Mechanics Behavior Analysis And Application Of CFST Frame Columns After Fire

Concrete-filled steel tube (CFST) members are applied in practical engineering more and more widely due to their excellent fire resistance and earthquake resistance properties. At present, mechanical behaviors of CFST at ambient temperature have been studied widely by Researchers both civil and abroad, but the researches about earthquake resistance properties of CSFT after fire are comparatively less, consequently, There are no sufficient academic backgrounds for strengthening and retrofitting of CSFT structures. With the widely use of CFST in high-rise buildings, there are practical engineering backgrounds to investigate CFST' s mechanical behaviors after fire.Mechanical behaviors of square and rectangular CFST columns are investigated in this paper. The main investigations are summarized as follows:Part 1. Finite elements method is adopted to calculate temperature field of square and rectangular CFST sections, which is subjected symmetrical fire. Calculating program is developed. Influence of section size, steel ratio and protection thickness to square and rectangular CFST' s temperature field are analyzed. Temperatures in the center of square and rectangular CFST are compared, whose areas are equal. The following conclusions are obtained: With the section size increasing, temperature fields decrease. Influences of steel ratio to temperature field are little. If the areas of square CFST is fixed on, the thickness of steel has no influence to section temperature. With the protection thickness increasing, The temperatures in section decrease rapidly. In the case of equal section areas, section areas increase, temperature in the center decrease. Temperature in the center of rectangular CFST is higher than that of square CFST. That is because short sides of rectangular CFST transmit heat more quickly. Differences of temperature in the center between square and rectangular CFST are greater with section areas of CFST increasing.Part 2. Stress-strain relations of steel and concrete after high temperature are chosen. The complete moment-curvature curves of CFST' s members are calculated by numerical method. Mathematical expressions of ultimate capacity of rectangular at normal temperature are obtained. Mathematical expressions of Infection parameter of ultimate capacity of square and rectangular CFST after fire are given. Simplified moment-curvature curve and stiffness of square CFST is obtained. The conclusions are follows: in the case of equal areas, ultimate capacity of rectangular increase with the ratio of length and wideness. Infection parameter of ultimate capacity is related to section size, time of fire and protection thickness.Part 3 Time history method is used to calculate horizontal earthquake effect of frame. Mechanics model is story shear-form model. Newmark' s integral method is used to solute dynamic equation, and Fortran program is offered. Simplified formula of P桝 framework curve of CFST is provided and make certain main parameters(elastic stiffness, yield load, yield displacement). Bilinear(Double linearity) hysteretic model is adopted. Elasto-plastic earthquake responses of three frames (8-story, 12-story, 16-story) are computed. Theconclusions are follows: The results calculated by simplified formula of P桝 curve is well accorded with numerical results. All the three designed frames satisfied with seismic resistance regulations. Analysis from earthquake responses of three frames, we can draw conclusions: at ambient temperature, weak-stories of structures with even mass and even stiffness locate in bottom two stories. After fire, weak stories usually locate in fire stories. So story of easy catching fire must strengthen design to resist fire. We can judge weak-stories from time history curve or extreme response figures.The achievements in this paper can be used as the basis of seismic design of square CFST frames after fire, and may provide theoretical basis for CSFT frames' strengthening and retrofitting after fire. The results establish basis for further studying earthquake res...