This paper studied the mechanical behavior of hybrid SCFC sections
under axial compression, also providing comparisons with
CFST and CFFT sections. The failure mode, axial load–displacement
relationships and axial load–strain responses were determined.
Additionally, the effects of key design parameters on major
mechanical characteristics, namely initial compressive stiffness,
peak strain, residual load-bearing capacity, and ductility, were
evaluated. The following conclusions are drawn:
(1) The SCFC section as a combination of three constituent
materials was demonstrated to be effective in improving
mechanical performance in axial compression, when compared
with most existing hybrid sections. The inner FRP tube
layer wrapped closely in the hoop direction provided effective
confinement to the core concrete, while the outer steel
tube layer provided a good ductile response with a postpeak
stable residual load-bearing capacity. The FRP tube
was circular in design, to alleviate concentration stress on
the corners of the outer square steel tube; the outer steel
tube was square in design, to provide convenience for
beam-to-column connections.
(2) Overall good agreement of initial stiffness between the
experimental results and calculated values was found. It
appeared that different components in the SCFC specimens
were in a condition of strain equilibrium during the initial
linear stage. At this stage, only minor interaction mechanisms
were activated among the different components.
(3) In the yielding process of the SCFC specimens’ load–strain
curve, several major responses occurred, namely the yielding
of steel, the volumetric change of concrete reversing
from compression to dilation, and the activation of confinement
by the FRP tube. Compared with the yielding processes
of the CFST and CFFT load–strain curves, the yielding process
of SCFC indicated enhancement of yielding strain and loadbearing
capacity. To evaluate the ductility, a method to
determine yielding point was proposed.
(4) From the hardening stage, the load enhancement was found
to increase with the confinement ratio, with the ratio of peak
load to yielding load ranging from 1.0 to 2.33. Additionally,
the effects of steel wall thickness and the confinement ratio
of FCCC on load enhancement in the hardening stage were
investigated. Results showed that 6–27% load enhancement
was effected due to the effects of steel wall thickness, further
load enhancement being due mainly to the effects of the
confinement ratio of FCCC, which was a governing factor in
effecting strength enhancement in the hardening stage.
(5) Peak strain was an important property in characterizing the
ductile response of SCFC sections. The peak strain depended
primarily on the confinement ratio of FCCC, and the strain
enhancement ratio increased with the decrease of steel wall
thickness.
(6) The SCFC sections showed excellent residual load-bearing
capacity, corresponding to 60–88% of the peak load, or 61–
175% of the yielding load. This property is beneficial to aseismic
design. It was also demonstrated that SCFC sections
improved the residual load-bearing capacity in comparison
to CFST sections, through the introduction of FRP within
the section.
(7) The SCFC sections showed excellent ductility. The ductility
ratio quantified by peak strain and yielding strain ranged
from 1.0 to 19.4 for the tested SCFC sections, and ranged
from 16 to 44 if such values were quantified by the ratio
between ultimate strain and yielding strain.