This report reviews the literature on the dynamic response of a Transmission Line (TL) system under a conventional atmospheric boundary layer as well as non-synoptic wind loading (downbursts and tornadoes). Gust-induced response for the conductors and towers is covered and limitations in the current structural design codes for wind loading are identified. Four central sections are considered in this study covering synoptic wind loading, downburst, tornado loading, and conclusions and recommendations.
For the case of synoptic wind events, four design codes (ASCE 74 2010, AS/NZS 2010, BS 2001, IEC 2003) specific to TLs are considered for comparison. Using the ASCE 74 (2010) as a datum for normalization, a code ratio (CR) is evaluated for various parameters to assess discrepancies between the codes. The code ratio for conductor forces CRFc is found to range between 81% and 144%. For the tower force code ratio CRFt, a discrepancy range of 68% and 185% is noticed. The study highlights the main reasons behind these discrepancies. For the case of non-synoptic localized wind events (downbursts and tornadoes), the study reveals that the event’s size and relative location to the tower lead to a number of critical load cases that need to be considered.
The report provides important design considerations for both synoptic and non-synoptic winds. At the end of the document, a list of the major existing gaps in the current design codes is provided as well as recommendations for their revision.
Dynamic response, Transmission Line, Conductors, Synoptic Winds, Downburst, Tornado, Gust Factor, Buffeting
A numerical model capable of predicting the dynamic response of a multi-span transmission line system under both the mean and fluctuating components of synoptic wind loads was previously developed in house at The University of Western Ontario. The model is validated experimentally using results of a previous aero-elastic test of a multi-spanned transmission line system conducted at the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario.
In this investigation, a parametric study is conducted on five different transmission line systems, namely: guyed steel lattice tower, self-supported steel lattice tower, H-framed steel tower, cantilever steel pole, and pre-stressed concrete pole. For each system, a full nonlinear dynamic analysis in the time domain is conducted under properly synthesized turbulent synoptic wind to evaluate time histories of the system’s responses. Peak responses of each system, such as tower and conductor deflections, base shear force and conductor reactions, are determined from this analysis. Such peak responses are due to the mean, background and resonant components. A similar analysis is also repeated in the quasi-static manner to identify peak responses due to mean and background components. The ratio between peak responses, including mean, background and resonant effects, to the peak responses, including mean and background effects, is evaluated and referred to as the Dynamic amplification factor (DAF). Such a DAF is used to identify the cases where the dynamic effect is important for the line design. The results of the study are also used to assess the adequacy of the gust response factor adopted in the ASCE to account for both the background and resonant components of the response of the transmission line system.
Transmission lines, Synoptic wind, Turbulent wind, Dynamic analysis, Quasi-static analysis