Transmission tower networks play an important role in the infrastructure system of many countries around the world. Many studies have been directed towards the evaluation of aerodynamic coefficients ...of the vertical tower body sections of transmission towers. Most of these studies obtain drag coefficients of tower sections from wind tunnel tests with yawed wind. However, strong winds like hurricane, downburst, tornado etc., which are the leading causes of transmission tower failures, often attack the towers with both yaw and tilt angles. Transmission towers constructed on the top or slope of mountains may also suffer the action of skew winds with both yaw and tilt angles. This paper addresses wind loads of transmission towers under winds with both yaw and tilt angles. By performing wind tunnel tests with multi-balance synchronous force measurement, the wind loads acting on a lattice tubular steel transmission tower under skew winds were measured, and the drag coefficients under 19 yaw angles and 13 tilt angles were obtained. Besides providing a more accurate modified formula for the skewed wind load factor, concepts of tilted and combined wind load factors are proposed with recommended formulas. By comparing the experimental and standard-calculated values of skewed wind load factor under a tilt angle of 0°, it is found that the standards underestimate the skewed wind load factor by at least about 10%, whereas the error of the proposed formula is less than 4% under the design key angles. In this study, the values of tilted wind factor under a yaw angle of 0° and combined wind load factor are recommended as 0.3 and 0.2, respectively. When the recommended values are used in calculation, the deviations between calculated drag coefficients and wind tunnel test results are less than 5%.
•Wind tunnel tests on transmission towers under winds with both yaw and tilt angles.•A new formula for skewed wind load factor validated with test results.•Combined wind load factor proposed to consider both yaw and tilt effects.
Summary
This paper studies the wind load on 1,000 m‐high super‐tall buildings and provides basic reference for design, including the utilization of passive and active control devices. High‐frequency ...force balance wind tunnel tests of super‐tall buildings with different height are carried out to investigate the effects of building height and wind flow on the wind load. Both monsoon and typhoon climate wind flows are simulated based on target models suggested in literatures. The simulation of typhoon climate wind flows is carried out by a newly developed technique. The analysis of the experimental results confirms that the aerodynamic force is very sensitive to both building height and wind flow. In monsoon climate, the turbulence intensity decreases on increasing the height above ground. Thus, on increasing the building height, vortex shedding becomes increasingly intense and excites stronger structural vibrations in the across‐wind direction, though the across‐wind fluctuating overturning moment coefficient is almost the same. In typhoon climate, both the mean and the fluctuating overturning moment coefficients increase with the building height. This is mainly caused by the decreasing mean wind speed. The vortex excitation becomes weaker on increasing the building height, and this phenomenon is different from that observed in the monsoon climate. In order to better explain vortex‐shedding excitation, a new parameter referred to as the characteristic turbulence intensity is defined herein as a weighted mean value of the turbulence intensity in the range of the building height. It provides a robust interpretation of the vortex excitation of super‐tall buildings located in different wind flow and climate conditions.
The accuracy of Large-Eddy-Simulation (LES) in modeling wind loads, especially components and cladding loads, does not currently fulfill its potential. The lack of extensive consideration for wind ...loading requirements and their direct influence on LES duration and mesh size are identified as research gaps. This study presents a validation metric for LES modeling of low-rise buildings, including the wind field and aerodynamics. Most of the proposed validation requirements are adapted from Boundary Layer Wind Tunnel (BLWT) testing requirements to highlight the challenges and opportunities for LES. The turbulence spectral frequency limits significantly contribute to the cost-accuracy trade-off in LES. A stationarity test of recorded time histories is proposed to determine the optimal duration, i.e., low-frequency limit, for statistical parameters other than the peak. An analytical expression is proposed for the LES mesh size estimation from the target maximum frequency limit and corresponding spectral energies. The proposed validation metric is demonstrated with LES models of a low-rise building targeting respective BLWT tests in three wind directions and in three exposures. Despite some limitations, the peak Cp error was within ±10% of the BLWT value range. The optimal normalized durations from LES were one order of magnitude shorter than those from the BLWT cases. The peak values did not show a significant difference in trend from BLWT; hence the duration dependence of peak uncertainties remains relevant for LES. The BLWT data supports the previously proposed high-frequency limit of two. LES models can produce reasonably acceptable wind loads with proper attention to the existing and new requirements, as shown in the present study.
