AFRP	Aramid fiber-reinforced polymer
BFRP	Basalt fiber-reinforced polymer
CCFT	Confined concrete-filled steel tube
C-CFRP-SCTWC	Circular CFRP-SCTWC
CFRP	Carbon fiber-reinforced polymer
CFST	Concrete-filled steel tube
DSTC	Double-skin tube column
D	Outer diameter of column
FRP	Fiber-reinforced polymer
FRPCSCTWC	FRP ring-confined SCTWC
GFRP	Glass fiber-reinforced polymer
HPC	High-performance concrete
HPC-CFRP	High-performance concrete CFRP
NA	Natural aggregate
PET	Polyethylene terephthalate
RA	Recycled aggregate
RAC	Recycled aggregate concrete
RCBA	Recycled clay brick aggregate
SCTWC	Steel–concrete composite thin-walled column
SEI	Strength enhancement index
ECFRP	Elastic modulus of CFRP
Ef	Elastic modulus of FRP
e	Elongation of steel
f′c	Compressive strength of standard concrete cylinders
fCFRP	Tensile strength of CFRP
ff	Tensile strength of FRP
fy	Yield stress of steel tube
Nu	Axial load-carrying capacity
Nu,c	Calculated axial load-carrying capacity
Nu,e	Experimental axial load-carrying capacity
εf	Elongation after fracture
L	Length of column
tf	Thickness of FRP layers
ts	Thickness of steel tube