Usenko Y. Stress-strain state of biaxial bent reinforced concrete Т-section beams (based on a complete diagram of concrete deformation)

The dissertation is devoted to the study of the stress-strain state of biaxial bended reinforced concrete Т-section beams based on a complete diagram of concrete deformation depending on the load level. In section 1 «Analysis of the biaxial bending impact on the work of reinforced concrete beams» the dissemination of reinforced concrete T-profile elements in which the phenomenon of biaxial deformation occurs, the causes of biaxial bending in such elements and its impact on structures are analyzed. As the literature and other information sources analysis shows, the phenomenon of biaxial bending is widespread among the elements of existing buildings and structures. The factors that cause it can be not only load but also technological, design and production nature. Biaxial bending causes changes of the stress-strain state in reinforced concrete elements. Its influence must be taken into account both in the calculations of the bearing capacity and suitability for normal work. Because even a small impact of biaxial bending changes the deformation nature of the reinforced concrete beam element. In section 2 «Principles of the stress-strain state modeling in normal sections of reinforced concrete T-beams under biaxial bending» the basic principles which are necessary for the stress-strain modelling of reinforced concrete T-section beams under biaxial bending are given. The choice of coordinate system for theoretical research is substantiated. To describe the stress-strain state of the bending element, it is necessary to know the position of the neutral line, which is characterized by certain parameters. All possible cases of the neutral line position in the normal section of the reinforced concrete T-section beam with systematization into three groups are analised. Formulas for determining the stresses in the compressed zone concrete are given. In Section 3 «Models of stress-strain state in the calculations of T-beams, under biaxial bending» the models of stress-strain state of biaxial bent reinforced concrete T-section beams with different geometric forms of the concrete compressed zone are shown: triangular (case 1.2), trapezoidal (case 1.1, 2.1, 3.1), trapezoidal (case 1.3, 2.3), pentagonal (case 1.4, 2.4, 3.4), pentagonal (case 2.2, 3.2), hexagonal (case 3.3). For each model the formulas of stress resultant Nc in the concrete compressed zone and its application coordinates x0,Nc and y0,Nc are given. A general approach to determining the parameters of the stress-strain state is presented. In section 4 «Methods of experimental research of reinforced concrete T-beams under biaxial bending » the way of preparation and research of reinforced concrete T -beams samples is described to determine experimental values of cross-sectional displacements and crack formation features. The construction of experimental beams samples, placement schemes of devices for measuring deformations and methods of testing T-beams samples are given. The crack formation features in the stretched cross-sectional zone and the destruction features of the compressed concrete zone are described. In section 5 «Practical implementation and verification of the developed theory of the stress-strain state description on the samples of T-beams» the stress-strain state of the T-beams samples is analyzed. Theoretical values of stress-strain state parameters are compared with the experimental data.