Similarity analysis of the fire-resistant behaviour of partially encased composite columns in different scales

As full-scale fire resistance tests commonly involve considerable expense and require specialized experimental apparatus, scale specimens are being employed in many investigations focusing on the performance of structural members under fire exposure. However, since the inherent thermal properties of materials are commonly independent of the geometrical scale factors, it is necessary to analyze the similarity among the thermal responses of scale specimens and their corresponding prototype in advance, in order to generalize the experimental conclusions to practice. In this context, the similarity of the fire-resistant behaviour of partially encased concrete (PEC) columns, a sort of structural member that is widely used in building and industrial structures, in different geometrical scales was studied in this work. A finite element model was developed and verified based on existing experimental results, and then used for the parametric studies. The similarities of the transient temperature fields, deformation states and the fire-resistant durations of the PEC columns with restrained thermal elongation in different scales were analyzed, which indicates that the mathematical relationship between the time-scale factor and the geometric scale factor in the classical similitude theory needs to be improved for such structural members with complex cross-sections. A modified time-scale equation was therefore proposed to ensure the fire endurance prediction accuracy of scale PEC models for the corresponding prototypes under fire, with the proportion of steel in the cross-section (steel ratio) and the aspect ratio well-considered, which could facilitate the temperature-related experiments and analyses of PEC structural members in the future.