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EVALUATION OF BOND STRESS OF CUT-OFF SECOND LAYER LONGITUDINAL BARS WITH ADDITIONAL STIRRUPS IN HIGH-STRENGTH REINFORCED CONCRETE BEAMS

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  • カットオフした2段目主筋先端に追加補強筋を配筋した高強度鉄筋コンクリート梁部材の付着応力度評価
  • カットオフ シタ 2ダンメ シュウスジ センタン ニ ツイカ ホキョウキン オ ハイキン シタ コウキョウド テッキン コンクリートリョウブザイ ノ フチャク オウリョクド ヒョウカ

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Abstract

<p> Aiming to increasing of workability and productivity of construction of high-rise reinforced concrete buildings, cut-off longitudinal bars in the second layer has been used. However, it is known that flexural cracks tend to open at termination point, and it leads to develop diagonal tension cracks. In addition, high-strength materials have been used for longitudinal and transverse reinforcements. In this study, to assess failure behavior, seismic performance of reinforced concrete beams having cut-off longitudinal bars with high-strength concrete and steels, test of beams which reinforcement ratio are 0.3−1.2% were carried out. Additional stirrups were installed to prevent crack around the head of the cut-off longitudinal bars.</p><p> For the test, four-teen 1/2.5-scaled test specimens of reinforced concrete beams were fabricated. The size of cross-section (250mm×400mm) is the same for all specimens as shown in Fig. 1(a). Five longitudinal bars were arranged in both the first and second layers, and the five longitudinal bars in the second layer is cut off. The main parameters are presence of cut-off longitudinal reinforcement, ratio of transverse reinforcement and location of additional stirrups. The parameters of beams are shown in Table1. The ratio of transverse reinforcements was arranged as follows: 0.3%, 0.6% and 1.2%. The additional stirrups were placed with two types of arrangement manner, which are placed in the head of cut-off longitudinal bars and in the region of middle of beams including in the head of cut-off longitudinal bar as shown in Fig. 1(c). Concrete compressive strengths of beams were from 54 to 77 N/mm2 . High-strength steel bars were used for longitudinal, transverse reinforcement and additional stirrups. The length of the beam was designed to 1600mm. The cut-off length of 680 mm corresponds to required length of cut-off longitudinal bars specified in AIJ standard.</p><p> Cyclic loading was applied with antisymmetric moment so that horizontal load is applied to the center of the beam as shown in Fig. 2. From the tests, following conclusions were obtained. 1) It is better to place the additional stirrups both in the head of cut-off longitudinal reinforcement and the region where longitudinal bar is placed in one layer to prevent undesirable cracks; 2) Smaller width of residual cracks of the beam with additional stirrups in the head of cut-off longitudinal bars was found. Shear capacity of the beam with additional stirrups was increased by arrangement of additional stirrups; 3) The maximum bond stress in the head of cut-off longitudinal bars was larger than bond strength calculated by AIJ standard; however, difference in maximum bond stress by arrangement of additional stirrups was not observed; 4) It is verified that maximum bond stress of cut-off longitudinal bars at the head region with additional stirrups can be predicted by present AIJ standard.</p>

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