Experimental study of concrete-filled steel tubular columns with inner stiffening parts exposed to fire

The mechanical properties and fire resistance of concrete-filled steel tubular columns with inner stiffening parts can be improved with longitudinal reinforcement and steel tube. With three main parameters of load ratio, cover thickness and steel ratio, ten slender concrete-filled steel tubular columns with stiffening part which has three section forms with longitudinal reinforcement, inner and outer steel tube with the tubes filled with or without concrete were designed, and axial compression experiment exposed to standard fire was carried out, by which failure characteristics, section temperature-time and axial deformation-time curves of specimens were analyzed. The critical or limiting temperature and the fire resistance of the columns were given. The impact of various parameters on the axial deformation and each sectional point of temperature was researched. The results indicate that comparing with the common concrete-filled steel tubular column, the concrete-filled steel tubular column with inner stiffening parts has different temperature field, stronger limiting temperature and better fire resistance. When columns reach equal loading ratio, the low temperatures of the internal stiffening parts encased by concrete can result in the increase in fire resistance. Loading ratio has a great influence on fire resistance, while the influence of cover thickness and steel ratio affecting is limited.