抄録
Although tensile reinforcement is placed in three layers in deep R/C beams like foundation beams due to requirement of large stress, the AIJ standard1) does not provide how bond around deformed bars in third layer should be verified. In these days, loading tests of R/C beams with double layers of deformed bars were conducted. When shear cracks occurred at end regions of the beam subjected to anti-symmetric bending, bond stress became quite small in the cracked area. This behavior is called tension shift. However, when bars in the second layer were terminated in the span, the tension shift in the cut-off bars did not appear clearly. Meanwhile bond stress around the extreme layer was small in the range where the cut-off bars exist, and was large in other range where no bars remain in the second layer. While the experimental studies on double layered R/C beams have progressed, experimental tests on triple layered beams are limited. In the AIJ standard, the effective bond range must be reduced by subtracting the tension shift range from the embedment length of the bar. And the tension shift range is defined as the same length as effective depth of the beam. That tension shift range is not reasonable for cut-off bars in the deep beams. As mentioned above, triple layered arrangements are often chosen for the deep beams.
In this study, anti-symmetric bending tests and FE analyses of R/C deep beams with triple layered arrangements were carried out in order to make clear the influence on bond behavior by terminating the bars in the span. Especially, tension shift in the cut-off bars around the beam end was focused on. Two foundation beam specimens were prepared. The tensile reinforcement was placed in three layers in the both beams. High-strength shear reinforcement and normal strength concrete were used for the specimens. The parameter of test was whether the bars in the third layer were terminated in the span or not. In discussion on the test and numerical results, where a previous test result7) was added, an equilibrium model5)6) between bond stress and shear reinforcement stress was used. In this model, it is assumed that moment of the tensile stress in shear reinforcement takes balance with moment of the bond stress around longitudinal bars in the range from the beam end to a distance of beam effective height, which is named the beam end region in this paper. Consequently, the following conclusions were obtained.
(1) Bond stress around all longitudinal bars in the beam end region decreased once due to cracks. And the bond stress increased again as tensile stress in shear reinforcement increased. In addition, ratio of bond stress in the third layer to the bond stress of all the bars increased by terminating the bars in the third layer.
(2) In the beam end region, after shear cracking, moment of the bond stress around cut off bars in third layer increased as moment of the tensile stress in shear reinforcement increased until yielding of longitudinal bars.
(3) When bond stress was calculated in a state that the longitudinal bars almost yielded, tension shift range of d/2, which is a half length of effective beam depth, gave better accuracy than the tension shift range of d, which is required in the AIJ standard.
