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.

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.

In this article, to solve the main problems, we determined the temperature regime of heating the steel beam, which took into account the fact of loss of integrity of the fire-retardant lining due to the thermal effects of fire. When calculating the temperature, the time of exposure to the standard temperature of the fire and the value of the heating temperature of the steel beam with mineral wool lining at which the latter loses its integrity was determined. Taking into account the geometrical parameters of the cross section of the studied I-beam, according to the finite-element scheme, the steel beam was divided into four elements of SHELL type with five points of integration in thickness in the Belichko-Tsai formulation. After the calculation, the corresponding results were obtained in the form of graphs of changes in the maximum deflection of the beam and the rate of increase of the maximum deflection depending on the time of exposure to the standard temperature of the fire. The critical values of the occurrence of the limit of fire resistance according to the graph of maximum deflection and the graph of the rate of increase of deflection were determined. The difference between the indicators shows that the time of the limit state of loss of bearing capacity is 70 min less, if not taking into account the loss of fire-retardant capacity of mineral wool fire-protection due to loss of integrity.

In the article, experimental fire tests on establishing maximum fire resistance of the steel columns with fire-retardant mineral wool covering are analyzed. The scientific novelty of this study are the new method developed for manufacturing and shaping samples of the steel columns with fire-retardant mineral wool covering, and the method developed for conducting fire tests on establishing patterns of dependence of the moment when steel columns with the fire-retardant cladding lose their bearing capacity down to the limit. The subject of the study was an influence of design parameters of the fire-retardant cladding on the results of the estimated fire resistance of the steel columns under the influence of the standard fire temperature mode. The main task of the research described in this article was to determine a moment of the peak critical temperature of fire-retardant steel structure depending on the thickness of the mineral wool covering. The test results of samples of steel columns with different thickness of fire-retardant cladding based on mineral wool covering are also described in the article; the tests were carried out in accordance with the developed methodology of experimental studies. The repeatability of the time indicators of the bearing capacity loss by the tested samples of steel columns with fire-retardant cladding depending on the duration of the sample exposure to the action of the fire standard temperature is shown. The obtained research results will allow to create a mathematical model for predicting a dependence of the moment when a steel structure reaches its critical temperature depending on the thickness of the fire-retardant cladding. The constructed mathematical model can be considered as a scientific basis for the creation of new engineering methods of calculation for assessing fire resistance of the steel structures with fire-retardant mineral wool covering. These engineering and calculation methods allow improving the regulatory acts for designing of the fire-resistant steel structures.

No abstract