As the second part of an integrated theoretical and experimental study of earthquake-resisting performance of the reinforced concrete columns confined by square steel tube, this paper presents experimental results conducted over twelve reinforced concrete columns confined by square steel tubes having five kinds of wall thickness. The columns were tested under reversed cyclic lateral force while subjected to constant axial load to study the effect of wall thickness of the square steel tubes on the seismic performance of the columns. It is shown that the effect of wall thickness on the earthquake-resisting capacity of the confined columns depends on the applied axial load level; the higher the axial load was, the more significant the effect of wall thickness. Experimental results also indicate that strengthening the square steel tube with inner stiffeners in the potential failure regions of the column could enhance confinement effectiveness of the steel tube significantly. Even under high axial load with axial load ratio n=0.67, the column confined by strengthened square steel tube by inner stiffener with B/t ratio of 118 exhibited very stable response in a manner similar to the columns confined by much thicker square steel tube with B/t ratio of 43.
