reversals.