Static Performance of UT-Type Semi−Rigid Joints Considering Loss of Bolt Pretension

To investigate the static behavior of UT−type assembled semi−rigid joints and the effects of bolt pretension loss, two representative joint configurations, UT250 × 150 and UT400 × 200, were studied by combining full−scale tests with refined finite element analysis using ABAQUS. Pure bending, bending−shear, and constant−axial−force−coupled loading conditions were considered, with particular attention paid to the effects of single−bolt and multiple−bolt pretension loss on moment capacity, initial rotational stiffness (Ky), interface slip, and the failure mode of the joints. The results show that the UT−type joint mainly fails through concentrated plastic yielding in the joint zone, and its ultimate moment (Mu) is 12.3–18.7% higher than that of a conventional bolted−welded joint, satisfying the design principle of “strong joint and weak member”. Loss of pretension in a single bolt has only a limited influence on the yield moment (My) and ultimate moment (Mu), with a maximum reduction of 8.0% in the ultimate moment (Mu) under negative pure bending; however, it causes clear degradation in the initial rotational stiffness (Ky), and pretension loss in the upper bolt produces a greater stiffness reduction than loss in a single lower bolt, with a maximum reduction of 33.43%. Multiple−bolt pretension loss exhibits a pronounced coupling effect. Simultaneous loss in lower bolts on the same side is the most unfavorable case, leading to a maximum stiffness reduction of 67.78% (coupling coefficient of 1.17), whereas diagonal loss is relatively controllable and generally keeps the stiffness reduction within 7%. When the axial compression ratio does not exceed 0.3, the mechanical response of the joint remains relatively stable, and the adverse effect of pretension loss can be alleviated to a certain extent; further increases in the axial compression ratio accelerate the degradation of both stiffness and load−carrying capacity. The present study provides a useful reference for the design optimization, construction quality control, and in−service maintenance of UT−type semi−rigid joints.