本研究之目的在於將高溫引起之材料性質劣化納入軟化桁架模型(RA-STM)與可變傾角桁架模型(variable-angle truss model)之中，據此評估鋼筋混凝土梁在火害後的容許剪力與撓曲強度之衰減。
在常溫下，對於a/d≧2.5的梁，兩種桁架模式都能合理預測梁之剪力強度，可變傾角桁架模型假設混凝土斜壓桿之角度α需介於30°至45°之間，分析結果與試驗值之比較皆屬合理，但以接近30°為佳。對於2.0≦a/d≦2.5的梁，軟化桁架模型預測之剪力強度會有稍為高估的現象，以可變傾角桁架模型分析時，傾斜角接近30°時結果會較準確。在溫度400℃以內的混凝土抗壓強度所對應之應變ε0增加趨勢並不明顯，經分析及驗證結果，將兩種桁架模型延伸至梁火害後剪力與撓曲破壞強度預測，以前人試驗之11支試體印證分析，皆有不錯的結果。本研究並模擬實尺寸鋼筋混凝土梁柱複合構件，針對梁的部分進行火害後強度分析。在加載外力維持不變的情況下，RA-STM分析結果顯示負彎矩區之撓曲與剪力強度衰減較正彎矩區為嚴重，若於火害後進行破壞試驗，RA-STM與可變傾角桁架模型兩法預測之破壞值相當接近。

The purpose of this study is to incorporate the residual mechanical properties of fire damaged concrete and steel into the rotating-angle softened truss model (RA-STM) and variable-angle truss model for the prediction of shear and flexural strengths of reinforced concrete beams after fire exposure.
Both models can reasonably predict the shear strengths for beams with span-to-depth ratio a/d≧2.5 under room temperature, within the orientations of diagonal strut between 30 degree and 45 degree. The RA-STM slightly over estimated shear strengths of beams with 2.0≦a/d≦2.5. The variable-angle truss model, assuming the inclined angle of diagonal strut of 30 deg, gives a good agreement with the test results. The compressive strain ε0 corresponding to peak strength of concrete subjected to fire test less than 400℃ did not increase significantly. Applying these two models to predict the shear and bending strength of fire damaged concrete beams, good agreements were found between the analytical and experimental results.
The full-scale reinforced concrete beam-column assemblage specimens were employed to assess the strengths after fire damage. Analytical results of RA-STM theory show that specimen under the constant applied load exhibits greater shear and bending strength deterioration in the hogging region than in the sagging region. After fire testing, the strengths predicted by the RA-STM and the variable-angle truss model are very close.

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