Annular reinforced concrete(RC) members are commonly used in bridge structures and offshore platforms. These RC members often fail under the combined actions of axial force, bending moment, shear force and torsion load in hazards of earthquake and wind. It is very important to study the failure mechanism of annular RC members under combined actions. This study proposes a model to analyze the ultimate strength of annular RC members under combined actions using limit failure theory. A new method is established to determine the geometric parameters of the warped failure surface, and the new calculation model for the ultimate strength is obtained using the equilibrium conditions based on the geometric parameters and the stress distribution on the failure surface. The proposed model calculations are compared with a series of experimental results of annular RC members, and they correspond well with the experimental results. The proposed model is feasible for engineering application.

This paper uses experimental investigation and theoretical derivation to study the unified failure mechanism and ultimate capacity model of reinforced concrete (RC) members under combined axial, bending, shear and torsion loading. Fifteen RC members are tested under different combinations of compressive axial force, bending, shear and torsion using experimental equipment designed by the authors. The failure mechanism and ultimate strength data for the four groups of tested RC members under different combined loading conditions are investigated and discussed in detail. The experimental research seeks to determine how the ultimate strength of RC members changes with changing combined loads. According to the experimental research, a unified theoretical model is established by determining the shape of the warped failure surface, assuming an appropriate stress distribution on the failure surface, and considering the equilibrium conditions. This unified failure model can be reasonably and systematically changed into well-known failure theories of concrete members under single or combined loading. The unified calculation model could be easily used in design applications with some assumptions and simplifications. Finally, the accuracy of this theoretical unified model is verified by comparisons with experimental results.

Under complex loads, especially for earthquakes and wind loads, RC I-shaped members are usually subjected to load combinations of axial forces, bending moments, shear forces, and torsion. In this paper, we deduce the relationships between the external forces and the cracking degrees. Then, a model for the evaluation of the ultimate strength of RC I-shaped members is established based on the ultimate equilibrium of twist failure surface. Finally, the model results are compared with the experimental results of former researchers. The model coincides well with the experiment.

Rectangular reinforced concrete members are widely used in building structures and bridge structures. The failures of these members usually occur under the combined loading actions of axial force, bending, shear and torsion. In this paper, a calculate model is established by determining the shape of warped failure surface and reasonably assuming the stress distribution on the failure surface. Then through the comparison of calculation results and experiment, the validation is verified for the calculation model of ultimate capacity of reinforced concrete members with rectangular section under combined actions.

Reinforced concrete elements with a rectangular section are commonly used in frame structural buildings and bridges. Such reinforced concrete structures failed often with the force combination of axial load, bending, shear and torsion caused by wind and earthquake. However, there are few researches on four loading combination of tension/compression, bending, shear and torsion, especially for the axial force effects on the failure mechanism and bearing capacity of reinforced concrete members. In this paper, a theoretical study and deduction of the unified failure model on the reinforced concrete members with rectangular section under combined axial force, bending, shear and torsion were carried out with the ultimate equilibrium theory. The results of a theoretical analysis were compared with the experimental results of RC members under combined loading actions. Both of the theoretical results and test results showed a good correlation.

The geometrical size and modulus of fibers are the main factors influenced the mechanical performances of fiber reinforced concrete. The use of different type of fiber in a suitable combination may potentially improve the overall mechanical performances of concrete and result in performance synergy. This paper studied experimentally the synergic mechanical properties of hybrid fiber reinforced concrete (HyFRC) with different kinds and percents of steel fibers and polypropylene fiber, to find out the higher performance of mechanical properties of hybrid fiber reinforced concrete mixtures. Four groups of test specimens with macro and micro-fiber combination of steel fiber and polypropylene fiber were used to investigate the HyFRC's compressive strength, split tensile strength, bending strength and impact resisting performance.

An impact bending test method for concrete and test equipments were developed compared with impact test methods suggested by the ACI,and the impact mechanical properties of hybrid fiber reinforced concrete(HyFRC) with different kinds and percents of carbon fiber,steel fibers and polypropylene fiber were investigated,and the effect of fiber types and contents on HyFRC's impact performance was analyzed.A hybrid factor to evaluate quantitatively the positive hybrid effect of concrete anti-impact behavior was suggested.The study results showed that hybrid carbon fiber reinforced concrete has better impact resistance capacity compared with that of ofher concrete.