The concrete in the stiffened tubes (such as S9 in Fig. 9)was damaged in local concrete crushed mode, i.e. no obvious diagonal cracks but local concrete trivial cracks and crush, which means the compressive strength of concrete was exploited.
The crushed area and crushed depth were more than those of the non-stiffened specimen.
The crushed concrete was located between two adjacent stiffener welds, while concrete near welds showed little
damage. The interaction between concrete and steel tube can also be detected by tapping the buckling locations of the steel tube after test, for the tapping sound revealed that there were almost no separation between concrete and steel tube.
The stiffeners enhance the constraint effect for concrete provided by steel tube and improve the concrete from a brittle failure mode to a more ductile failure mode.