An evaluation of an actual fire resistance limit of non-protected steel structures for different temperature regimes of fires

Introduction. A normative document Set of Rules 2.13130.2012 (item 5.4.3) states that if an required fire resistance limit of structures is equal to R15 (RE15, REI15) an application of non-protected steel structures is possible. But an actual fire resistance limit depends both on a reduced thickness of the structure and a temperature regime of a fire (for example “cellulose” or hydrocarbon regime). This study is aimed on a numerical evaluation of the actual fire resistance limit of the non-protected steel structures for the standard “cellulose” and hydrocarbon temperature regimes of fires at various reduced thickness of the structures.

Methodology and results. A numerical modeling of a heating of the structures was carried out using the software FDS 6. The non-protected steel structures having the reduced thickness δ_cr from 3 to 60 mm were considered. The fire resistance limits were determined as a time interval from a beginning of the fire to the time moment when the temperature of the structure reaches a value of 500 °C. Dependences of the actual fire resistance limits on δ_cr were obtained. These limits for the hydrocarbon fire were substantially lower than for “cellulose” one. A dependence of a ratio of the fire resistance limits for the hydrocarbon and “cellulose” fires on the reduced thickness of the structure was found.

Conclusion. The results of this study confirm the requirement of the normative document Set of Rules 2.13130.2012 (item 5.4.3) concerning a possibility of the application of the non-protected steel structures in the case when the required fire resistance limit is equal to R15 (RE15, REI15), but only for the “cellulose” fire. But this requirement is not valid for the hydrocarbon fire.

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