The problem of fire resistance of metal building structures with a long service life is considered. The results of a study of the influence of a long service life of steel structures on the physical, chemical and mechanical characteristics (tensile strength, yield strength, relative elongation) at the elevated temperatures in relation to a steel structure with a service life of 92 years are presented. The steel structure was operated under stress conditions — bending under the load of a reinforced concrete flight of stairs. Due to long-term use, traces of the corrosion process are noticeable on its surface over its entire area. The results of the chemical analysis of test samples № 1, 2 are presented. According to GOST 27772—2021, it can be established that the steel under study corresponds to the standard content of the chemical composition of steel of strength class C345, with the exception of a 2-fold increased S content and an insufficient amount of manganese content. With increased sulfur content, steel becomes less strong and the steel resistance to brittle fracture decreases. This is confirmed by earlier work, which showed that increased sulfur content helps to reduce the mechanical characteristics of steel by 2 times. For the first time, the tests were carried out to assess changes in the mechanical characteristics of long-term steel structures under conditions of increased temperature (fire). A comparative analysis of the results of studies of mechanical tests at elevated temperatures for various steel grades was performed. It was established that for the studied samples of steels № 1, 2, long-term natural exposure at temperatures of 500 °C and 600 °C, the yield strength decreases by 23 % and 46 %, and the ultimate strength decreases by 42 % and 50 %, respectively. A tendency for a more intense loss of mechanical characteristics for steel structures of long-term natural aging under fire conditions was revealed.