BEHAVIOR OF SLENDER RC COLUMNS PLACED INSIDE PLANE FRAME IN FIRE: Study on fire resistance of slender RC columns Part 4

Reinforced concrete columns formed with high-strength concrete have been developed for use as axial force carrying members. They have a reduced cross section compared with conventional “short” columns and are known as “slender” columns. These columns allow for larger building interior spaces and offer architectural space with good visibility. In fire, the slender RC columns have possibilities for horizontal displacement from thermal expansion of beams and one side heating in a frame. Further, buckling length differs depending on jointing method and stiffness of a frame. In addition, there are initial unavoidable deformation in manufacturing and eccentric axial load generated by an eccentric joint of beams. They would lower the fire resistance performance of slender RC columns. This study shows fire resistance of slender RC columns using concrete with compressive strength of approximately 120N/mm2 in a plane frame. From thermal stress analyses conducted for several frames with combination of horizontal displacement from thermal expansion of beams, decentering axial load, initial deformation, one side heating, and buckling length as parameters, the following are obtained. 1. Induced joint translation angle which is less than 1/31 rad. from thermal expansion of beams has less influence on fire resistance of slender RC columns. 2. Axial load whose eccentricity ratio e/D is less than 0.3 has less influence on fire resistance of slender RC columns. 3. Initial deformation α/h which is less than 1/103 has less influence on fire resistance of slender RC columns. 4. Slender RC columns subjected to one side heating fail later than ones subjected to all circumferences heating. 5. Buckling length has significant influence on fire resistance of slender RC columns. 6. In fire, slender RC columns fail from forming second or third plastic hinge continuously after one plastic hinge is formed and horizontal displacement increases. The results indicate that the main factor which decreases fire resistance of slender RC columns is buckling length. When slender RC columns in a frame are designed, it is important that the buckling length is adequately evaluated as to influence of a frame.