Numerical modelling and analysis on axial loading behaviors of composite column-in-column system

Previous studies have showed that severe earthquake could cause damage to concrete filled steel tube (CFST) column. Although the energy dissipator can be used to improve the seismic performance of the CFST column, its enhancement is limited. Based on a new passive control strategy, the authors recently proposed a novel composite column-in-column (CIC) system to enhance the seismic performance of the CFST column. The CIC system was composed of an exterior column, an interior column, and a series of connecting springs and dashpots, and its load-bearing capacity and seismic mitigation effectiveness was experimentally and numerically demonstrated respectively. The test results revealed that under compression tilting might occur to the interior column when the interior column was not restrained by the springs and dashpots, which resulted in the underestimation of the ultimate axial loading capacity of the CIC system. This paper extends the previous experimental study by further conducting numerical investigation to clarify the restraint effect provided by the springs and dashpots on the axial loading behaviors of the CIC system. The influences of the length-to-diameter ratio, diameter-to-thickness ratio, concrete strength and steel strength on its axial loading capacity are comprehensively examined. Moreover, according to the codes EC4, AISC 360-16, GB 50936-2014 and T/CCES 7-2020, formulae for predicting the ultimate axial loading capacity of the system are developed and validated by the test and numerical results. Results show that, by employing the optimized springs and dashpots, the exterior and interior columns can work compatibly to avoid the tilting and global buckling in the CIC system, providing a good axial loading behavior with favorable ductility and post-peak performance. The prediction formulae are reliable for the CIC system design and analysis.