Plate girder bridges, desired for their adaptability across various spans and loading conditions, are typically evaluated through finite element method (FEM) simulations rather than costly full-scale laboratory testing. Steel structures often fail due to mechanisms like yielding, fractures, fatigue, and buckling, which reduce flexural stiffness. Flexural rigidity (EI) variations can indicate damage location, making it a valuable parameter for damage detection. This study employs a damage detection method to locate and quantify damage in composite and steel plate girders under unknown damage scenarios. To this end, two case studies involving steel and composite plate girders demonstrate this approach through detailed finite element models. This study addresses modelling and convergence challenges while evaluating the effectiveness of the applied damage detection method.