Abstract Reinforced concrete frames under spatial action exhibit distinct nonlinear behaviour. The material nonlinearity can be captured at cross-sectional level. Existing cross- sectional models can be classified into resultant models, truss models, uniaxial mod- els, wall models, as well as models based on finite element analyses. Here, the authors propose a new hybrid approach. The approach combines a uniaxial model with a wall model. The cross section is divided into two components. The stirrups, the adjacent concrete and longitudinal rebars form a notional, thin-walled cross section assembled from membrane elements. The second, uniaxially stressed component comprises the remaining concrete and longitudinal reinforcement. The strain state of the cross sec- tion follows from the equations of equilibrium, continuity, and the constitutive laws. Keywords: reinforced concrete, frame, nonlinear analysis, cross-sectional analysis, hybrid approach. 1 Introduction Many of the common structures consist of frame elements. New construction methods and progress in concrete technology render possible bolder and more slender concrete structures. Their safe and economical realisation calls for nonlinear and powerful computational methods. Currently used structural analysis software is suitable only to a limited extent for these highly stressed structures. Particularly with regard to spatial frames, the geometrical and material nonlinearities are not accurately considered [1]. Within beam theory the material nonlinearity can be captured at cross-sectional level. A cross-sectional analysis provides the strain state corresponding to a set of section forces. One application of a cross-sectional analysis are interaction diagrams. Interaction diagrams visualise the bearing capacity of a cross section under combined 1 Paper 224 A Hybrid Approach for the Non-Linear Analysis of Reinforced Concrete Cross Sections T. Löhning, J. Schenk and U. Starossek Structural Analysis and Steel Structures Institute Hamburg University of Technology (TUHH), Germany Civil-Comp Press, 2008 Proceedings of the Ninth International Conference on Computational Structures Technology, B.H.V. Topping and M. Papadrakakis, (Editors), Civil-Comp Press, Stirlingshire, Scotland The Ninth International Conference on Computational Structures Technology, 2-5 Sept. 2008, Athens, Greece