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CONTENTS
Volume 34, Number 2, February 2022 (Special Issue)
 


Abstract
This paper presents a failure type assessment curve method to judge the failure type of transmission tower segments. This novel method considers the equivalent static wind load characteristics and the transmission tower members' load-bearing capacities based on numerical simulations. This method can help judge the failure types according to the relative positions between the actual state points and the assessment curves of transmission tower segments. If the extended line of the actual state point intersects with the horizontal part's assessment curve, the segment would lose load-bearing capacity due to the diagonal members' failure. Another scenario occurs when the intersection point is in the oblique part, indicating that the broken main members have caused the tower segment to fail. The proposed method is verified by practical engineering case studies and static tests on the scaled tower segments.

Key Words
assessment curve; equivalent wind loads; failure type; transmission tower; ultimate load-bearing capacity

Address
Yue Li:College of Civil Engineering, Tongji University, China

Qiang Xie:College of Civil Engineering, Tongji University, China

Zheng Yang:College of Civil Engineering, Tongji University, China

Abstract
The galloping of iced conductors has long been a severe threat to the safety of overhead transmission lines. Compared with normal transmission lines, the ultra-high-voltage (UHV) transmission lines are more prone to galloping, and the damage caused is more severe. To control the galloping of UHV lines, it is necessary to conduct a comprehensive analysis of galloping characteristics. In this paper, a large-span 1000-kV UHV transmission line in China is taken as a practical example where an 8-bundled conductor with D-shaped icing is adopted. Galerkin method is employed for the time history calculation. For the wind attack angle range of 0°~180°, the galloping amplitudes in vertical, horizontal, and torsional directions are calculated. Furthermore, the vibration frequencies and galloping shapes are analyzed for the most severe conditions. The results show that the wind at 0°~10° attack angles can induce large torsional displacement, and this range of attack angles is also most likely to occur in reality. The galloping with largest amplitudes in all three directions occurs at the attack angle of 170° where the incoming flow is at the non-iced side, due to the strong aerodynamic instability. In addition, with wind speed increasing, galloping modes with higher frequencies appear and make the galloping shape more complex, indicating strong nonlinear behavior. Based on the galloping amplitudes of three directions, the full range of wind attack angles are divided into five galloping regions of different severity levels. The results obtained can promote the understanding of galloping and provide a reference for the anti-galloping design of UHV transmission lines.

Key Words
full range of attack angles; galloping characteristic; galloping region division; iced 8-bundled conductor; UHV lines

Address
Wenjuan Lou:Institute of Structural Engineering, Zhejiang University, Hangzhou 310058, P.R. China

Huihui Wu:Institute of Structural Engineering, Zhejiang University, Hangzhou 310058, P.R. China

Zuopeng Wen:Institute of Structural Engineering, Zhejiang University, Hangzhou 310058, P.R. China

Hongchao Liang:Institute of Structural Engineering, Zhejiang University, Hangzhou 310058, P.R. China

Abstract
This study aimed to analyze the wind-induced mechanical energy (WME) of a proposed super high-rise and long-span transmission tower-line system (SHLTTS), which, in 2021, is the tallest tower-line system with the longest span. Anew index – the WME, accounting for the wind-induced vibration behavior of the whole system rather than the local part, was first proposed. The occurrence of the maximum WME for a transmission tower, with or without conductors, under synoptic winds, was analyzed, and the corresponding formulae were derived based on stochastic vibration theory. Some calculation data, such as the drag coefficient, dynamic parameters, windshielding areas, mass, calculation point coordinates, mode shape and influence function, derived from wind tunnel testing on reducedscale models and finite element software were used in calculating the maximum WME of the transmission tower under three cases. Then, the influence of conductors, wind speed, gradient wind height and wind yaw angle on WME components and the energy transfer relationship between substructures (transmission tower and conductor) were analyzed. The study showed that the presence of conductors increases the WME of transmission towers and changes the proportion of the mean component (MC), background component (BC) and resonant component (RC) for WME. The RC of WME is more susceptible to the wind speed change. Affected by the gradient wind height, the WME components decrease, with the RC decreasing the fastest and the MC decreasing the slowest. The WME reaches the its maximum value at the wind yaw angle of 30°. Due to the influence of three factors, namely: the long span of the conductors, the gradient wind height and the complex geometrical profile, it is important that the tower-line coupling effect, the potential for fatigue damage and the most unfavorable wind yaw angle should be given particular attention in the wind-resistant design of SHLTTSs.

Key Words
finite element model; gradient wind height; transmission tower-line system; wind-induced mechanical energy; wind tunnel test

Address
Shuang Zhao:School of Civil Engineering and Architecture, Chongqing University of Science and Technology,
No. 20 Da Xue Cheng Dong Road, Chongqing 401331, P.R. China

Zhitao Yan:School of Civil Engineering and Architecture, Chongqing University of Science and Technology,
No. 20 Da Xue Cheng Dong Road, Chongqing 401331, P.R. China

Eric Savory:Department of Mechanical and Materials Engineering, University of Western Ontario, No. 1151 Richmond Street, London N6A 5B9, Canada

Bin Zhang:CMCU Engineering Co., Ltd., No. 17 Yu Zhou Road, Chongqing 400039, P.R. China

