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

1995年科比地震中，钢筋混凝土框架的内外柱可能承受由压向拉的复杂轴力变化，地震时竖向加速度大于假定值，从而影响了框架柱的抗剪承载力。然后进行了4个250 mm × 250 mm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。试验中改变了柱的轴向荷载和轴向荷载与侧向荷载之间的加载路径，即混凝土抗压强度比为0.49时的轴压不变，混凝土抗压强度比为0.29时的轴拉不变，侧向荷载作用后改变轴向荷载（称之为分载加载），侧向荷载作用后改变轴向荷载（称之为分载加载）。 ...更多信息 多路（称为同时加载）。结论如下。(1)变轴力作用下钢筋混凝土柱的抗剪承载力比恒轴力作用下的抗剪承载力分别降低了0.69倍和0.88倍。Arakawa公式和AIJ规定的钢筋混凝土结构极限强度设计不能考虑受拉柱的轴向荷载，高估了受拉柱在变轴向荷载作用下的抗剪强度。(2)在不同的轴向荷载作用下，两种加载路径下的抗剪强度基本相同，单独装载和同时装载。在轴向拉伸载荷下，同时加载的剪切强度，但是，在单独加载下的0.83倍。(3)无论柱轴力如何变化，桁架机构承担的剪力几乎不变，而拱机构的抗剪贡献随柱轴力的变化而急剧变化。(4)采用实测柱纵向钢筋沿着粘结应力和实测抗剪钢筋拉力计算得到的斜压杆倾角与受拉柱轴向荷载作用下的主压应变倾角相似，并假设斜压杆在桁架机构中均匀形成。因此，压应力不能通过斜向剪切裂纹传递。这也是钢筋混凝土柱在轴拉荷载作用下难以形成拱效应的原因。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