Study On Short-term Bending Stiffness Of Cracked Reinforced Concrete Beams Based On Elasticity Theory With Different Moduli

Generally speaking, reinforced concrete beams work with cracks, which will have a big impact on short-term bending stiffness of reinforced concrete beams. It is believed that the calculation of short term bending stiffness of reinforced concrete beams with cracks is a complicated issue, and the calculation formula of short term bending stiffness in current codes are achieved by the analysis of the experimental phenomena and data. On the basis of experimental data on flexural behavior of HRB335 and HPB235 grade reinforced concrete beams, the calculation formula of flexural stiffness are obtained in current reinforced concrete structure codes (GB50010-2002). It is worth discussing the applicability of the calculation formula of bending stiffness in current codes to concrete beams with high strength reinforcement.A large amount of experiments and studies have shown that flexural reinforced concrete beams are elastic during the process of the crack property on the tensile stress area to the yield of steel. Based on the homogeneous elastic material model in structured mechanics, the different tension and compression moduli is employed to study flexural mechanical behaviors of 15 HRB 500 and 2 HRB335 grade concrete beams with reinforcement only in the tensile end under the load of about 0.8 M u. We found that the concrete grade of 17 test beams approximately keep consistent with section size, both using the method of three-point loading. With no beams in the compression area, we can calculate in reverse the elastic mould value subjected to tension based on the experimental data, and use regression analysis to get analytical expressions of the elastic mould and the ratio of longitudinal reinforcement, then we further present the constitutive relation of the ratio of longitudinal reinforcement and some parameters such as short-term bending stiffness, the neutral axis, the crack's height, reinforcement stress, concrete stress and so on. Main contents in this thesis are as follows:The first chapter has a brief review of the research status on short-term bending stiffness of reinforced concrete beams.The second chapter describes the basic knowledge of reinforced concrete beams and basic theories of different tension-compression different moduli.The third chapter talks about the current calculation formula of short-term stiffness of reinforced concrete beams, and analyze influencing factors of short-term stiffness calculation by comparing current codes. In the fourth chapter, main creative researches are conducted as follows: we solved the pure bending beams with different tension and compression moduli, and we calculated in reverse the elastic mould value subjected to tension based on the experimental data, what's more, analytical expressions of the elastic mould and the ratio of longitudinal reinforcement are given by adopting regression analysis, then we further present analytical expressions of the reinforcement ratio and some parameters such as short-term bending stiffness and stiffness degradation, the neutral axis, the crack's height, stress in longitudinal tensile reinforcement and maxim stress in the compression area of concrete beams.The fifth chapter is the results and analysis including the discussion on calculations in the fourth chapter, and a comparative study on different moduli method and short-term bending stiffness and stiffness degradation in China, America and Europe.The sixth chapter gives the conclusion and expectation of this thesis.The last chapter is the acknowledgement and bibliography of the research with an appendix.To sum up,this thesis not only provides an innovative research idea to study reinforced concrete beams with cracks, but also is significant for the perfection of structure design theory of reinforced concrete beams.