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

1995年神户地震中，钢筋混凝土框架的外柱和内柱可能承受复杂的轴向荷载，由压变拉，由于地震时的竖向加速度比假定的大，从而对这些柱的抗剪能力造成不良影响。然后进行了四个250 mm×250 mm方形截面的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。混凝土抗压强度为275kgf/cm~2~298kgf/cm~2。试验中改变了柱轴向荷载和轴向荷载与横向荷载之间的加载路径；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，施加横向荷载后变化的轴向荷载(称为单独荷载)和随施加水平荷载而变化的轴向荷载si…更多次(称为同时加载)。得出如下结论：(1)钢筋混凝土柱在变轴向荷载作用下的抗剪强度比恒定轴压荷载下的分别降低了0.69倍和0.88倍。Arakawa公式和AIJ关于钢筋混凝土结构极限强度设计的规定不能考虑受拉柱的轴向荷载，高估了变轴向拉力作用下的抗剪强度。(2)在分别加载和同时加载两种加载路径下，受压变轴力作用下的抗剪强度基本相同。(3)无论柱轴向荷载如何变化，桁架机构承载的剪力几乎不变，而拱形机构对抗剪的贡献随着柱轴向荷载的变化而发生较大变化。(4)假定混凝土斜压杆在桁架机构中均匀形成，根据柱纵钢筋的粘结应力和剪力钢筋的拉力计算出的斜压杆的倾斜度与主压应变在张拉柱轴力作用下的倾斜度相似。因此，压应力不能通过斜向剪切裂纹传递。这就是钢筋混凝土柱在轴拉荷载作用下难以形成拱形机制的原因。较少

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