Comparison of Structural Performance of T-Flange Joints Using Different Bolt Grades Under Axial Tensile Loading Conditions

This study investigates the effect of bolt grade on the structural performance of T-flange joints used in wind turbine support structures. A nonlinear finite element model was developed to compare Grade 10.9 and Grade 12.9 bolts under identical loading and boundary conditions. The results showed that using Grade 12.9 bolts increased the reaction force by 12.3%, bolt stress by 21%, bolt strain by 18.7%, flange strain by 20.0%, tower shell strain by 18%, and tower shell stress by 11.2% compared with Grade 10.9 bolts. The external-load–bolt-force Z-Fs relationship, where Z denotes the applied external tensile load and Fs denotes the corresponding internal axial bolt force, was used to evaluate bolt-force development and load transfer during loading. The Z-Fs relationship further showed that the internal bolt force increased by 13.7% for Grade 12.9 bolts at the design external tensile load. These results indicate that Grade 12.9 bolts improve load-transfer capacity and joint resistance, but they also increase the stress and strain demand on the connected flange and tower shell. Therefore, higher-grade bolts should be evaluated within the complete joint system rather than based on bolt strength alone.