Review of Span and Gust Factors for Transmission Line Design

Current criteria for the calculation of synoptic wind load effects on transmission line components are based on Davenport's gust response factors, which take into account the different response of structures and long spans of wires to turbulent wind. The gust factors include resonant components that imply possible dynamic amplifications of the wind effect on the structures and wires, although the wire resonant response is generally ignored because of high aerodynamic damping. Although the gust response factor equations were derived for 1-hour mean wind records, the factors are being applied for winds with different sampling intervals but with reference to 10-minute winds. The conversion is based on Durst's velocity ratio curve, which was not derived from records of extreme winds but from low velocity storms. Due to these differences, and also other simplifications, there could be errors in calculated line or component reliability of at least one order of magnitude. While line design weather loading conditions are derived from synoptic wind records, it is well known within the industry that most line or structure failures are induced by non-synoptic storms, such as downdrafts or tornadoes generated by thunderstorms. Field evidence from these storms indicate that the 1-hour mean factors, even when extrapolated to very short-interval gusts, do not explain the observed effects and therefore new span and gust factors are needed for design. The paper reviews the basic concepts and history of span and gust factors and the impact of some of the current assumptions and simplifications. It also focuses on recent research on the actual effects of non-synoptic storms and the new factors that are derived from these experiences.