Experimental investigation of size and cross-sectional shape effects on strain distributions in concrete-filled steel tubes

Abstract

This paper presents a series of experiments on concrete-filled steel tubes (CFST) with different cross-sectional geometries, instrumented by distributed fibre optic strain sensors (DFOS) to reveal features of strain developments in concrete and steel at various load levels. Fifteen specimens of CFST with circular, octagonal, and square cross-sections with various steel ratios (7.5 % to 4.1 %) were subjected to axial compression, while the strain distributions along the axial and hoop (circumferential) directions were measured, both on the surface of the steel tube and inside the passively-confined concrete. The results reveal that the axial stress, peak axial strain, ductility index and strength index decrease with the cross-sectional size, and also as the shape of CFST changes from circular to octagonal and then to square, primarily due to their differences in confinement effects. The relationships between performance indices and confinement are supported by new data through DFOS, showing the variations in hoop strain patterns across the cross-sections for different shapes. The use of DFOS offers new insights into confinement effects on CFSTs, which had not been possible with traditional point-wise instrumentation. Such new insights can be valuable to future developments of theoretical and design models.