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Steel structures are widely used in modern construction in various structural forms. In particular, steel structures are common in quick prefabricated buildings. These are industrial and agricultural facilities (woodworking shops, warehouses of building materials, granaries, poultry farms, greenhouse), garages for special equipment, sports and recreation complexes, exhibition and entertainment centers, shopping pavilions, office buildings, etc.

Abstract. Imperfection of structure, metrological instruments and fuel and nozzle system control methods the conditions are created ensuring non-homogeneous distribution of temperatures throughout heated surfaces of tested structures and during their fire-resistance tests in combustion furnaces. The article contains results of numerical simulation of bearing walls fire resistance test performed using different configurations of combustion furnaces. Using computer-assisted gas and liquid flow simulating software temperature gradient for heated surfaces of bearing walls was created and temperature distribution for every minute of the computational experiment for each configuration was calculated. In temperature gradients 6000 to 7500 cells were located (depending on the particular configuration design), evenly distributed throughout the structure surfaces, containing temperature data in any moment during the computational experiment. As a result of processing of these data the value of temperature dispersion value was calculated. Difference between maximum and minimum temperatures on the surface of a reinforced concrete was also determined. Based on the curves representing temperature dispersion values at the surface of each of the simulated structures of the furnace chamber for every minute of the computational experiment the configuration with the most homogeneous temperature distribution throughout the heated surfaces of the bearing wall was defined, what allows reducing an error occurring due to temperature distribution non-homogeneity by heated surface structures during the fire resistance test. Problem statementWhereas testing in combustion furnaces [1,2] are carried out in the "standard" temperature mode, there arises a question concerning homogeneousness of a reinforced concrete structure heating depending on design and configuration of the combustion furnace used, since internal design of its chamber, layout of burners and combustion products exhausting holes affects homogeneousness of temperature distribution throughout the heated surface of vertical structures, in particular this refers to wall structures.

In the article, the processes of integrity loss of fire-retardant cladding from mineral wool of a steel I-beam are considered. The relation between fire-retardant cladding integrity and its thermal insulating ability was studied. Computer model of a steel beam with fire-retardant cladding of mineral wool, which takes into account its own weight, current mechanical load and temperature mode of heating under the heat-insulating effect of the cladding, using the computer system LS-DINA, is developed in this article. It is shown that the fire-retardant cladding loses its integrity under the thermal influence of fire long before the onset of the limit state of loss of fire resistance in terms of bearing capacity. The conditions under which fire-retardant cladding loses its integrity and fire-retardant ability are investigated. Having considered the time of fire-retardant cladding integrity loss the fire resistance limit can be significantly clarified and allows to avoid early collapse of building structures cause by fire.