A comparative study on steel‐concrete composite columns for use in RC Tall Buildings in Seismic Areas

Usable/rentable floor areas are crucial for the building owners in reinforced concrete (RC) tall buildings. To address this issue, concrete‐encased (CE) or concrete‐filled (CF) composite columns are proposed as alternative vertical load carrying elements by providing the required strength and ductility, but with smaller dimensions. Comparing the performance of columns with different configurations can be done using moment (M)‐axial load (N) interaction curves. This paper summarizes the approaches proposed by Eurocode 4 and AISC 360‐16 for both types of composite columns, presenting simplified and true curves for a specific class of sections. Two case studies involving hybrid RC tall buildings in Istanbul, which utilized composite columns, are presented along with a comparison with steel‐concrete composite and RC solutions. Numerical results indicate that employing steel‐concrete composite columns resulted in approximately 24% reduction in total cross‐sectional areas for one of the case studies, leading to increased usable/rentable areas. Additionally, concrete‐encased and concrete‐filled composite columns exhibited similar performance in terms of axial load‐moment capacities. The impact of longitudinal rebar ratios is also investigated, revealing that the AISC 360‐16 approach yields smaller bending moments and compression capacities compared to Eurocode 4 due to the significant strength reduction factor considered in calculating the concrete contribution in composite sections.