Securing Steel Davit Arms: When and How?

Tubular steel poles are a popular structure choice for high voltage transmission line applications because of their simple appearance, small footprint, and ease of installation (as compared to latticed towers). With modern welding technologies, they can also be a good option for extra high voltage lines. But because of the inherent slenderness and the low self-damping characteristics of the davit arms, the unloaded arms can be prone to vortex-induced vibration (VIV). In this context, “unloaded” means the arm is installed on the pole, but no weights, tie-down cables, insulators, conductor, or other physical load have been placed on the arm. This condition is not uncommon during the construction phase of a line and can last from a few hours to several months. Industry guidance recommends that all unloaded arms be secured, immediately after the arm is installed to mitigate VIV induced damage, by hanging weights, or tying the arm tips back to the pole shaft with cables. The details (cable types and sizes, weights) of this guidance are nonspecific. Experience has also shown that not every arm need be secured; the actual requirements depend on arm geometry and local wind conditions. This paper describes an AEP analytical study supplemented by testing that investigates the effects of wind-induced vibration on (various sizes and shapes of) davit arms to determine critical wind speeds (CWSs). These, in turn, lead to guidelines for when and when not to secure an arm. This paper also proposes an engineering method to determine the appropriate weights and cable sizes to use when necessary.