ELEMENTAL EXPERIMENT OF DISK SHEAR-KEY UNDER TENSILE FORCE AND SHEAR FORCE AND ITS SHEAR STRENGTH EVALUATION

Stress transmission mechanism and mechanical behavior of the joints for external seismic retrofitting : Part 2

Abstract

 The disk shear-key is a composite shear resistance system composed of a steel disk and an anchor bolt. The disk shear-key has high stiffness and shear strength as compared with a general post-installed anchor. In the previous study, it is suggested that the expected shear strength can not be demonstrate when tensile force acts on the disk shear-key. Therefore, in the current design and construction method for the external seismic retrofitting, the disk shear-keys are placed in the center of the span and it is assumed that the shear force is to be borne on average. Then, for the tensile force due to the eccentric moment, post-installed anchors are placed at both ends of span. In order to make effective use of the disk shear-key, it is necessary to grasp the mechanical behavior under combined stress and arrange it efficiently. Therefore, in this study, the disk shear-key was tested under cyclic shear force along with constant tensile force.

 In Chapter 2, the outline of the experiment is indicated. The eight specimens were built with various parameters, which is, diameter of the steel disk R_d, embedded length of the anchor bolt l_e, and tensile force ratio η’. Here, the tensile force ratio is the ratio of the tensile axial force to the tensile strength of the disk shear-key.

 The experiment results are described in Chapter 3. The shear strength of the specimens in this experiment was less than the ultimate shear strength according to the current evaluation formula. This is considered to be caused by the different loading condition in the vertical direction from the previous experiment. When the vertical stress generated at the joint surface is zero (η’=0), the shear strength of the disk shear-key can be roughly estimated by the design shear strength according to the current design evaluation formula. However, in the range of η’=0 to 0.5 conducted in this experiment, the relative horizontal displacement of joint at maximum shear strength was within 2mm, which is the allowable value of the external seismic retrofitting joints. Also, the relative vertical displacement of joint increases as the tensile force is applied. Furthermore, in the case of application to external seismic retrofitting, the relative vertical displacement becomes large even when no tensile force is applied. It is suggested that the steel disk slips out, and it is believed that the shear resistance decreases due to the strength reduction in the bearing stress area of the concrete. In addition, the curvature of the anchor bolt is maximized at the position near 2d_a away from the joint surface on the existing side, and the value reaches the yielding region. However, curvatures within the elastic range occur at the other measurement points, and bending deformation hardly occurs at the retrofitting part.

 In Chapter 4, the shear strength evaluation formula under constant tensile force was constructed. The evaluation formula was modified in consideration of slipping out of the steel disk and the reduction of restraint effect by anchor bolt due to the tensile force. It was evaluated almost accurately with the experimental results.