Related research reveals that, in non-stiffened square CFSTs (S1 and S5), cross-sectional constraint region (shaded area in Fig. 16) distributes near the corners of composite tube and the center of concrete, and carries out transition to the non-constraint regions (blank areas) adjacent to the steel plate center (in Fig. 16(a)).
For these stiffened specimens, the stiffeners restrain steel tube's local buckling at welds, thus the number and location of the welds influence the constraint region.
The boundaries between non-constraint regions and constraint region are described by second order parabola curves.
Fig. 16(b)–(d) displays the constraint region (shaded area) and non-constraint regions (blank areas) in stiffened
cross section of specimens. Fig. 17 shows longitudinal distribution of concrete constraint region (shaded area) and the non-constraint regions (blank areas).
Sections 1–1 is the stiffened cross section in which stiffeners are welded with the steel tube; Sections 2–2 is the weakest cross section between two adjacent stiffened cross sections.
Detailed calculation method can refer to author's previous research [1].