The increase in concrete strength (fck) brings linear growth of ultimate load, however, the brittle concrete of high strength makes ductility drop (Fig. 20(b)).
As expected, the increase of yielding strength of steel tubes (fck) brings linear growth of ultimate load and ductility (Fig. 20(c)).
Closer longitudinal and horizontal spacing of adjacent stiffener welds also promotes higher ultimate resistance, for the improved confinement for concrete core, thus providing more economical efficiency of materials.