With increases in the height and span of structures, improvement
of the load-bearing capacity and ductility of columns under
compression is a constant pursuit [1,2]. A number of studies have
investigated hybrid columns by combining different material components
in a single column section to enhance structural performance.
Typical hybrid columns include concrete-filled steel tube
columns (CFST) [3–5], concrete filled fiber-reinforced polymer
(FRP) tube columns (CFFT) [6–10], steel–concrete double skin
tubular columns [11,12], and hybrid FRP–concrete–steel doubleskin
tubular columns [13].
Since the 1960s, concrete-filled CFST columns have been presented
and their mechanical performance extensively studied
[14,15]. The advantages of large load-bearing capacity and high
ductility [16,17] have led to the wide use of CFST in engineering
practice [18,19]. Based on the CFST section, a steel–concrete double-
skin tube column (steel–concrete DSTC) has been proposed
and a series of studies conducted [20]. In such sections, the inner
circular steel tube is designed to provide confinement of the core
concrete, thereby achieving high load-bearing capacity and stiffness.
However, the axial stress–strain curve of such columns shows
softening behavior rather than a hardening behavior after sectional
yielding, as illustrated by the stress–strain relationship of CFFT section
columns [21–25]. The stress–strain relationship of CFFT columns
under axial compression presents bilinear ascending
behavior, leading to a load-bearing capacity superior to most previous
hybrid column sections [26,27]. In most such columns, the
FRP acts primarily as a jacket, confining the inner concrete in the
hoop direction to achieve a desired load-bearing capacity [28–32].
Experiments have been conducted to understand the mechanical
performance of CFFT sections and the confinement mechanism of
FRP tubes. It was found that although satisfactory lateral confinement
can be provided by the external FRP, the failure mode of CFFT
columns is often induced by fracture of the tube under a bi-axial
state of stress, and the failure process is therefore brittle and
explosive [10]. This may cause potential safety issues and such
construction should be avoided owing to the brittleness [33].
With further exploration of hybrid sections, hybrids of steel,
concrete and FRP have multiplied in recent years. In 2011, a novel
FRP-encased steel–concrete composite column was proposed by
Karimi et al. It utilizes a glass FRP composite tube that surrounds
a steel I-section, the tube being subsequently filled with concrete.
This resulted in enhanced compressive behavior under axial loading
[34]. A hybrid FRP–concrete–steel double-skin tubular column
(FRP–concrete–steel DSTC) [13] was also proposed, that consisted
of either circular or square FRP (as the outer skin) and a steel tube
(as the inner skin) as confinement layers, with concrete in
between. This proposal took advantage of both steel and FRP to
deliver good structural and ductile performance [13], in comparison
to most existing columns. However, the FRP–concrete–steel
DSTC cross-section may display relatively low bending stiffness
and difficulty in joint configuration of beam to column. Apart from
this, hybrid sections have been used in the strengthening field.
Feng et al. [35,36] proposed a strengthening approach to improve
the buckling resistance of steel members in compression through
FRP and steel composite sections, and enhanced load-bearing
capacity and ductility were obtained.
In this paper, a novel hybrid column section is proposed as a
composition of three constituent materials, namely steel, concrete,
and FRP. A FRP-confined concrete core (FCCC) is encased in CFST.
Its layout is composed of a square steel tube as the outer layer
and a circular filament-wound FRP tube as the inner layer, with
concrete filled between these two layers and inside the FRP tube
(SCFC in short, as shown in Fig. 1(a)). This section is expected to
optimally combine the merits of FRP, concrete, and steel and to
take advantage of the interaction mechanisms among these layers.
This construction can simultaneously achieve better load-bearing
capacity and ductility without losing its residual load-bearing
capacity. In such sections, the FRP tube, in which the fiber lies at
a small intersection angle to the hoop direction, is expected to provide
confinement to the core concrete to improve load-bearing
capacity; meanwhile the outer steel tube layer is designed to resist
dilation of FCCC to enhance ductility. Compared to steel inner tube,
FRP can provide sustained and higher confinement to concrete
under the triaxial stress state by virtue of its anisotropy and high
strength, and the property of FRP on corrosion prevention allows
the core concrete adopting environmental materials such as seasand
concrete [37]. Moreover, this section displays improved bending
stiffness and flexural bearing capacity, as well as the
convenience of beam-to-column joint configuration. To examine
the performance of the proposed SCFC column section, compressive
experiments are conducted on 18 stub column specimens,
with different concrete strength, FRP thickness and steel thickness.
The effects of such parameters on the main stages of axial load–
strain curve are investigated, notably the initial linear stage, the
secondary linear stage (hardening stage), the post-peak stage,
and the process of yielding. Further, the mechanical characteristics
of initial compressive stiffness, peak strain, residual load-bearing
capacity, and ductility are studied. Moreover, one CFST specimen
and three CFFT specimens are also examined for comparison