| Summary: | 博士 === 國立成功大學 === 建築學系碩博士班 === 94 === Concrete structures generally behave well in fires. Most fire-damaged concrete buildings can be repaired and reused even after severe fires. Certainly, they must be repaired to meet the seismic requirements specified by building code. When concrete is exposed to heat, chemical and physical reactions occur at elevated temperatures, such as loss of moisture, dehydration of cement paste and decomposition of aggregate. These changes will bring a breakdown in the structure of concrete, affecting its mechanical properties. Therefore, it is important to evaluate the residual strength and stiffness of RC members after fire events and to understand the effect of temperature on the mechanical properties of concrete, especially the stress-strain relationship used to predict the behavior in a future strong earthquake.
An experimental research is performed on the residual compressive stress-strain relationship for concrete after heating to temperatures of 100-800˚C. All concrete specimens are standard cylinders,Ø15cm×30cm, made with siliceous aggregate. From the results of 108 specimens heated without pre-load, the relationships of the mechanical properties with temperature are proposed to fit the test results, including the residual compressive strength, peak strain and elastic modulus. A single equation for the complete stress-strain curves of unheated and heated concrete is developed to consider the shape varying with temperature. Furthermore, a total of 54 specimens heated under pre-load are carried out to study the effect of stress level on the residual compressive stress-strain curves. For split-cylinder tests, a total of 54 specimens are tested to provide the splitting tensile strength for different temperatures.
For fire-damaged RC member analysis, the heat conduction equation is solved by using the finite difference method to calculate the maximum temperature distribution in the cross section exposed to a fire. Based on the assumption that plan section remains plan, and utilizing the residual stress-strain curves of concrete and steel after exposure to high temperature, the finite element method is introduced to calculate the sectional stress and strain distributions which satisfy the equilibrium and compatibility equations. To verify the accuracy, 12 full-size columns are constructed and subjected to uniaxial or biaxial bending after exposed to the CNS Standard fire for 0, 2, 4 hours. Not only the crack pattern due to heating but the decrease of flexural strength, stiffness and ductility after fires are investigated. Comparing with the experimental load-deflection curves, the analytical results show a good agreement.
For seismic evaluation of RC building, a 2-story pure framing RC structure of a shaking table test under biaxial motion, tested by Oliva, M. G., is analyzed. Furthermore, a numerical example for evaluating residual seismic resistant performance of a 5-story fired-damaged RC structure is presented in this paper.
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