## 4 Test Results  
### Load-Displacement Relationship and Failure Mode  
Figure 6 shows the lateral load-displacement (\(V\)-\(\Delta\)) relationship and failure mode of the unstiffened specimen N00-150. During the push in the positive direction where the chord member was under compression, the out-of-plane rotation or torsional deformation of the plane truss occurred largely. Such torsional effects were attributed to inappropriate lateral supporting: for N00-150 that was tested first, the location of ball jigs was different from Figure 5 and was concentrated at the center region (horizontal distance 1000 mm) of the chord member. Consequently, as lateral load exceeded 150 kN, the plane truss rotated and the strength was no longer increased. As shown in Figure 6b, out-of-plane deformation or distortion occurred significantly at the plate-to-RHS K-connection, and thus, loading in the positive direction was stopped. During the pull in the negative direction where the chord member was under tension, on the other hand, loading was continued until \(\Delta\) = -50 mm. The maximum load \(V_{u}\) = -402 kN was reached at \(\Delta\) = -27 mm (later drift ratio = -1.9%). At the load level of about \(V\) = -150 kN, the stiffness decreased as slip occurred at the bolted joint of branch members. As shown in Figure 6b, the unstiffened K-connection of N00-150 underwent severe deformations of the connecting chord face (\(t\) = 4.5 mm and \(b\)/\(t\) = 33.3) and longitudinal plate (\(t_{p}\) = 15 mm). This shows that the governing limit state was chord face plastification. There was no weld rupture. At the bolted joint of the branch members, scratches on the faying surface were clearly shown and, due to subsequent hole bearing action, threaded marks or dents were left on the inner face of the hole. Figure 7 shows the test results of the stiffened specimen S15-150. For S15-150, out-of-plane rotation and torsional effects were greatly reduced. As shown in Figure 7a, after robust initial elastic behavior, the stiffness decreased due to slip at the bolted joint of branch members at the load level of about \(V=\pm 200\) kN; however, the strength and stiffness began to increase again, as the bolt was in contact with the hole edge and bearing action was activated. The maximum load in the positive direction, \(V_{u}=+487\) kN, was less than that in the negative direction, \(V_{u}=-593\) kN, due to torsional effects during the push in the positive direction. Note that, during the pull in the negative direction, S15-150 was not fully tested until its ultimate limit state due to the loading limit of the actuator (i.e., 650 kN). At the load level of \(V=\pm 400\) kN, the load was sharply reduced as the reaction frame slipped on the floor; however, the load was recovered soon after. Figure 6: Test results of unstiffened plate-to-RHS connection: N00-150: (**a**) Lateral load-displacement relationship; (**b**) Failure mode. As shown in Figure 7b, the deformations of the connecting chord face (\(t\) = 4.5 mm and \(b/t\) = 33.3) and longitudinal plate (\(t_{p}\) = 15 mm) were significantly reduced at the stiffened K-connection of S15-150 (compare with N00-150 in Figure 6b). There was no weld rupture, and the bolts had little or no deformation. At the bolted joint of branch members, scratches on the faying surface due to slip and, due to subsequent bearing action, threaded marks or dents on the inner face of the hole were clearly shown. This indicates that the governing limit states were the slip and subsequent hole bearing at the bolted joint in addition to chord face plastification. Figure 8 shows the test results of the stiffened specimen S25-200. For S25-200, after robust linear elastic behavior until \(V\) = \(\pm\)200 kN, the stiffness decreased as slip occurred at the bolted joint of branch members. The decrease in stiffness was more pronounced under positive loading due to torsional effects. After the slip, the load increased again as the bolts provided additional resistance through bearing action.