No abstract

In the development of the ideas and approaches to the analysis of the force resistance of concrete of V.M. Bondarenko, the initial prerequisites for the model of the thermomechanical state of concrete under short-term sharp high-temperature exposure, characteristic of fire conditions, are formulated. The separation of force deformations into components is carried out on the basis of the connection with the accumulation of damage in the structure of the material, based on the principle of independence of the limiting structural stresses from temperature and the mode of force action, which makes it possible to establish basic thermomechanical relationships and determine the deformation parameters of concrete operating under conditions of unsteady heating in a loaded state. Based on the extension of the hypothesis of entropy damping of nonequilibrium processes to the area of action of an active destructive factor, the principle of normalization was formulated and a kinetic equation was proposed, from the solution of which exponential dependences having a single structure were obtained, which make it possible to describe the basic temperature parameters of concrete, the relationship of stresses with deformations, and other nonlinear characteristics. The application of the proposed principles creates a reliable theoretical basis for describing the mechanisms of thermal resistance of concrete and greatly simplifies the modeling of the effect of high temperature on the properties of concrete in the practical implementation of methods for the numerical calculation of reinforced concrete structures.

The work of a beam with partial limitation of displacements on supports under conditions of unsteady high-temperature heating up to destruction is shown. Depending on the level of limitation of the rotational stiffness, the sequence of the formation of plastic hinges on the supports and in the span was revealed, which affects the resistance time of the beam. The conditions are formulated under which restrictions on the displacement of supports lead to the formation of adaptive resistance mechanisms. The time during which the beam resists bending and the ultimate resistance time depend on the combination of the stiffness level of the axial ka and rotational kr of the anchors. An increase in the level of limitation of the rotational stiffness kr in most cases leads to an increase in both the time of resistance to bending of the beam and the limiting time of resistance. With the provision of structural measures for the stiffness levels of the support fastenings ka and kr within certain limits, it is possible to significantly increase the fire resistance of the beam due to the formation of adaptive resistance mechanisms.

Проведен анализ работы тонких плит в условиях характерного для пожара неравномерного высокотемпературного нагрева. Показано, что повышенное искажение формы прогиба, возникающее из-за высоких уровней тепловых вариаций, имеет существенные последствия для развития сжатого бетонного кольца и растянутой арматурной сетки. реализующие пространственную работу плиты при нагреве, которые не будут учитываться предположением о равномерности температуры среды, используемым обычно при проектировании. Из полученных результатов видно, что линейное изменение температуры среды вызывает концентрацию деформаций дробления в углу, наиболее близком к максимальной температуре; деформации уменьшаются в менее нагретых температурных областях.