Although the depth-to-thickness ratio (b/t=57) of specimen S5 is beyond the limits in Euro code 4 in which the effects of local buckling can be neglected, specimen S5 buckled at 2500 kN after its tube reached yielding resistance 2211 kN in the experiment.
Compared to specimen S5, no buckling was developed in specimens S6, S7, S8, S9, and S10, which failed by material failure beyond peak resistance, rather than local buckling failure in elastic and plastic stage.
It can be indicated that, without thickening the tubes, the stiffeners further upgrade classification of composite
sections to Class 2 grade, in which the cross sections can develop their limited plastic resistances and deformation because of local buckling.