The thermal analysis of fire test results obtained for loaded cast iron tubing used to line subway tunnels, their rational fire protection and pre-set fire resistance

Introduction. The article addresses the fireproofing limit of the cast-iron tubing that lines subway tunnels. These structures have been fire tested neither in our country, nor abroad, and therefore, no fire test results have been analyzed, although this analysis is necessary to guarantee structural performance. Purpose and objective. The purpose of the study is to analyze the results of two fire experiments, including the testing of the cast iron tubing without fire proofing, and the testing of the tubing protected by a fireproof plate. The objective of the study is to choose and use a rational thermal analysis methodology. Methods. A standardized methodology was applied to test the fire resistance of loaded full-scale tubing specimens in a fire furnace, where thermocouple measurements of tested specimens were taken. The thermal analysis of these measurement results was conducted, using the methods and software for the numerical calculation of non-stationary temperature fields inside fireproof structures in one- and two-dimensional settings. Results. It has been established that the fire resistance limit of the 5.6-25-NU cast-iron tubing tested under constant static loading (150 kN) without any fire protection is 54 minutes, which corresponds to classification R 45, while the fire resistance limit of the same tubing, fireproofed by PROSASK fire panel plates, that are 25 mm thick, is, at least, 121 min (R 120). Calculations allow to prognosticate a change in the fire resistance, if account is taken of the difference between the test environment and the one of the tubing when in operation. The authors present the results of calculations for various fire protection options and show that the fire protection and fire resistance limits of the cast-iron tubing are high, if PROSASK fire plates are used. The authors also demonstrate the projectability of the fire resistance values, if the modes of exposure differ from the standard temperature mode. Conclusions. The thermal analysis of the results of two fire experiments, conducted to evaluate the fire resistance of the cast-iron tubing as the tunnel lining allows to obtain the information that is essential for the fire protection and pre-set fire resistance of critical subway structures as well as the further development of this area of experimental and theoretical research. The authors demonstrate the efficiency of thermal calculations as an instrument for the evaluation of fire protection parameters/fire resistance of tubing and the reduction in the number of costly fire tests.