Fire behaviour of pin-ended concrete-encased concrete-filled steel tube (CECFST) slender columns under concentric and eccentric compression

Concrete-encased concrete-filled steel tube (CECFST) columns are widely recognised for their superior structural performance in mega construction structures. Despite extensive research under normal conditions, limited attention has been given to their performance in fire conditions. In this study, five CECFST slender columns were tested under transient fire conditions to investigate their fire behaviour. Key test findings included the failure mode, temperature distribution, structural response and fire resistance. Experimental results revealed that fire-exposed CECFST columns generally showed global buckling accompanied by extensive tensile cracking, demonstrating excellent ductility. Explosive spalling was observed in two specimens despite adding 0.2 vol% PP fibres. Moreover, compared to conventional concrete-filled steel tube (CFST) columns, CECFST columns exhibited superior fire resistance, highlighting their effectiveness as a ductile structural member under fire exposure. In addition, a calculation model was developed based on finite difference method (FDM) to predict the temperature distribution and strength reduction for CECFST columns in fire conditions. The calculation model can be easily implemented via MATLAB, and the code is open source. The proposed model was verified with test results, showing reasonable accuracy. This study provides insights and practical tool for evaluating fire performance of CECFST columns, particularly in super high-rise buildings and infrastructure projects where the maximum fire resistance may be required by the code.