| Reinforced concrete structures have a reputation of good behavior in fires because of concrete having a low thermal conductivity. But when exposed to fire for a long time, catastrophic collapses of the reinforced concrete structures that were caused by the mechanical properties of concrete and steel degraded with increasing temperature were not so rare. So it is important to strengthen and improve the fire resistance design of reinforced concrete structures and enhance the fire safety of structures through the rational design. For this purpose, based on a lot of numerical analysis calculations and theoretical researches, the fire resistance design methods of all kinds of reinforced concrete members are established in this paper. The main contents and results in this paper include:(1) A set of finite element equations for nonlinear transient temperature field analysis of reinforced concrete members are established. In analysis, the temperature-dependent thermal properties of concrete are considered, and the iterative process of heat flow equilibrium is used for improving solution precision of temperature field. A computer program TFARC (Temperature Field Analysis of RC) written in MATLAB is developed. This program can be used to analyze steady state thermal problems, linear transient thermal problems and nonlinear transient thermal problems. At last, the comparison study between numerical results of the program with test data (or exact solutions )of all sores of temperature field problems shows that the program used for the temperature field analysis of reinforced concrete members is reliable.(2) Based on a lot of numerical analysis calculations by using the program TFARC, the conception of mean strength reduction factors of materials is introduced, the rules of material mechanical properties that vary with time are analyzed, and then the calculation formulas of mean strength reduction factors of concrete and steel under four-face heating condition are established.(3) The effects of concrete spalling and slenderness ration of member on fire resistance properties of axially loaded reinforced concrete members are analyzed, by using the formulas of mean strength reduction factors of materials, and imitating the design method for members at ambient temperature, the calculation formulas of the ultimate fire-resistant load-bearing force are proposed for axially loaded reinforced concrete members.(4) The analytic formula of additional eccentricity and the limit for judging large and small eccentricity failures are developed for eccentrically loaded reinforced concrete members at elevated temperature, and then the equilibrium equations for the failure cross-section are established by using the formulas of mean strength reduction factors of materials and additional eccentricity, these equations can be used for the fire resistance design of eccentrically loaded reinforced concrete members.(5) Based on a lot of numerical analysis calculations by using the program TFARC, the relations between strength reduction factors of steel bar located in different place and fire exposed time under three-face heating condition are established, and then the determination of yield strength of steel bar at elevated temperature need not depended on the temperature of steel bar. Based on the failure mechanism of reinforced concrete flexural members at elevated temperature, the calculation formulas of the ultimate load-bearing force for flexural members are proposed by using the foreside time-dependent strength reduction factors of steel bar.The distinct merits of the fire resistance design methods established in this paper consist that the fire-resistant load-bearing force of members is not directly depended on the analysis of temperature field of members, but only determined by the fire exposed time of members, so it is very convenient for the fire resistance design of reinforced concrete structures. Finally, the validity of these methods is verified by a lot of test data and some criterion methods.
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