Influence of Bolted-Joint Slippage on the Response of Transmission Towers Subjected to Frost-Heave

Slippage of bolted joints is an important factor in the behavior of transmission towers. Field inspections of towers located in the northern regions of Canada show substantial frost-heave induced displacements. The conventional structural analysis software solutions for tower leg axial forces, based on idealized joint behavior under field-observed frost-heave induced displacements, are excessively large, implying tower failure in some cases. However, field inspections show structurally stable towers, and design engineers often consider joint slippage as the main reason for this discrepancy. In this paper, the experimental slippage behavior of transmission-tower bolted joints investigated by Ungkurupanian (2000) is incorporated into a non-linear joint finite element and applied to study the behavior of transmission towers under working loads by using the finite element method. The analysis shows that tower-leg joint slippage has a significant influence on tower behavior by either reducing the load carrying capacity or significantly increasing the deflections under working loads. On the other hand, joint slippage has a positive effect on towers subjected to frost-heave induced displacements, as the resulting member axial forces are much lower than those corresponding to rigid joints.