<p>This study applies the CS-MITC18+ 3-node triangular flat shell element to analyze stiffened plate and shell structures under static loading and evaluates its effectiveness in comparison with the counterpart MITC18+  element. The MITC18+  flat shell element is an extension of the MITC3+ flat shell element, with Allman-type drilling degrees of freedom added. In this formulation, the in-plane displacements are approximated using Allman-type displacements, while the out-of-plane behavior is enriched by a cubic bubble function. The transverse shear strains are separately interpolated using the MITC3+ shear-removal technique. By averaging the membrane and bending strains of the MITC18+  element over sub-triangular domains defined by segments connecting the element nodes and centroids, the membrane and bending stiffness matrices of the CS-MITC18+  flat shell element are integrated along the boundaries of the smoothed domains rather than through surface integrals. The presented flat shell elements and 2-node Timoshenko beam elements are used to simulate several plates and shells reinforced by stiffeners. The numerical studies show that the proposed approach can reproduce the reference results and agrees well with other comparable elements. Although it incorporates cell-based smoothed membrane and bending strains and requires more computational time, the CS-MITC18+  element does not demonstrate improvement over the MITC18+  element. In other words, the MITC18+  flat shell element is a strong candidate for the analysis of ribbed plates and shells.</p>