STUDY OF FIRE RESISTANCE OF REINFORCED CONCRETE BARS ACCORDING TO SPECIFIED CONCRETE STRENGTH INDICATORS

Abstract

The article presents the results of determining the limit of fire resistance of reinforced concrete crossbars, taking into account the study of identification of strength characteristics of concrete based on the results of fire tests. During the fire tests, two serial reinforced concrete crossbars with the same structural parameters were investigated. The experimental setup made it possible to study full-size elements. The preparation, method of conducting and obtained results of fire tests are described in detail in the previous article. During the fire tests, temperature indicators were obtained at the control points of the cross-section of the tested elements and the value of their maximum deflection depending on the test time. The purpose of this article is based on determining the time of onset of the limit state as a sign of the loss of load-bearing capacity of reinforced concrete crossbars according to the specified concrete strength indicators, calculated based on the results of fire tests. Based on the obtained experimental temperature indicators, temperature interpolation was carried out and temperature distributions in the cross-sections of reinforced concrete crossbars were constructed. The obtained results are adequate, since the F criterion does not exceed the table values and is equal to 0.734. To clarify the strength properties of concrete, a deformation model was used, which is based on the use of equilibrium systems of internal layers in the section. The mathematical apparatus presented in the work allows rewriting the deformation model into a system of linear algebraic equations, the unknowns of which are the concrete strength reduction coefficients. Based on the value of the maximum deflection of the crossbars, the problem of strength was solved and the limit of fire resistance was determined according to the specified coefficients of concrete strength reduction. It is shown that the onset of the limit state due to the loss of bearing capacity is not reached in all cases and is determined according to the technical conditions (approximately 0.3 of the destructive load) М = 50 кНм.