Analysis Of Fire Performance For Reinforced Concrete Filled Steel Columns (RCFS)

Reinforced concrete filled steel columns are concrete filled steel columns which have steels in the core concrete with high fire-resistance, They could reach the fire-resisting limit without any other measure of fireproof. These columns not only have superior performance like the normal concrete filled steel columns, such as higher bearing capacity, better plasticity and ductility, superior earthquake resistant capability etc, but also have better fire-resisting performance. Therefore the reinforced concrete filled steel columns are widely used overseas. In this paper, the performance of reinforced concrete filled steel columns when exposed to fire was analyzed using the combination of theoretical analysis and nonlinear finite element simulation. The obtained results can be used as basis for the fire-resistance design process of reinforced concrete filled steel columns. Several conclusions can be obtained from the results of this paper:At first, the bearing capability of concrete filled steel tube columns subjected to axial compressive loading was studied based on the Twin Shear Unified Strength Theory. The calculation formula was derived and the effect of the intermediate principal stress, the confinement effect between the strain-constrained concrete cylinders and the steel tube, was considered sufficiently. Good agreement can be observed from the comparison between the theoretical results and the experimental data in the literature. The values of the factors for the lateral pressure and for strength of material were discussed and verified. This formula was generalized to reinforced concrete filled steel tube columns. It is applicable for both the square and the circular reinforced concrete filled steel tube columns.Second, the formula of Rankine was introduced, and the formula of the bearing capacity of the reinforced concrete filled steel tube columns at normal temperature was used in the formula of Rankine. Then, the calculation formula of fire resistance of steel columns and reinforced concrete filled steel tube columns was derived. In this formula, the fire resistance was expressed by the capacity of plastic failure and elastic buckling of columns at normal temperature. An example of a square reinforced concrete filled steel tube column was illustrated. Comparison between the theoretical results and the experimental data in the literature was conducted and it can be concluded that the formula of Rankine could provide a true and safe answer.Third, the fire performance of reinforced concrete filled steel tube columns were analyzed in term of instantaneous thermal analysis of ANSYS 8.0. On the base of the thermal performance parameters and the constitutive relationship of the material, the finite-element models were set up using 3D thermal conduction element. Then, the temperature field and the thermal-structure coupling analysis were simulated. The temperature field could express that planar precarious temperature field which has the first boundary condition and does not have heat fountainhead in the columns. The distributing of the temperature field at different time was derived. It shows that the equal thermal line of section fringe is close to the figure, the isotherm of the section inside is approximately circular along with the temperature. Finally, the thermal stress was calculated using the finite element model. In this process, the thermal elements are turned to be structure elements accordingly. The proper material attributes were selected and the result of thermal analysis was loaded as body load on the model.The fire resistance and the temperature field of the reinforced concrete filled steel tube columns when exposed to fire were investigated based on theoretical analysis and numerical simulation. The results are useful for the further development of the concrete filled steel tube columns.