Strut and Tie Models with elastic reinforcement

• Applicant: Professor Dr.-Ing. Mike Schlaich
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495079204

The object of the research project is the investigation of the deformational and loadbearing behavior of elastically reinforced discontinuous regions (d-regions) in reinforced concrete (RC) structures. It aims to provide a qualified statement about the applicability of the method of strut and tie models (STM) for these types of RC-structures.Due to the growing significance of sustainability and durability in construction industry, there is an increasing interest in noncorrosive fiber-reinforced-polymer-reinforcement (FRP-reinforcement). In order to fully exploit the potential of this linear elastic reinforcement, a comprehensive understanding of the loadbearing behavior of such reinforced cross-sections is essential. Herein lies the relevance of the evoked project.The STM method idealizes the complex internal force distribution of the highly indeterminate d-regions into a free ideal truss. It is based on the lower bound theorem of plasticity, according to which, assuming elastic-ideal plastic or rigid-plastic material behavior, any load for which a stable statically permissible state of stress can be specified is not higher than the bearing load. Due to the decoupling of stresses and distortions, the angles of inclination in the STM can be assumed to be constant regardless of the load. The requirement for a stable equilibrium is thus fulfilled.In the case of strain-dependent material behavior (linear elastic reinforcement), forces generate strains. These cause rotation in the nodal zones, which results in a growth of the forces within the STM. Therefore, the required stable stress state cannot be achieved. In order to ensure a stable equilibrium, the compatibility of the rotation within the STM need to be verified. The strains must not cause a change of the inclination angles.Strictly speaking, this also applies to RC cross-sections. In reality, Steel exhibits strain-hardening whereas concrete presents strain-softening at larger strains. According to the literature, limiting the strength of concrete and the "skillful" choice of the internal load-bearing system (STMs) is intended to ensure that the assumed stress distribution can still occur. An assessment of the rotational capacity of the STM not only allows the extension of the STM method to elastically reinforced concrete cross-sections, but also closes the existing definition gap for steel RC-concrete at the same time. The idea is to reduce the complex load bearing mechanisms of the relevant nodes to the utilization of geometric reserves and multi-axial stress states. The description of the rotational capacity by tapering the horizontal stress fields in the discrete compression nodes is a promising approach. By shifting the resultant force of the compression struts outwards, strains can be absorbed without changing the angle of inclination. Validation of this approach is carried out in several series of experiments with different nodal geometries.

• DFG Programme: Research Grants