Comparative Analysis of Code Provisions for Built-Up CFT Columns with High Strength Concrete under Varied Loading Conditions
Abstract
Steel-concrete composite structures are gaining popularity for their advantages over traditional materials, yet the behaviour of concrete-filled tube (CFT) columns, particularly built-up sections with high strength concrete (HSC), remains underexplored. This study investigates the impact of concrete strength, cross-sectional dimensions, and loading types on the ultimate load capacity of built-up CFT columns. Seven specimens with concrete strengths of 48 MPa and 53 MPa and cross-sections of 100x100 mm, 125x125 mm, and 150x150 mm were tested under concentric and eccentric loading. Results showed a 46% strength increase compared to bare steel columns due to concrete infill, while higher concrete strength had a minimal effect on overall capacity. Eccentric loading reduced load capacity by approximately 50%. Analysis of design codes revealed that AISC-2010 is 14% less conservative than EUROCODE-4 for concentric loads, while for eccentric loads, failure points fell within the AISC-2010 safe zone, indicating underestimation of risks. EUROCODE-4 demonstrated better reliability, with failure points lying outside its safe zone. These findings underscore the need for more robust code provisions for built-up CFT columns under varied loading conditions.
Conclusion
Composite columns demonstrate significant advantages over bare steel columns, carrying an average of 45% more load for the same cross-sectional area. For concentrically loaded columns, a 5 MPa increase in concrete strength results in a 10% gain in load capacity. However, under eccentric loading, the same strength increase yields only a 5% improvement. Loading type has a substantial impact, as eccentric loading reduces load capacity by approximately 48% compared to concentric loading. In terms of stiffness, composite columns exhibit about 30% greater axial stiffness than bare steel columns. Notably, a 5 MPa increase in concrete strength enhances stiffness by 34%, underscoring the material's significant contribution to stiffness, even though its effect on load capacity is less pronounced. However, eccentric loading markedly reduces stiffness, emphasizing the critical role of load application conditions. Regarding design code predictions, the AISC-2010 guideline is not significantly conservative for concentric loads. Moreover, for eccentrically loaded columns, failure points were found within the code's predicted safe zone, indicating that AISC-2010 does not adequately account for the risks posed by eccentric loading. This highlights the need for incorporating a greater safety margin when using this code for eccentrically loaded CFT structures. In contrast, EUROCODE-4 provides more conservative predictions for concentric loads and demonstrates greater reliability under eccentric loading. For all eccentrically loaded specimens tested in this study, failure points were located outside the predicted safe zone in the P-M interaction curves, reflecting the code’s ability to better capture the behaviour of composite columns under complex loading conditions. Given these findings, EUROCODE-4 is recommended for designs where uncertainty is higher or where eccentric loading is expected