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

Svitlana FEDCHENKO; Serhii POZDIEIEV; Ihor FEDCHENKO; Andrii SUBOTA

Svitlana FEDCHENKO Cherkasy Institute of Fire Safety named after Chornobyl Heroes of the National University of Civil Protection of Ukraine
Serhii POZDIEIEV Cherkasy Institute of Fire Safety named after Chornobyl Heroes of the National University of Civil Protection of Ukraine
Ihor FEDCHENKO Cherkasy Institute of Fire Safety named after Chornobyl Heroes of the National University of Civil Protection of Ukraine
Andrii SUBOTA State higher educational institution "Uzhhorod National University"

Keywords: fire resistance, reinforced concrete, crossbar, test, strength, temperature, calculation

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.