Effects of Bolt-Tightening Torque and Rubber Interlayer Thickness on the Slip Resistance of Nylon Coupler–GFRP Circular-Tube Joints

To characterize the slip-resistant behavior of nylon-coupler joints in prefabricated GFRP circular-tube crossing frames, a two-factor experimental investigation was conducted considering bolt-tightening torque and rubber interlayer thickness. Four interface configurations were examined: no interlayer and rubber interlayers with thicknesses of 1.0, 1.5, and 2.0 mm. Six bolt-tightening torque levels of 5, 7, 9, 11, 13, and 15 N·m were considered. Three replicate tests were conducted for each parameter combination, yielding 72 test records. The relative axial displacement measured by D1, the transverse displacement recorded at D2, and load-transfer-beam strain were monitored to characterize the loading response. Because most load–relative displacement curves did not reach a distinct post-peak descending branch, the maximum recorded load within each recorded test window (Fmax) was used only as a comparative index and was supplemented by the loads at fixed D1 relative axial displacements of 5, 10, and 15 mm. The maximum transverse displacement recorded at D2 remained below 3 mm for all specimens, indicating that the transverse response at the monitored point remained limited during the tests. When the bolt-tightening torque increased from 5 to 15 N·m, the mean Fmax values of joints with no interlayer and with 1.0, 1.5, and 2.0 mm interlayers increased by 133.0%, 200.1%, 167.3%, and 171.7%, respectively. Two-way analysis of variance indicated that bolt-tightening torque, interlayer thickness, and their interaction were all statistically significant (p < 0.001), accounting for 56.35%, 34.34%, and 5.41% of the total sum of squares, respectively. The 1.0 mm interlayer consistently produced the highest mean Fmax at each torque level. The fitted slope of its Fmax–torque relationship was 0.542 kN/(N·m), approximately 2.43 times that of joints without an interlayer. The results suggest that an interlayer of appropriate thickness can improve interfacial conformity and the effectiveness of frictional load transfer, whereas increasing interlayer thickness may introduce greater compressive and shear deformation. Within the specific rubber material, joint configuration, bolt-tightening torque range of 5–15 N·m, and nominally symmetric short-term monotonic loading conditions examined in this study, the 1.0 mm interlayer exhibited the best overall performance among the tested configurations.