Multi-axis testing of concrete-filled steel tube columns forming ductile soft-story in multi-story buildings

Abstract

In regions of low-to-moderate seismicity, the design of multi-story buildings with strong-beams weak-columns are quite common, which triggers the formation of a soft-story collapse mechanism during rare ground motions. A design approach to overcome this issue is introducing a ductile soft-story at the first level, such that the damage is controlled, while the above stories remain in the elastic response range. In this approach, the ductile soft-story acts as an isolator and limits the damage at only the first-level, thereby reducing the cost and time of repair and improving the safety and resilience of these buildings. The use of concrete-filled steel tube (CFT) columns can be considered as an attractive solution to construct such ductile soft-stories. To validate this concept, the multi-axis substructure testing (MAST) system at Swinburne University of Technology is used to conduct a set of 3D quasi-static cyclic tests on CFT columns with both circular (CCFT) and square (SCFT) cross-section shapes. A combination of bidirectional horizontal load protocol with vertical load variation is applied to the specimens. The data obtained from these experiments are used to study the physical damage progression and hysteresis response of CCFT and SCFT columns under complex 6-DOF boundary forces. The obtained experimental data are then used to calibrate FE numerical models to conduct performance-based fragility analysis of the soft-story buildings, at the system level, and vulnerability assessment of CFT columns, at the component level. The results show confidence in the superior seismic performance of CFT columns and highlight the advantages of using these elements in the construction of seismic resilience and sustainable buildings.