The article demonstrates the both theoretical and actual fire resistance limits of the composite I-shaped and box-shaped thin-walled steel structures in compression conditions under the standard fire load. The calculation was based on the Eurocode 3 and finite element modeling of high-temperature fields in SOFiSTiK PC. The experimental tests were carried out on the basis of design data to validate the results of both the calculation and modeling. It is shown that the static part of the calculation of the critical temperature, upon irreversible plastic deformations occur, is solved not completely correctly by means of regulations. In average the calculated critical temperature exceeds the actual one on 50-80 °C. It is shown that the assumption of a critical temperature equals to 350 °C is unreasonably low. The complex graphs of the temperature growth for each steel construction are given according to the paragraphs of normative documents, the finite-element modeling and results of thermocouple indicators for the fire tests. The solution of thermophysical part of calculation according to Eurocode 3 showed good convergence with the results of the experimental data, including the samples with effective fire protection, but strongly depend on the step of calculation. The accurate results were reached only when the time step equals 1 sec. The finite element modeling predicted the correct time to achieve the critical temperature of the tested sample without any additional assumptions. The MBOR-16F material produced by TIZOL JSC was used as a flame protection. This is new material, which has not been previously studied yet. The recommendations on application of the finite element programs are given in the thermophysical part of the fire resistance calculation.