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CONSIDERATION ON SHEAR STRENGTH OF R/C BEAM WITH BOND SPLITTING AND YIELD OF SHEAR REINFORCEMENT

  • NISHIMURA Koshiro
    Assoc. Prof., Faculty of Engineering, Hokkaido University, Dr.Eng.
  • ONISHI Naoki
    Assist. Prof., Faculty of Engineering, Hokkaido University, Dr.Eng.

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Other Title
  • 付着割裂とせん断補強筋の降伏を伴うRC梁のせん断強度に関する考察
  • フチャクカツレツ ト センダンホキョウキン ノ コウフク オ トモナウ RCリョウ ノ センダンキョウド ニ カンスル コウサツ

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Abstract

 The AIJ standard and guidelines require checking bond stress around reinforcing bars in R/C beam, which targets are not only cut-off bars but also bars placed through the span and anchored at the both ends of the beam. On the other hand, there are several studies on shear strength of R/C members fail in bond splitting, and shear strength formulas for the bond splitting failure were proposed. One is a practical method which origin is Ohno & Arakawa formula, which was based on experimental studies and proposed in 1960. However, application of the Ohno & Arakawa formula to high strength R/C member is not examined sufficiently. The others are based on shear resistance models those are superposition of arch and truss mechanisms and are solved by the theory of plasticity. The models can be divided into two types, which are called type A and B hereafter, according to assumption of concrete strut angle in the truss mechanism. These types are summarized in “Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings Based on Ultimate Strength Concept” published by AIJ in 1990. In the type A, the concrete strut angle is decided as the solution gives the largest value, and shear strength under a yields mode of bond splitting and yielding of shear reinforcement can be calculated. However, the bond splitting strength and shear strength are evaluated separately in the present design method. It is observed in some experiments that R/C beams fail in bond splitting with shear cracks and yielding of shear reinforcement. If it is verified by test results that considering the yield mode of bond splitting and yielding of shear reinforcement is significant for shear strength, it means necessity of reviewing the present design method.<br><br> In order to investigate significance to consider the yield modes of bond splitting and yielding of shear reinforcement when shear strength is calculated, calculations based on the types A and B models are examined by comparing with test results in the previous studies. The calculations are given by applying the bond splitting strengths to the shear resistance model that consist of arch and truss mechanisms. The bond splitting strengths are simplified by transforming into strengths of shear stress on bond splitting plane. Four yield modes can be represented by adopting the type A, which are states: shear reinforcement yields; bond splitting plane yields; the reinforcement and the splitting plane yield; and the both components don't yield. The type B does not include the state that both the reinforcement and the splitting plane yield. When the type A model is compared with the type B model, the type A model shows better agreement with the test results in the yield modes and the shear strength than the type B. As a result of the comparison, it can be said that effects of increasing shear reinforcement ratio or yield strength of shear reinforcement are different in the yield mode of the bond splitting and the yielding of shear reinforcement. In order to examine the Ohno & Arakawa formula on evaluation of shear reinforcement, calculation result of the formula is compared with the type A model, and these two calculations show good agreement. This result implies that influence of the bond can be evaluated by Ohno & Arakawa formula. Although the yield mode of bond splitting and yielding of shear reinforcement can be expressed by the type A model, there are some problems to evaluate shear strength accurately, for instance: combination of material strength of concrete and reinforcement; influence of cyclic loading and inner supplementary ties; dowel effect of longitudinal bars; and so on.

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