The accurate behaviour assessment of complete structural systems is essential in developing Performance Based Engineering methodologies. In this aspect, structural models based on beam-column elements are still a competitive option compared to solid-FEM models; mainly because of the practical data generation, results interpretation and the direct use of internal forces in design. Nevertheless, BC-elements are developed under plane sections hypothesis and under null or simple imposed shear deformation along cross sections allowing for reasonable non-linear evaluation of normal internal forces (axial force and biaxial bending moment) but not for tangential internal forces (biaxial shear and torsion) whose influence on the behaviour of structural systems with irregular distribution of stiffnesses, masses or resistances has proved to be determinant. A NL-fibrewise sectional model for concrete structures capable of simulating the total interaction between all six beam internal forces and deformations with arbitrary shaped cross section and longitudinal reinforcement and stirrups arrangement has been developed. Crack concrete is simulated as a 3D orthotropic material which in combination with steel reinforcements yields, in general, a full constitutive matrix coupling all stress and strain components. The model enables an explicit expression of the sectional stiffness matrix considering the interaction of the complete set of internal sectional forces on a cracked section. A reinforced concrete cross section is analysed for increasing levels of combined bending and shear loads. Noticeable dependencies on the concomitant load are shown on the overall performance of both momentcurvature and shearing force-deformation diagrams.
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