Behavior of concrete-filled double steel tube stub columns under partial compression

This paper reports the investigation on the behavior of stub columns having two nested steel tubes filled with concrete, abbreviated as CFDST, consisting of core concrete, inner and outer steel section and sandwich concrete. A total of twenty-seven stub columns were fabricated and tested. The experimental parameters included the wall thickness of the outer steel tube, strength grade of the concrete, ratio of the partial compressive area and the cross-sectional shape of the bearing plate. The typical failure modes, load-displacement relationships, and stressstrain relationships were studied. The results showed that outward bulging of the upper end of the outer steel tube occurred for the CFDST columns, and the degree and extent of bulging increased with the ratio of the partial compressive area. The bearing capacity of the columns loaded on the partial compressive area increased with the wall thickness of the outer steel tube and concrete strength. When the wall thickness of the outer steel tube increased, the partial bearing capacity increased by 25.3% on average. Reduction of the ratio of the partial compressive area improved the compressive bearing capacity. However, the shape of the bearing plate had a minor effect of the bearing capacity. Based on the experimental results, the FEM model was developed for analyzing the bearing capacity of the CFST and CFDST columns. The results of the analyses were found consistent with the experimental results.