Notes: 1.
α1,
a2,
b1,
b2—the width of each side, as shown in Fig. (
2). 2.
t—the thickness of steel tube. 3.
fys,
fck—the yield strength of steel and the characteristic compressive strength of concrete prism, calculated by
fck =0.76
fcu,k. 4.
Nb,
εb—the load and corresponding average longitudinal strain when the local buckling occurred, it means the local buckling appeared at the post-peak stage when they are marked by “()”, and when local buckling was not found, they are marked by “—”. 5.
Nu,
εu—the experimental maximum load and corresponding average longitudinal strain. 6.
ε85—the average longitudinal strain when the load falls to 85% of the maximum load. 7.
Nun—the nominal strength of the specimens,
Nun=
fckAc+
fysAs, where
Ac is the area of concrete,
As is the area of steel tube. 8.
DI— ductility index, defined as
DI=0.75×(
ε85/
ε75) [
18], where
ε75 is the average longitudinal strain when the load reaches 75% of the maximum load in the pre-peak stage.