Abstract
The current study investigates the dynamic effects in the tornado-structure response of an aeroelastic self-supported lattice transmission tower model tested under laboratory simulated tornado-like vortices. The aeroelastic model is designed for a geometric scale of 1:65 and tested under scaled down tornadoes in the Wind Engineering, Energy and Environment (WindEEE) Research Institute. The simulated tornadoes have a similar length scale of 1:65 compared to the full-scale. An extensive experimental parametric study is conducted by offsetting the stationary tornado center with respect to the aeroelastic model. Such aeroelastic testing of a transmission tower under laboratory tornadoes is not reported in the literature. A multiaxial load cell is mounted underneath the base plate to measure the base shear forces and overturning moments applied to the model in three perpendicular directions. A three-axis accelerometer is mounted at the level of the second cross-arm to measure response accelerations to evaluate the natural frequencies through a free-vibration test. Radial, tangential, and axial velocity components of the tornado wind field are measured using cobra probes. Sensitivity analyses are conducted to assess the variation of the structural dynamic response associated with the location of the tornado relative to the lattice transmission tower. Three different layouts representing the change in the orientation of the tower model relative to the components of the tornado-induced loads are considered. The structural responses of the aeroelastic model in terms of base shear forces, overturning moments, and lateral accelerations are measured. The results are utilized to understand the dynamic response of self-supported transmission towers to the tornado-induced loads.

Key Words
aeroelastic; dynamic analysis; response spectrum; tornado; transmission tower; wind

Address
Nima Ezami:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Ashraf El Damatty:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Ahmed Hamada:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Horia Hangan:WindEEE Research Institute, The University of Western Ontario, London, ON, Canada

Abstract
Majority of transmission line system failures at many locations worldwide have been caused by severe localized wind events in the form of tornadoes and downbursts. This study evaluates the structural response of two different transmission line systems under equivalent F2 tornadoes obtained from real incidents. Two multi-span self-supported transmission line systems are considered in the study. Nonlinear three-dimensional finite element models are developed for both systems. The finite element models simulate six spans and five towers. Computational Fluid Dynamics (CFD) simulations are used to develop the tornado wind fields. Using a proper scaling method for geometry and velocity, full-scale tornado flow fields for the Stockton, KS, 2005 and Goshen County WY, 2009 are developed and considered together with a previously developed tornado wind field. The tornado wind profiles are obtained in terms of tangential, radial, and axial velocities. The simulated tornadoes are then normalized to the maximum velocity value for F2 tornadoes in order to compare the effect of different tornadoes having an equal magnitude. The tornado wind fields are incorporated into a three-dimensional finite element model. By varying the location of the tornado relative to the transmission line systems, base shears of the tower of interest and peak internal forces in the tower members are evaluated. Sensitivity analysis is conducted to assess the variation of the structural behaviour of the studied transmission lines associated with the location of the tornado relative to the tower of interest. The tornado-induced forces in both lines due to the three different normalized tornadoes are compared with corresponding values evaluated using the simplified load case method recently incorporated in the ASCE-74 (2020) guidelines, which was previously developed based on the research conducted at Western University.

Key Words
computational fluid dynamics; finite element; tornado; transmission line system; wind

Address
Nima Ezami:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Ashraf El Damatty:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Ahmed Hamada:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Mohamed Hamada:Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada

Abstract
Transmission lines systems are important components of the electrical power infrastructure. However, these systems are vulnerable to damage from high wind events such as hurricanes. This study presents the results from a 1:50 scale aeroelastic model of a multi-span transmission lines system subjected to simulated hurricane winds. The transmission lines system considered in this study consists of three lattice towers, four spans of conductors and two end-frames. The aeroelastic tests were conducted at the NSF NHERI Wall of Wind Experimental Facility (WOW EF) at the Florida International University (FIU). A horizontal distortion scaling technique was used in order to fit the entire model on the WOW turntable. The system was tested at various wind speeds ranging from 35 m/s to 78 m/s (equivalent full-scale speeds) for varying wind directions. A system identification (SID) technique was used to evaluate experimental-based along-wind aerodynamic damping coefficients and compare with their theoretical counterparts. Comparisons were done for two aeroelastic models: (i) a self-supported lattice tower, and (ii) a multi-span transmission lines system. A buffeting analysis was conducted to estimate the response of the conductors and compare it to measured experimental values. The responses of the single lattice tower and the multi-span transmission lines system were compared. The coupling effects seem to drastically change the aerodynamic damping of the system, compared to the single lattice tower case. The estimation of the drag forces on the conductors are in good agreement with their experimental counterparts. The incorporation of the change in turbulence intensity along the height of the towers appears to better estimate the response of the transmission tower, in comparison with previous methods which assumed constant turbulence intensity. Dynamic amplification factors and gust effect factors were computed, and comparisons were made with code specific values. The resonance contribution is shown to reach a maximum of 18% and 30% of the peak response of the stand-alone tower and entire system, respectively.

Key Words
aerodynamic damping; aeroelastic modeling; buffeting response; dynamic amplification factors; gust effect factors; system identification; transmission lines; wall of wind

Address
Ziad Azzi:DDA Claims Management, Miami, FL, U.S.A

Amal Elawady:Department of Civil and Environmental Engineering, Florida International University, Miami, FL, U.S.A

Peter Irwin:Extreme Events Institute of International Hurricane Research Center, Florida International University, Miami, FL, U.S.A.

Arindam Gan Chowdhury:Department of Civil and Environmental Engineering, Florida International University, Miami, FL, U.S.A.

Caesar Abi Shdid:Department of Civil Engineering, Lebanese American University, Beirut, Lebanon


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