## 4 Results
### Load-Displacement Curve and Stress-Rotation Curve
Figure 11 shows stress-rotation curves that were converted from strain rates according to the rotation angles of the connections in the first and second specimens for each rotation direction, as measured by strain gauges attached to the steel pipes and pockets. In the case of the downward rotation (Figure 11a), continuous compressive stress occurred after applying a load at the point of the strain gauge (SG-V) attached to the horizontal member steel pipe. The maximum compressive stress at the location where the strain gauge was attached to the horizontal member steel pipe (DN-1-SG-H: 316.7 MPa, DN-2-SG-H: 335.8 Mpa) was less than 50% of the yield strength of the horizontal member steel pipe material (698.0 Mpa). In the case of the measurement point where the strain gauge (SG-H) was attached to the vertical member steel pipe, compressive stress occurred in the initial rotation part, but after the maximum compressive stress, the stress reversed and became tension. The tensile stress that occurred afterward increased continuously and attained a yield strength (639.0 Mpa) around the point where the connection's maximum moment was reached. It was found that plastic deformation occurred in the steel pipe as the lower part of the pin pressed on the vertical member steel pipe due to the rotation of the connection, but it was determined to be a minor degree of deformation compared to the deformation of the pin when the shape of the connection was examined after the end of the experiments (Figure 8). The stress that occurred at the strain gauges attached to both sides of the top part of the pocket (SG-R, L) was smaller than the stress that occurred in the steel pipe, and the difference in the stress that occurred on the left side and the right side was small in both the DN-1 and DN-2 specimens. Tensile stress occurred at the initial rotation angle, and after around 0.1-0.2 rad, it reversed and became compressive stress. It was determined that the pocket was under tension owing to the pin at the beginning of the experiments, and the stress at the SG-R and L stress measurement locations at the left and right top part of the pocket began to decrease as tearing occurred at the top of the front part of the pocket. In addition, the compressive stress that occurred in the latter part of the experiments was determined to be affected by the tensile stress that occurred owing to the hammering performed after the pin was inserted in the pocket and before the strain gauges were attached. Based on the deformation shapes of the pocket-type connections and the level of stress that occurred, it was found that the main deformation in the connections of the downward rotation specimen occurred in a part of the top of the pocket (the top of the face that makes contact with the pin and which is subjected to direct force) and the pin. The section in which the rotational stiffness rapidly decreased was the rotation section where the pin and pocket began to yield, and it was found that the yielding of the pin and the pocket had the primary influence on distinguishing the pocket-type connection's initial rotational stiffness and post-yield rotational stiffness. pipe of UP-2. The tensile force transmitted to UP-2-SG-H decreased due to the occurrence of cracks in the welded joint. However, in the specimen of UP-1, the crack did not occur. Since the specimen with the upward rotation has a relatively strong rotation-resistant capacity, the connection welding of the pin and the horizontal steel pipe can be broken in some specimens. It can be seen that the stiffness of the upwardly rotated connection was influenced more by the material of the vertical steel pipe than the pocket, which is a direct component of the connection. The stress in the laterally rotated connection (Figure 11c) was less than each material's yield strength at all locations where stress was measured.