Concrete Encased Steel (CES) column is one typical form of composite columns synergizing the merits of steel and concrete, which has been widely used in the construction of high-rise buildings. Employing high strength materials in CES columns is a promising solution to improve space efficiency and reduce the greenhouse gas emission. However, current international design codes only permit the design of normal strength composite columns. The primary issue of high strength concrete lies in the inherent brittleness that raises ductility concerns. In addition, the plastic strength of high strength steel sections may not be fully developed due to the early failure of concrete. Therefore, this project aims to explore the structural behaviour of high strength CES columns, assess the applicability of current design codes, and propose new design methods if necessary.

A series of experimental investigations was carried out in this project, i.e. the concentric compression test on CES stub columns to assess the cross-sectional axial capacity, four-point bending test on CES beams to assess the cross-sectional moment capacity, eccentric compression test on CES short columns to assess the axial-flexural capacity, as well as the CES slender beam-column test to assess the overall buckling behaviour. All the test results were compared with the predictions using EN 1994-1-1 and other design codes including AISC 360-16 and JGJ 138-2016. It was found that EN 1994-1-1 does not always give conservative predictions, especially the axial capacity of high strength CES columns. To extend the existing EN 1994-1-1 to cover CES columns with high strength concrete and high strength steel, some modifications were proposed. Aside from experimental study, statistical analysis and numerical simulation were also performed to further explore the behaviour and load-carrying capacity. Statistical analysis was conducted by collecting available test data from published literatures, which covers CES columns subjected to axial compression and coupled compression and bending. The fibre element analysis was employed as the numerical technique. Taking into account of concrete cover spalling, concrete confinement effect, reinforcing bar buckling and steel section strain-hardening, the fibre element approach can reproduce the load-deformation relation with satisfactory accuracy.

In conclusion, this study fills the gap of missing information on high strength CES composite column, and makes significant contribution towards the understanding of its structural behavior and failure mechanism. The findings reported in this research work provide a valuable reference and guidance to the engineering practice. Finally, based on the published 8 journal papers, a new design method is proposed as an extension of existing EN 1994-1-1. The new design method is applicable for CES columns with concrete class up to C90/105 and steel grade up to S550.

1. Lai BL, Liew JYR, Wang TY. Buckling behaviour of high strength concrete encased steel composite columns. Journal of Constructional Steel Research 2019; 154: 27-42.
2. Lai BL, Liew JYR, Xiong MX. Experimental Study on High Strength Concrete Encased Steel Composite Short Columns. Construction and Building Materials 2019; 228: 116640.
3. Lai BL, Liew JYR, Xiong MX. Experimental and analytical investigation of composite columns made of high strength steel and high strength concrete. Steel and Composite Structures 2019; 33(1): 67-79.
4. Lai BL, Liew JYR, Akshay Venkateshwaran, Li S, Xiong MX. Assessment of high-strength concrete encased steel composite columns subject to axial compression. Journal of Constructional Steel Research 2020; 164:105765.
5. Lai BL, Liew JYR, An LH, Xiong MX. A unified approach to evaluate axial force-moment interaction curves of concrete encased steel composite columns. Engineering Structures 2019; 201: 109841.
6. Lai BL, Liew JYR, An LH. Behavior of high strength concrete encased steel composite stub columns with C130 concrete and S690 steel. Engineering Structures 2019; 200: 109743.
7. Lai BL, Liew JYR. Axial-Moment Interaction of High Strength Concrete Encased Steel Composite Columns: Design Recommendation. Journal of Constructional Steel Research 2020; 170: 106136.
8. Lai BL, Liew JYR. Design and Testing of Concrete Encased Steel Composite Beam-Columns with C90 Concrete and S690 Steel Section. Engineering Structures 2020; 220: 110995.
9. Lai BL, Liew JYR. Axial-Moment Interaction of High Strength Concrete Encased Steel Composite Columns: Experimental investigation. Journal of Constructional Steel Research (under revision).