Polishchuk M. Stress-strain state of bending elements made of glued wood with a combined reinforcement

The dissertation investigates the work of a new design of bending elements made of glued wood, namely beams, the reinforcement of which involves a combination of steel and composite reinforcement. For these samples, diagrams of the distribution of deformations and stress graphs within the height of the calculated cross section in the middle of the span from the beginning of loading to failure under the action of direct pure bending were constructed. New experimental data about the change of stiffness and load-bearing capacity of reinforced beams in comparison with non-reinforced specimens of similar dimensions under the action of one-time short-term loads are obtained and the corresponding diagrams of deflections and deformations are constructed. The theoretical load-bearing capacity of the investigated beams was calculated using the deformation model, which takes into account the specifics of stress distribution in the stretched and compressed zones of the beam, on the basis of the deformations obtained during the tests. It was also made and tested additional wooden samples of prisms of structural sizes that allowed determining such important characteristic of wood as an initial modulus of elasticity and in accordance to the separate methodology the change of its value during loading was estimate. In the introduction the relevance of the research topic is substantiated, also the purpose and main tasks of the dissertation are described, the object, subject and methods of research are indicated, the scientific novelty, personal author contribution are pointed. The first section is devoted to an overview of works that reveals the use of wooden reinforced structures in construction. The history of the first application and development of glued laminated timber structures in construction and implementation of reinforced wooden structures, which were reinforced first with steel and later with composite reinforcement, are considered. Wide possibilities, modern tendencies and prospects of application of laminated timber structures at erection of various kinds of buildings and constructions are described. The second section describes the method of experimental research in order to establish the stress-strain state of glued timber elements with combined reinforcement under the action of one-time short-term loads for bending work. The volume of all experimental samples that were tested in the work is indicated. The processes of their production, arrangement of metal and composite reinforcement are described. More detailed attention is focused on the arrangement of composite materials, because according to the technology it took place in several stages. The scheme of experiment plant for testing reinforced beams is given. The location and purpose of all measuring instruments used in the research are described. The sequence of the test method is proposed. In accordance with the developed method, additional samples are made in the form of wooden prisms in size 45˟45˟250 mm to determine the modulus of elasticity of wood.The technique of production and testing of these samples is also described. The third section presents the results of experimental and statistical studies of reliable estimation of the initial modulus of elasticity. The dependence of the secant modulus of elasticity change on the stress level in wood is determined using a statistical method. It is determined how its value changes from the beginning of samples loading to destruction. Influence of moisture on the change of elasticity modulus and strength of solid wood prisms has also been studied. As a result of testing the samples, the value of compressive strength of the wood from which the beams are made was established. The rupture test results of steel rods, which were used to reinforce the compressed zone of beams, are given. The fourth section highlights the results of experimental studies of reinforced beams. As a result of the transverse bending test, new experimental data were obtained on the deformation and stiffness of glued beams due to the use of combined reinforcement. The values of deformations that occurred during loading in all layers of wood, as well as in steel reinforcement of the compressed zone and composite in the tensile zone were determined. Based on these values, the corresponding deformation diagrams are constructed. In the fifth section, the theory of calculation of reinforced glued timber beams with the use of a deformation model is given. This model takes into account the nonlinear distribution of stresses along the height of calculated cross section and allows designate the formation of folds in compressed zone. With this technique, the values of stresses were calculated and the character of their distribution in the height of the cross-section in the zone of pure bend of all experimentally investigated samples was established.