Experiments and 3-D FEM analyses were performed on high-strength concrete columns laterally prestressed by the shear reinforcements to study the influence of the active confinement upon shear strength and crack behaviors. The lateral prestress has been introduced in proportion to thc strength of concrete, and the width of every crack over transverse reinforcements has been measured by a microscope. By increasing thc lateral pressure as mentioned above, shear crack strength and ultimate shear strength have increased proportionally, however, the effectiveness of active confinement has been weakened with decreasing axial force ratio. The FEM analyses can evaluate the shear crack strength and ultimate shear strength, and explain the effect of the active confinement on shear behaviors by evaluating the intensity of confinement in tri-axial state of stress with minor principal stress and equivalent confining pressure.
