This study presents an in-depth investigation of a cold-formed steel portal frame structure, originally designed in the 1950s, through advanced nonlinear analysis. The research aims to compare traditional structural modelling approaches with more detailed nonlinear modelling techniques while evaluating various retrofitting strategies. A comprehensive nonlinear analysis was conducted, accounting for all sources of nonlinearity. Multiple modelling versions were explored, including beam elements with rigid and pinned connections, as well as a hybrid model incorporating 3D columns modelled with shell or solid elements, and beam elements for diagonals and roof structure, utilizing multipoint constraints for node connections. The connections between cold-formed steel profiles were modelled using fasteners that exhibit non-linear force-slip behavior. The structure's response was analyzed at both service and ultimate limit states, leading to the proposal of various retrofitting methods, such as the use of Ultra-High Performance Concrete (UHPC) for columns, additional gusset plates, and profile doubling. The primary output of this study is a comparative analysis between traditional structural modeling approaches and more detailed nonlinear modeling techniques, providing insights into the accuracy and efficiency of different strategies for cold-formed steel structures. This research contributes to the understanding of advanced modeling techniques for cold-formed steel structures and offers practical insights for engineers and researchers involved in structural retrofitting and sustainable construction.