Experimental and numerical study of the axial stiffness of bolted joints in steel lattice transmission tower legs

Steel lattice transmission towers are constructed through the bolted assembly of various sizes of angle steel members and joint slip is inevitable; ignorance of this slip, which is the prevailing practice in transmission tower design engineering, can lead to an overestimation of the axial stiffness of bolted joints. This study investigates the impact of joint slip effects under tensile loading through a series of 30 tensile tests of 10 typical bolted joint configurations (3 specimens for each tested configuration) used in classic 500 kV lattice steel transmission line towers in China. Finite element analyses were also conducted as a complement and validation of concepts. Particular focus was placed on the load-deformation/slip curves of the tensile tests. The experimental results show that the joint slip was realized through a complicated process consisting of elastic deformation caused by the frictional load transfer and the asynchronous joint slip. Meanwhile, when subjected to an incremental axial tension, the axial stiffness of the bolted joints was highly nonlinear with regard to the elastic deformation. A simplified multi-linear strain-stress model was created using the experimental data to represent this nonlinearity in finite element analysis. The results from finite element analysis of the 30 tensile tests using this multi-linear strain-stress model show that the model can well predict the failure load level and positions of rupture of the angle members. The actual average axial stiffness of the tested bolted joints during the elastic deformation process of load transfer by bolt bearing is approximately 30% of the theoretical value. Finally, a general nodal element is proposed to describe the joint slippage effects and introduce this important factor in the structural analysis of lattice transmission towers in view of safer and more reliable design.