Han, Chen, Liao, Tao, and Uy 2013

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Experimental Study, Results, and Discussion

Five full-scale concrete filled stainless steel tubular columns under axial compression were tested under standard fire conditions. Stainless steel was chosen for its ductility, higher post-yielding strength, and good corrosion resistance. Self consolidating concrete was used for the concrete columns, and three to four thermocouples were placed along the specimens before casting the concrete. The columns were tested in a column furnace at the Tianjin Fire Research Institute in China, and the furnace has dimensions of 3 m x 3 m, with a maximum height of 4 m. A servo-controlled hydraulic jack with load capacity of 1500 tonnes was used to apply the axial load, and a force transducer was used to control the magnitude of the constant applied load. A displacement transducer was utilized to measure the displacement of the jack. Spherical hinges were used in order to model pinned-pinned end conditions. The specimens all failed in compression, and the maximum fire resistance was greater than 240 minutes (for one specimen, the remaining four specimens failed before 240 minutes). For the specimens with square cross sections, local buckling was observed along the entire length, as well as weld fracture. For the specimens with circular cross sections, local buckling was also observed, however there was less bulging than that of the square cross sections. Once the outer steel tubes were removed, concrete spalling was observed at the corners of the concrete, and longitudinal cracks were also observed. At areas where the weld fractured, the concrete was crushed as it was exposed directly to the fire. The circular specimens also exhibited cracking, however there was no damage in some areas, thus the overall integrity of the concrete in the circular specimens was better than that of the square specimens. The larger the load level, the greater the local buckling and weld fracture in the steel tubes, and a more significant crushing of concrete was observed. In both the circular and square specimens, a larger load level caused a decrease in fire resistance time.

Analytical Study

A finite element model was developed using ABAQUS, and a thermal-stress analysis procedure was used for the heat transfer analysis and structural analysis. For the thermal analysis, heat is transferred to the outside of the stainless steel tube using convection and radiation, and into the inner concrete tube with conduction. The specific heat of concrete was increased in order to account for the moisture content based on the water evaporation in the concrete. For the structural analysis portion, the axial load was applied first at ambient temperature, and then the temperature is applied based on the thermal response analysis. The load is kept constant while the temperatures are applied until failure of the column. Axial deformation was categorized into three stages: expansion, gradual contraction, and sharp contraction. During fire exposure, the temperature of the outer tube rises quickly and expands (stage I), quicker than the concrete core. The load ratio on the outer tube decreases, whereas the overall specimen continues expanding slightly. During stages II and III, the load carried by the outer tube decreases until the fire resistance time is reached, and the concrete core takes most of the load. The FE model was further used to study the difference between stainless steel and carbon steel columns. The fire resistance increased from 48 to 82 minutes when the stainless steel was used, and the strength and stiffness of the stainless steel reduce slower as compared to the carbon steel specimens.

References

Han, L.-H., Chen, F., Liao, F.-Y., Tao, Z., and Uy, B. (2013). “Fire performance of concrete filled stainless steel tubular columns.” Engineering Structures, 56, November, pp. 165-181 doi:10.1016/j.engstruct.2013.05.005