Influence of Thermal Gradients on the Resistance of Axially Loaded Steel Columns Subjected to Localised Fires

The definition of the resistance and the associated failure time of unprotected steel membersin fire is crucial to the safety of steel structures. For this purpose, design standards allow for prescriptive approaches that rely on nominal fire curves that do not represent the physics of a real fire, for instance, a localised fire. Indeed, the thermal actions can be significantly different from the ones based on prescriptive approaches and non-negligible thermal gradients, both in the section and along the length, can develop in the structural members affecting the resistance of steel members. In this paper, three design procedures enabling increasing complexity of the temperature field were compared by investigating axially loaded steel columns characterised by a range of slenderness and load utilisation factors subjected to localised fires. The procedures consisted: i) in exploiting the recently proposed analytical LOCAFI model to compute single steel temperatures at different column heights and calculating the failure time with EN1993-1-2 considering the highest temperature; ii) in determining the failure time with beam finite element analyses in SAFIR by employing the temperatures at the different column heights, iii) in using a more refined version of the LOCAFI model included in SAFIR, which directly allows for the thermal gradients in the cross sections at the different heights of the column. The analysis showed that simplified procedures may predict both higher or lower failure times and therefore the employment of an advanced modelling is recommended.