Abstract：Based on the pseudo static tests of two reinforced concrete (RC) columns without fire, two RC columns after fire without strengthening and ten RC columns after fire and strengthened with thin-walled steel tubes, the effects of axial load ratio, shear span ratio, steel tube thickness and strengthening modes on the seismic performance of concrete columns strengthened with thin-walled steel tubes were investigated. A calculation formula for the shear bearing capacity of concrete columns after fire and strengthened with thin-walled steel tube was developed. The results show that the reinforcement by the thin-walled steel tube can significantly improve the shear capacity, the limit deformation capacity and the cumulative energy dissipation capacity of concrete column after fire, and the secant stiffness of the strengthened specimen is similar to that of the unfired specimen. The bearing capacity of concrete columns after fire and strengthened with thin-walled steel tube is high than, about 41.8%~47.0% and 38.5%~74.4%, that without strengthening when the shear span ratios are 1.78 and 3.0, respectively. The axial load ratio has a significant influence on the shear bearing capacity, the stiffness, the energy dissipation capacity and the ductility, but has a small influence on the limit deformation of the strengthened specimens. The thickness of the thin-walled steel tube influences the energy dissipation capacity more than the shear bearing capacity of the strengthened specimens. The reinforcement mode with welding angle and bolt anchorage at the root of thin-walled steel tube can improve the strengthening effect.
