As per the previous research, the concrete in square CFST is divided into a constraint region and non-constraint regions.
The non-constraint concrete is described with uni-axial stress–strain relation curve of plain concrete referred to GB50010-2002 or author's previous published paper [1].
The constraint concrete employs the modified Mander concrete model (in Fig. 15)proposed by Mander, Priestley and Park in 1988 [16].
The favorable influence of transverse confinement on the axial stress–strain relation is taken into consideration in the constraint concrete.