Strength design of steel-concrete composite columns considering size effects and concrete cover spalling

Current design standards, such as AS/NZS 2327, Eurocode 4, and AISC 360, do not adequately account for two critical behaviours in steel-concrete composite columns: the size-dependent reduction in concrete strength and premature concrete cover spalling in high-strength concrete-encased steel (CES) columns under axial loading. Overlooking these phenomena can lead to unsafe designs for large size/high-strength composite columns. This study develops size effect factor equations for both concrete-filled steel tubular (CFST) and CES columns to address the issue of strength reduction. For high-strength CES columns, 0.5 % steel fibre reinforcement by volume is suggested to prevent premature concrete cover spalling, ensuring the full utilisation of the concrete cross-section. Alternatively, an effective concrete area can be adopted to account for spalling effects. The proposed modifications, including the size effect factor and the concrete cover spalling consideration for CES columns, were rigorously validated against a comprehensive experimental database of 1764 CFST and 162 CES column tests. Integrating these changes into the existing AS/NZS 2327 design equations substantially improved prediction accuracy, leading to safer and more reliable designs for composite columns. Furthermore, a reliability analysis demonstrated that the concrete capacity factor in AS/NZS 2327 can be safely increased from 0.65 to 0.7 after incorporating these advancements. These proposed modifications offer significant potential for integration into other international design standards.