Seismic performance of concrete-filled steel tube columns using ultra-high-strength steel under long-period ground motion demands

The long-period ground motions observed in recent subduction-zone earthquake events (e.g., 2011 Tohoku earthquake in Japan) have subjected high-rise buildings to large numbers of lateral cyclic deformations. Concrete-filled steel tube (CFT) columns, which are often utilized in high-rise buildings in Japan, have been studied under lateral demands with a few loading cycles. However, their seismic performance under a larger number of repeated loading cycles is a considerable concern for future seismic events. Moreover, the use of ultra-high-strength steel materials in CFT columns has recently gained popularity. However, studies on CFT columns made using ultra-high-strength steel materials are still limited. In this study, the seismic performance of CFT columns made using conventional steel or ultra-high-strength steel were investigated experimentally under repeated lateral loading cycles. Four cantilever CFT column specimens were tested with a combined constant compressive axial loading and cyclic symmetric lateral loading. Each specimen was tested with two different lateral loading protocols: the conventional protocol with two cycles at each drift amplitude level, and a second protocol with twenty cycles at each amplitude level to represent the lateral drift demand under a long-period ground motion. The effects of the number of loading cycles on the seismic performance of the CFT columns are discussed. Based on the experimental observations, design recommendations are also presented. Moreover, an approach is proposed to estimate the onset of local buckling of the CFT columns under seismic loading.