This lecture covers different model fidelities for nonlinear analysis of structures, introducing concepts like plastic hinges, elastic beam elements, and concentrated plasticity. It discusses advantages and disadvantages of concentrated plasticity models, as well as the numerical modeling of structures using elastic beam elements. The lecture also delves into the shape functions, strains, and curvature within a beam element, explaining the virtual displacements formula for element stiffness and stress matrices. Examples are provided to illustrate the construction of global stiffness matrices and the application of rotational springs in elastic beam elements.