Torsional behavior of horizontal joints in prestressed concrete towers under combined compression, bending and torsion

Horizontal joints are critical regions in prestressed concrete towers (PCTs). Existing design codes neglect the effect of bending-moment variation when predicting their torsional capacity, leading to biased predictions. To address this limitation, a new N-M-V-T multi-point combined loading system was developed and validated, enabling independent application of axial force, bending moment, shear force, and torque while overcoming the limitations of conventional single-point bending loading. Using this system, six tests on PCT horizontal joints were conducted under compression-torsion, compression-bending-torsion, and compression-shear-bending-torsion conditions, and corresponding FE models were established. The test and FE results show that the pure-torsion specimen mainly failed by torsion-shear failure induced by global relative slip at the horizontal joint, whereas the compression-bending-torsion specimens mainly exhibited local compression-shear failure. Bending moment significantly reduced the torsional capacity of the horizontal joint by altering the circumferential stress distribution and contact state at the joint. Parametric analysis indicated that the tower outer diameter and prestress ratio were the governing parameters for torsional capacity. Based on the test and FE results, a four-stage torsional-capacity model was proposed to account for the evolution of the horizontal-joint contact state from closure to opening, covering the fully compressed, closed under bending, locally opened, and fully opened states. The prediction errors of the model were controlled within 5% for the test results and within 15% for the extended parametric cases.