Kozak O. Stress-strain state, strength and crack resistance of nodes of monolithic frames and continuous beams with reinforcement stressed on concrete

The dissertation is devoted to the study of the stress-strain state, strength and crack resistance of nodes of monolithic frames and continuous beams with curved reinforcement steel, stressed on concrete. This paper reviews the literature and analyzes the methods of calculating the nodes of monolithic frames and continuous beams with pre-stressed reinforcement (without adhesion to concrete). On the basis of the conducted analysis it is established that the methods of calculation (according to current norms) of strength and crack resistance of inclined sections of nodes of continuous beams and frames with pre-stressed curved reinforcement steel (without adhesion with concrete) are not sufficiently perfect and do not take into account the individual faults. To determine the effect of pre-stressed curved reinforcement steel on the strength (crack resistance) of nodes of continuous beams and frames, prototypes were designed and constructed, which allowed to analyze the stress-deformed state, the process of cracking and bearing capacity, depending on the different tilt angle of the pre-stressed reinforcement (without adhesion to concrete). The developed test procedure of the test specimens made it possible to obtain the necessary strength and deformation characteristics. According to the results of experimental studies, the presence of pre-stressed curved reinforcement steel (without adhesion to concrete) significantly affects the shape of fracture, the nature of cracks and the overall bearing capacity of the supporting sections of monolithic continuous beams and frames. After processing the results of the experimental studies, based on the current normative documents, an improved method of calculating the strength of the inclined sections of the elements with pre-stressed curved reinforcement steel is proposed, taking into account the vertical unloading force from the pre-stressed reinforcement Vdis. Also presented is an advanced technique for calculating the fracture toughness of inclined sections, which additionally takes into account the bending moment, which creates the unloading force Vdis, from pre-stressed curved reinforcement steel. A method for calculating the crack opening width based on the current norms is proposed, taking into account the compressive stress in concrete from pre-stressed reinforcement (σср). The proposed calculation methods allow to take more accurate account of the stress-strain state of the nodes of monolithic continuous beams and frames with pre-stressed curved reinforcement steel (without adhesion with concrete), and their results are closer to the real work of the structure. The theoretical calculation of the prototypes in the "LIRA PC" was performed, taking into account the physical nonlinearity, the results of which gave a fairly good convergence with the experimental data. In general, experimental and theoretical studies have confirmed the effectiveness of the use of pre-stressed curved reinforcement steel (without adhesion to concrete) at the nodes of monolithic continuous beams and frames. The use of pre-stressed curved reinforcement steel allows to increase the bearing capacity and the crack resistance of the inclined sections of the support units.