Keywords: object-oriented programming, non-linear analysis, reinforced concrete, cross section, frames.

New construction methods and progress in concrete technology render possible bolder and more slender structures. While the non-linear computation of plane frame structures usually shows good accordance with experiments, many analysis programs still fail to adequately model reinforced concrete frames under spatial loading. The material non-linearity usually is captured at cross-sectional level. The cross-sectional analysis provides the relationship between the sectional forces and the generalised strains. Most programs consider the effects of interacting normal and bending forces on the cross-sectional stiffness. However, for instance the interaction between torsion and bending is mostly neglected, or considered by simplified methods. Existing cross-sectional models can be classified into resultant models, truss models, uniaxial models, wall models, as well as models based on finite element analyses. A hybrid cross section model developed at the institute of the authors combines a uniaxial model with a wall model and thereby allows for interactions between all six internal forces of a spatial frame element [1]. An object-oriented approach is chosen for the implementation of the model into a software module. For the analysis of frame structures, the module then is integrated into the finite-element framework OpenSees [2].

First numerical computations at cross-sectional level show good results, both for linear material behaviour under combined bending, shear and torsional forces, as well as for non-linear material behaviour under combined bending and normal forces. Also the integration of the cross section module into the framework seems to work well for plane frame structures. In the next step, both the cross section module and the combined program are to be tested with non-linear materials under arbitrary combinations of all six force components. In the future, the approach might allow for a unified design procedure, limiting the principle concrete strains and the reinforcement strain produced by all six force components. However, for this new design concept further research is necessary.

1

T. Löhning, J. Schenk, U. Starossek, "A Hybrid Approach for the Non-Linear Analysis of Reinforced Concrete Cross Sections", in "Proceedings of The Ninth International Conference on Computational Structures Technology", B. Topping, M. Papadrakakis, (Editors), Civil-Comp Press, Stirlingshire, United Kingdom, paper 224, 2008. doi:10.4203/ccp.88.224

2

M.H. Scott, G.L. Fenves, F. McKenna, F.C. Filippou, "Software Patterns for Nonlinear Beam-Column Models", Journal of Structural Engineering, 134(4):562-571, 2008. doi:10.1061/(ASCE)0733-9445(2008)134:4(562)

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