Evaluation of Transverse Reinforcement Requirements for High-Strength Concrete Columns

The use of high-strength concrete (HSC) is continuously increasing due to its mechanical and durability advantages over normal-strength concrete (NSC). In high-rise buildings, HSC can reduce the dimensions of the lower-stories columns, which makes it a more cost-effective choice for builders than NSC. Studies have shown, however, that HSC is more brittle in compression than NSC and that the confinement provided to HSC is less effective than in NSC. Therefore, greater confinement is required for columns made from higher strength concrete to achieve similar strength and ductility enhancements. This behavior is the main obstacle to HSC's widespread use. While the ACI 318 standard does not limit the concrete compressive strength, the Eurocode 2, the New Zealand standard 3101, and the CSA A23.3 standard limit the maximum strength that can be used for seismic design to 90 MPa, 70 MPa, and 80 MPa, respectively. Moreover, studies have shown that the axial load level has a significant influence on the ductility of concrete columns subjected to cyclic flexure and constant axial loads. A large number of studies were performed to examine the behavior of HSC columns subjected to compressive loading and to combined constant axial load and reversed cyclic flexure simulating earthquake loading. The main drive for this study was to give recommendations for the confinement reinforcement ratios required for HSC columns in a real 60-stories high-rise building. First step in the study was to compare the confinement requirements of several international standards. Since it is the first experience in Israel to use HSC for a commercial RC multistory building, the next step was to design and perform experimental study on large-scale HSC circular columns subjected to compressive loading with different volumetric transverse reinforcement ratios. The results show the differences between the standards requirements and give an insight on the recommended transverse reinforcement ratio for columns that behave as secondary structural elements, which are not designated as part of the seismic-force-resisting system.