Size effect behavior of square concrete-filled steel tubular short columns under axial compression

Fourteen square concrete-filled steel tubular (CFST) specimens were fabricated and tested to investigate the size effect behavior under axial compression. The sectional widths of the specimens ranged from 156 mm to 1040 mm, encompassing both laboratory and practical engineering scales. The failure modes, stress state at peak load, and the vital mechanical characteristics, like composite elastic modulus, peak axial stress and strain, and the ductility coefficient, were analyzed to highlight the influences of both sectional width and steel ratio of square CFST columns. Compared to plain concrete, the size effect of square CFST columns in peak axial stress is weakened thanks to the contribution of steel tubes. Moreover, the peak axial strain, the strain at a residual bearing capacity of 0.85N_u, and the ductility coefficient also present a decreasing trend as the sectional width increases. Finally, a formula for predicting the bearing capacity is proposed by analyzing the axial stresses of the steel tube and concrete core, which is validated by the test data. The prediction accuracy is also compared with that from the current codes of ACI, EC4, and GB50936.