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This paper presents results from numerical studies on the effect of critical factors governing the axial compression mechanical properties on square cross-section concrete-encased concrete filled steel tubular (CFST) stub columns after fire exposure. A validated three dimensional finite element model is applied for evaluating axial compression mechanical properties of concrete-encased CFST stub columns after fire exposure under different factors. The factors varied in the parametric study include fire exposure time, steel tube area ratio, longitudinal bar ratio, compressive strength of concrete, yield strength of steel tube, and sectional core area ratio. Results from parametric studies show that axial compressive strength and axial stiffness of square concrete-encased CFST stub columns after fire decrease significantly. The decline of axial stiffness is higher than that of axial compression ultimate bearing capacity. Fire exposure time and sectional core area ratio have significant influence on the reduced level of axial compressive strength and axial stiffness of square concrete-encased CFST stub columns after fire exposure. Steel tube area ratio and reinforcement ratio have some influence on the reduced level. Compressive strength of concrete and yield strength of steel tube have limited influence on the reduced level.

The temperature distribution of concrete filled steel tube reinforced concrete (CFSTRC) square short columns was analyzed. Test results of four CFSTRC square short columns were reported in this paper. The specimens exposed to the ISO-834 standard fire without initial loads. The fire test lasted 180 minutes. The influential factors included the side length of cross-section, slenderness ratio and steel tube area ratio. Finite element method (FEM) simulation was used to calculate temperature distribution of CFSTRC square short columns at elevated temperature. The test results show that the larger side length of cross-section, the lower temperature inside the specimens with the same fire exposure time and steel tube area ratio and height of specimens. Under the same fire exposure time and side length of cross- section and steel tube area ratio, the slenderness ratio was larger, and the temperature inside the specimens was lower. With the same fire exposure time and side length of cross-section and slenderness ratio, the steel tube area ratio had slight influence on temperature distribution inside the specimens. The FEM simulation results agreed well with the experimental results. The FEM simulation was capable of calculating temperature distribution of CFSTRC square short columns with an accuracy that was sufficient for design purposes.

In order to research the calculating method of flexural strength of fire-damaged reinforced concrete (RC) beams with carbon fiber reinforced polymer (CFRP) sheets strengthening, the flexural strength of fire-damaged beam was analyzed with section analysis. The fire exposed region includes tensile region and compressive region. A procedure was developed for quantifying the amount of bending moment of fire-damaged RC beams section with an external strengthening CFRP sheets. An equation of CFRP sheets of effective strain was recommended. The proposed method was verified under the experimental results as well. The results show that the proposed approach is capable of predicting bending moment of fire- damaged RC beams with an accuracy that is sufficient for design purposes.