Non-linear FE-Analysis of stress redistribution in a deep beam

Design of discontinuity regions in reinforced concrete is today often performed with Strut and Tie Method (STM) or linear finite element analyses (LFEA). However, it is questionable to what extent these design methods are appropriate with regard to resistance, ductility, serviceability and economic reinforcement arrangement. In design with STM a simple equilibrium model based on plasticity theory is used, based on load paths selected by the engineer. The question is then whether the assumed stress field is possible to achieve with regard to the available plastic deformation capacity. It is also uncertain if the serviceability demands are satisfied. The aim of this research is to clarify the effect of alternative reinforcement configurations in discontinuity regions in reinforced concrete and give recommendations for design with regard to load bearing capacity, serviceability and material efficiency. The present investigation is conducted with a nonlinear finite element analysis (NLFEA) on one selected deep beam in one span with a specific geometry and reinforcement configuration designed on the basis of LFEA or with alternative STM. The NLFEA shows considerable stress redistributions and energy dissipation due to concrete cracking. At the load level when the reinforcement yields, the compression resultant in the midsection is located along the top edge of the deep beam. The stress redistribution due to yield of the reinforcement steel is small. The increase in load corresponds in principle to the strain hardening. For the studied case, a design based on LFEA or STM with strut inclination according to common recommendations underestimates the load bearing capacity in relation to NLFEA