Reinforced concrete and steel (RCS) structures are an exceptional combination when exploited thoughtfully; the advantages of composite structures over pure steel or reinforced concrete have been demonstrated through studies and real tests. Nonetheless, the industry often struggles to adopt this valid option and nonlinear modeling strategies are currently rare, given its complexity. This Thesis is focused on the development of a Python-based library with simulation tools for steel, reinforced concrete and composite structures and on the proposition of a reinforced concrete and steel subassemblage model that captures the main failure modes of the real structure.