The fiber model [1] of the cross-section is a very popular approach for the elastoplastic analysis of beams and frames. It consists in evaluating the material response in terms of normal stress at a discrete number of integration points (fibers) over the section for a given increment of the strain field according to the rigid section assumption. The stress field is then integrated numerically for obtaining axial force and bending moments. The interaction with tangential stresses is usually neglected at the fiber level, due to the inaccurate representation of the shear strain over the section based on the rigid section model. In this work, we propose a full coupling of normal stress-shear stresses at the fiber level by exploiting the accurate strain field coming from the De Saint Venant problem [2]. A closest point projection provides then the full stress state at each fiber and, then, all the stress resultants and moments with a full interaction among all of them. This model of the cross-section is implemented in an accurate mixed finite element of 3D beam with equilibrated stress field, with the possibility of including also non-uniform warping. Numerical tests confirm an excellent accuracy compared with a solid reference solution and the efficiency of the proposal, very similar to the standard fiber beam model. REFERENCES [1] Spacone, E., Filippou, F.C. and Taucer, F.F., Fibre beam-column model for non-linear analysis of r/c frames: part I. formulation. Earthquake Engng. Struct. Dyn., Vol. 25, pp. 711-725, 1996. [2] Genoese A., Genoese A., Bilotta A., Garcea G, A mixed beam model with non-uniform warpings derived from the Saint Ven`ant rod. Computers & Structures, Vol. 121, pp. 87-98, 2013.