Shear Strength of Reinforced Concrete Column Subjected to Varying Axial Load due to Vertical Motion

1995年神户地震中，钢筋混凝土框架内、外柱承受的轴向荷载由压向拉的变化较为复杂，竖向加速度大于预期值，对框架内、外柱抗剪能力产生不利影响。然后进行了4根250mm × 250mm方形截面柱试件的抗剪强度和变形试验。抗剪配筋率为0.39%。剪跨比为1.0。混凝土抗压强度范围为275至298 kgf/cm^2。试验中柱的轴向载荷和柱的轴向载荷与侧向载荷之间的加载路径发生了变化；恒定轴压对应于混凝土抗压强度比0.49，恒定轴拉对应于混凝土抗压强度比0.29，施加侧向荷载后变化轴向荷载（称为单独荷载）和同时施加侧向荷载时变化轴向荷载（称为同时荷载）。结论如下：(1)变轴向荷载作用下，R/C柱抗剪强度分别比恒轴向荷载作用下降低0.69倍和0.88倍。Arakawa公式和AIJ规定的R/C建筑极限强度设计，不能考虑受拉柱轴向载荷，过高估计了受拉轴向载荷变化下的抗剪强度。(2)分别加载和同时加载两种加载路径下，不同轴向载荷作用下的抗剪强度基本相同。同时加载下轴向拉伸荷载下的抗剪强度是单独加载下的0.83倍。(3)无论柱轴向荷载如何变化，桁架机构所承受的剪力几乎不变，而拱机构的抗剪贡献随柱轴向荷载的变化而急剧变化。(4)采用实测柱纵筋粘结应力和实测抗剪配筋受拉力计算的假定桁架机制下均匀形成的混凝土对角受压支板的倾斜度与受拉柱轴向荷载作用下主压应变倾斜度相似。因此，压应力不能通过斜剪裂缝传递。这是钢筋混凝土柱在受拉轴向荷载作用下拱机制难以形成的原因。少

Exterior and interior columns in reinforced concrete (R/C) frames may be subjected to axial load varying complicatedly from compression to tension in 1995 Kobe Earthquake, resulting in ill influence on shear resistant capacity of these columns, since the vertical acceleration during the earthquake was larger than that as supposed. Then four column specimens with square cross section of 250mm x 250mm were tested to study the shear strength and deformation. Shear reinforcement ratio was 0.39%. Shear span ratio was 1.0. Concrete compressive strength ranged from 275 to 298 kgf/cm^2. Column axial load and loading path between the axial and lateral load to columns were varied in the tests ; constant axial compression corresponding to concrete compressive strength ratio of 0.49, constant axial tension corresponding to concrete compressive strength ratio of 0.29, varying axial load after applying the lateral load (called separate loading) and varying axial load with the lateral load applied si … More multaneously (called simultaneous loading). Conclusions were drawn as follows.(1) Shear strength of R/C columns under varying axial load was reduced up to 0.69 times and 0.88 times that under constant axial load in tension and compression, respectively. Arakawa formula and AIJ provision for ultimate strength design of R/C buildings, which can not account for tensile column axial load, overestimated shear strength under varying axial load in tension.(2) Shear strength subjected to varying axial load in compression was almost same in two loading paths, i.e., separate loading and simultaneous loading. Shear strength in tensile axial load under the simultaneous loading, however, was 0.83 times that under the separate loading.(3) Shear force carried by truss mechanism was almost constant regardless of varying column axial load whereas contribution of arch mechanism for resisting shear changed drastically with the variation of column axial load.(4) The inclination of diagonal compression struts in concrete assumed to be formed uniformly in truss mechanism, which was computed using measured bond stresses along column longitudinal bars and measured tensile forces in shear reinforcement, was similar to the inclination of principal compressive strain under tensile column axial load. Therefore compressive stress could not be transferred across diagonal shear cracks. this is the reason why arch mechanism was difficult to develop in R/C columns under tensile axial load. Less