Xu and Liu 2013

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This paper develops an experiment for studying the effects of fire on concrete filled steel tube reinforced concrete columns (CFSTRC). These columns consist of an inner concrete filled steel tube, surrounded by outer reinforced concrete. This combination leads to have higher stiffness and fire resistance as compared to the conventional concrete filled steel tube.

Experimental Study, Results, and Discussion

Four CFSTRC columns were subject to ISO-834 standard fire and included two square and two circular cross sections. Two of the main parameters were cross section tye, and core area ratio which measures the ratio of the overall cross section to the area of the inner core. Loading was applied to the specimens before and during the test, and the size of the loading was maintained. The test was performed in the fire test Laboratory of Suzhou University of Science and Technology. The furnace, along with six K-type thermocouples which were placed along the specimens were used for this experiment. Two hydraulic jacks were used for the loading, which had capacity of 3000 kN, and were located at the top of the columns outside of the furnace chamber. Two displacement transducers were placed on the up-side of the column, and two more were placed at mid-height and measured lateral deflection, outside of the furnace. The failure criteria (specified in ISO-834) and state that if the total axial contraction reaches .01H (mm), or if the rate of contraction reaches .003H (mm/min), the specimen has failed. All four specimens failed due to local buckling, along with local concrete crushing. The spalling and buckling were more serious for square columns as they experiences a longer fire exposure time. Longitudinal cracking was observed at the compression side of the four specimens, whereas transverse cracking was observed in the tension side. The outer concrete was removed to view the inner core for failure of the inner core. Local buckling was observed of the reinforcement at the position of maximum transverse deflection, and no deformation was observed at the stirrups. The temperatures of the circular columns is higher than those of the square column at the same fire exposing time, as the core ratio is greater for circular columns. Overall, the fire resistance of the square cross section is much higher than that of the circular, due to load ratio, cross section area, and core area ratio.

Analytical Study

A finite element model was developed using ABAQUs to calculate the temperature field at different heating times, which was verified using experimental data. The thermal properties of steel and concrete were follow the proposal of Lie (1994). The effects of water vapor on concrete thermal properties was taken into account. Heat was transferred from the environment to the outer surface through radiation and convection. The temperatures predicted from the model are compared with the experimental data, and show good agreement.

References

Xu, L., Liu, Y.-B..(2013). “Concrete Filled Steel Tube Reinforced Concrete (CFSTRC) Columns Subjected to ISO-834 Standard Fire: Experiment.” Advances in Structural Engineering, 16 (7), pp. 1263-1282doi: 10.1260/1369-4332.16.7.1263