Stressed-skin diaphragm action occurs when cladding, typically thin steel sheets or two sheets encapsulating insulation, significantly contributes to a structure’s global stiffness. Ignoring this effect in design can lead to inaccurate assessment, particularly under hori-zontal loads. The Equivalent Cantilever Beam Method (ECBM), considered an extension of the Component Method, models the flexibility of connections using equivalent cantilever beams composed of standard finite element members available in engineering analysis software. Earlier applications of the ECBM were limited to the elastic range of connector behaviour. This study introduces a nonlinear extension of the method by incorporating material yielding into the equivalent beams, enabling plastic redistribution of forces without specialised nonlinear springs. A hierar-chical validation is presented: single-connector behaviour, a diaphragm panel against tests and ECCS predictions, and a full 3D building against an advanced reference model. Results demonstrate excellent agreement, indicating that the proposed method provides a practical and deployable solution for including stressed-skin effects in routine structural design models. The ECBM still follows the analytical framework of ECCS Recommendations No. 88, extending its applicability while maintaining its original simplifications and assumptions. Despite these limitations, the method may support the practical assessment of clad steel structures, including cases involving parasitic diaphragm action and the possible reduction or elimination of roof-plane bracing.