Beam elements are considered as flexural elements with shear deformations with no axial deformations as beams belong to inextensional diaphragms and rigid zones at the ends to account for the stiffness increase at the joint, if needed. The element stiffness matrix varies throughout the analysis due to plasticity effects. In order to simulate such effects a hysteretic law and a spread plasticity model are introduced. The hysteretic model is formulated based on an initial moment-curvature relationship which represents the backbone skeleton curve. Such skeleton curves must be defined for each edge section of all elements. These curves can be either user defined or can be computed using a fiber
model and certain given properties of the section under consideration such as geometry, concrete and reinforcement properties.