EXPERIMENTAL STUDY ON FIRE RESISTANCE OF CFT COLUMNS WITH FIRE PROTECTION AT THE STANDARD FIRE (PART 1): BEHAVIOR AND LOAD SUPPORT ABILITY OF CFT COLUMNS WITH FIRE PROTECTION USING ORDINARY AND HIGH STRENGTH MATERIAL

CFT columns are used in middle and high-rise buildings as members with excellent structural performance and fire resistance. CFT columns without fire protection have been used in fire resistance design based on many experimental studies, but have various limitations, so many CFT columns have coated with fire protection. However, as a cause CFT column with fire protection was recognized as a fire resistance structure with temperature limitations of steel tube, there were few experimental studies on the behavior and the load support ability at the fire. In this study, the ratio of the compressive strength of steel tube to the compressive strength of filled concrete at room temperature “axial strength ratio” is proposed as an index to show axial strength in a high temperature of CFT columns with fire protection. Therefore, by taking the “axial strength ratio” as the main experimental variable, centrical load bearing fire tests were performed by applying a standard fire and an axial force equivalent to the long-sustained strength to CFT columns with fire protection using ordinary and high strength materials. From this experimental study, the following was found. ・During the fire, steel tube and filled concrete behave while sharing the axial force, and behavior in the fire of CFT columns is classified by “axial strength ratio”. ・It is assumed that the collapse time of CFT columns with fire protection was shortest at “axial strength ratio” for each fire protection thickness. ・Similarly, average surface temperature of the steel tube at the time of collapse also showed the lowest value when “axial strength ratio” is about 1.0 to 1.2. ・Thus, it was confirmed that “axial strength ratio” is an effective index to axial strength in a high temperature of CFT column with fire protection using ordinary and high strength materials. ・It is considered that these are due to the influence of the temperature distribution in the horizontal section of steel tube and filled concrete. ・The surface temperature of the steel tube at the time of collapse of CFT column with fire protection was 430 to 600°C.