Experimental behaviour of circular concrete filled steel tube columns and design specifications

• Strength Index (SI) of column decreases as the core concrete strength increases.
• Brittleness of 107 MPa concrete is shown by sharp transitions from pre-peak to post-peak region in load-shortening curves.
• The application limits of EC4 can be widened to cover columns with high strength properties.
• Confinement effect of the circular tubes should be revised in AISC 360-10 design formulations.

This paper presents 18 tests conducted on short, medium and long circular Concrete Filled Steel Tube (CFST) columns. To explore the impact of column parameters and confinement effect three L/D ratios, two D/t ratios, two steel qualities and three concrete classes were employed. Some specimens have properties within application limits of EC4 and AISC 360-10 whereas others have properties beyond the application limits. Since new, large and efficient structures require adoption of high strength materials, it is compulsory to push the limits of design specifications. It is shown that 56 MPa and 66 MPa concretes provide very smooth and ductile load-shortening curves which imply high deformation capacity of such concrete classes. Brittle nature of 107 MPa concrete is shown by very sharp transitions from pre-peak to post-peak region and sudden discharge of loading in load-shortening curves. Additionally, 239 experimental data were collected from literature to assess EC4 and AISC 360-10 predictions within application limits and beyond application limits. Instead of focusing on a narrow region of configurations, this paper examines the performance of prediction methods on short, medium and long CFST columns. EC4 predictions indicate much better agreement with the test results. However AISC 360-10 predictions are conservative for all combinations of parameters. The application limits of EC4 can be widened to cover solutions of columns with broader properties. Confinement effect should be handled elaborately in AISC 360-10 formulations. L/D and relative slenderness are key parameters and have direct impact on column behaviour. However D/t does not have direct impact on column behaviour.