Temporary Support of Lattice Steel Transmission Towers

Lattice steel transmission towers have been used to support transmission lines over the past 120 years. Many events have been known to damage these towers to an extent to which they must be repaired structurally. These events include but are not limited to vehicle impacts, agricultural equipment damage, corrosion damage, vandalism, and initial fabrication / construction errors. It has been the standard practice of transmission utilities in the past to produce the steel detail drawings in house, and either have their own line maintenance crews make the steel changeout or to have an electrical contractor perform the work. Most often, the responsible engineer at the utility will specify a load that must be supported during the work. There is an old wives tale prevalent among electrical contractors that the towers are "designed" to stand on three legs. This no doubt comes from the fact that many of the foreman and supervisors in the industry have personally seen the towers stand with one leg removed. In fact, many tangent and light angle towers will stand with one leg disconnected with no temporary support. However, with the many combinations of tangent, light angle, heavy angle, and dead end configurations, wire tensions, and construction / design practices, it is imperative to communicate that a general "blanket" statement that the towers will stand on three legs not true. In fact, allowing this blanket statement to be used in the industry is incorrect. It is the purpose of this paper to discuss the correct procedure(s) for temporary support of towers and outline the correct safety factors that are given by OSHA, ASME, and the governing utility. The focus will be on cranes supporting towers and the applicable safety factors. Additionally, alternative temporary support options shall be discussed.