Investigation on the required minimum shear reinforcement ratio for reinforced concrete members with different moment/shear force-combinations

• Applicant: Professor Dr.-Ing. Duc Tung Nguyen
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 520431381

Shear failure of reinforced concrete members without shear reinforcement is brittle and characterized by a formation of the critical shear crack. To prevent such kind of failure that usually occurs without preliminary signs, a minimum ratio of shear reinforcement rho w,min for beams is required. To determine this ratio, the truss analogy is normally used, in which the tie force provided by the minimum shear reinforcement must be equal to the shear capacity of the same beam without shear reinforcement. Having the same approach, the values of rho w,min amongst the current design provisions are very similar and formulated only as a function of material strengths. This approach is convenient for the practical design, but it does not correctly reflect the behavior in beams with low shear reinforcement ratios, since shear failure of beams with low shear reinforcement is still observed by a formation of a single shear crack that does not satisfies the main assumption of the truss model. Most recent experimental investigations on beams with different moment/shear force-combinations (M/V-combinations) showed that lower ratios of shear reinforcement than rho w,min have no influence on the shear behavior of simply supported beams with concentrated load but can improve the shear resistance for more than two times of beams or cantilevers with distributed loads. These observations cannot be explained using the approaches included in the current design provisions. Inspiring from the most recent observations, this research project aims to investigate the flow of forces and shear transfer mechanisms in beams with low ratios of shear reinforcement and different M/V-combinations. Since the number of tests on beams that differ from conventional shear tests available in the literature are very limited, an experimental program including 26 tests on beams with different M/V-combinations and with low ratios of shear reinforcement is planned. Within the experimental investigation, the location, form and geometry of the shear crack that leads to failure should be captured. In addition to the experimental investigation, numerical simulation will be carried out. The numerical models are verified by the experimental observations. The verified numerical models is then used to analyze the flow of forces in beams and to supplement information that cannot be directly deducted from the experimental investigation. In a next step, a parameter study is conducted using the verified numerical simulation. In these numerical tests the location of shear failure, the shear capacity is investigated with a broad band of system parameters and shear reinforcement ratios. Based on the experimental and numerical investigations, mechanical modelling will be developed. The mechanical model quantifies the shear mechanisms acting on the critical shear crack. Comparing the shear capacity obtained from the mechanical model, a new formulation for the minimum shear reinforcement ratio is deducted.

• DFG Programme: Research Grants