Load-transferring mechanism and calculation theory along engaged threads of high-strength bolts under axial tension

Abstract

Bolted connections are widely used in steel structures such as steel buildings, bridges, transmission line steel towers, in which the stress behavior of the bolt is very complex, including tension, shear and bending moment. The load-transferring mechanism along engaged threads is difficult to grasp, especially under the alternating loads or dynamic loads. Therefore, a three-dimensional finite element model with helical threads being considered is established, whose accuracy is validated through the corresponding test. Then, the load-transferring behavior along the engaged threads under axial tension, such as the stress distributions, axial load distributions along the thread, are all investigated. Finally, a modified equation that refers to the elastic thread mechanics proposed by Yamamoto is proposed based on the numerical results, and the impact factor R defined as the ratio of actual force to average axial tension is adopted for the evaluation of load distributions along bolt threads. The results show that during the whole loading process, the load-sharing proportions of 1st and 2nd bolt threads increase first and then decrease, and that of the 3rd thread has no obvious change, and those of other threads decrease first then increase linearly. The slippages in engaged threads zone 0.22P apart from the nut loaded surface do not occur, and the slippages of the zone from 0.22P to 0.61P rapidly increase and get to maximum, after that decrease linearly. Besides, the modified formula, which takes the effects of the plastic deformation and contact surfaces into account, is proved to be more reasonable.