Review of Design Approaches for Steel Columns Subjected to Localised Fires: Insights from the 2nd Generation Eurocode Model

Complex temperature fields may develop inside structural steel elements in fire, severely affecting the local and global response of the structure. However, simplified yet well-accepted design procedures allow for uniform temperature distributions in the steel elements. The second generation of Eurocodes provides advanced models for localised fires where non-negligible thermal gradients both inside the sections and along the elements are expected to influence the resistance of steel elements, especially those critical to the stability of the entire structure, such as vertical members. This paper sheds some light on the consequences of employing procedures with increasing complexity in the design of unprotected axially compressed steel columns subjected to localised f ires for which the effect of the smoke layer is not significant. Five procedures were compared, considering a refinement in both the thermal and the mechanical models, culminating in the application of a procedure enabling complex temperature fields both in the section and along the column, namely the LOCAFI model, and exploiting numerical simulation in SAFIR. A parametric study was conducted, encompassing different fire scenarios and structural characteristics, resulting in 27468 analyses. It was shown that in the case of localised fire scenarios that induce significant thermal gradients in the structure advanced modelling is recommended because it better captures the thermomechanical response. Detailed considerations on relevant outcomes, such as the temperature at failure, were provided. Finally, the influence of the surrounding structure on the effectiveness of the procedures was investigated extending the parametric analysis to columns with various degrees of axial restraints.