•Tandem comparison of input wind field and output wind load is crucial for validation of LES.•LES mesh can be designed based on existing high frequency requirements.•Statistical parameters other than peak converge slightly faster in LES than in BLWT.•The dependence of peak on duration remains similar in LES as in BLWT.
•Technique for fragility curves estimation for transmission towers in power networks.•Nonlinear static and dynamic analyses of 3D FEM of towers with joint details.•Efficient simulation of wind field ...and forces transferred from cables to towers.•Three-level probabilistic simulation to enhance accuracy/efficiency trade off.•Component-level failure linked to global failure via progressive collapse analyses.
The fragility model of transmission towers is one of the key elements for the hurricane risk and resilience assessment of regional power utility infrastructure. The current methods for fragility curve assessment of transmission towers rely on either static analysis or dynamic analysis with simplified demand parameters and limit state functions. The details of tower connections, such as joint eccentricities, rotational stiffness and slippage are often overlooked in fragility analyses due to the associated complexity of modeling and large computational effort. The purpose of this study is to model the realistic performance of transmission towers under hurricanes and develop a fragility curve estimation framework balancing between accuracy and efficiency.
The correlated multivariate wind load time histories on the tower are simulated, and the load transferred from the cables to the cross-arms of the tower are generated using an effective analytical model. Two types of three dimensional finite element models of transmission towers are developed in the OpenSees platform. One is a complex model with all the joint details, which requires a very large computational effort to conduct nonlinear dynamic analyses, and the other one is a simple model without all the joint details, which is more computationally efficient. A small-scale dynamic probabilistic simulation with the two models is conducted, and a method to replace the complex model with the simple model is developed and validated. Using a high-performance computing cluster, medium-scale nonlinear dynamic time history analyses are carried out to obtain the demands of individual structural members and their correlation. Component-level failures are obtained via large-scale numerical simulations and are linked to global collapse of the tower through element removal and progressive collapse analyses. A case study of two transmission towers under hurricane is carried out. The results indicate that, compared to a traditional tip displacement method, the proposed component-based method with detail driven finite element model and large-scale simulations leads to a more accurate assessment of the transmission tower fragility curve.
This paper studies reliability-based design optimization (RBDO) of a 5-MW wind turbine blade for designing reliable as well as economical wind turbine blades. A novel dynamic wind load uncertainty ...model has been developed using 249 groups of wind data to consider wind load variation over a large spatiotemporal range. The probability of fatigue failure during a 20-year service life is estimated using the uncertainty model in the RBDO process and is reduced to meet a desired target reliability. Meanwhile, the cost of composite materials used in the blade is minimized by optimizing the composite laminate thicknesses of the blade. In order to obtain the RBDO optimum design efficiently, deterministic design optimization (DDO) of the 5-MW wind turbine blade is carried out first using the mean wind load obtained from the wind load uncertainty model. The RBDO is then initiated from the DDO optimum. During the RBDO iterations, fatigue hotspots for RBDO are identified among the laminate section points. For an efficient RBDO process, surrogate models of 10-min fatigue damages
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at the hotspots are accurately created using the Kriging method. Using the wind load uncertainty model and surrogate models, probability of fatigue failure during a 20-year lifespan at the hotspots and the design sensitivities are calculated at given design points. Using the probability of fatigue failure and design sensitivity, RBDO of the 5-MW wind turbine blade has been successfully carried out, satisfying the target probability of failure of 2.275 %.
Abstract As the main supporting structure of wind turbines, the service quality of towers directly affects the operational safety of wind turbines. Researching tower vibration characteristics, ...evaluating its service quality and early warning of damage is an important means to improve the operational safety of wind turbines. Firstly, the external load conditions of the wind turbine tower were determined through wind load calculation, and the load and dynamic theory of the variable pitch torque wind turbine tower were summarized. Secondly, based on the Euler-Lagrange energy equation, the Lagrange equation of the simplified dynamic model of the wind turbine tower is established, and the vibration characteristics of the tower under different wind speeds are analyzed. The research results can provide a theoretical basis for the research on the characteristics of the wind turbine tower.
A study of the aerodynamic characteristics of steel tubular butterfly-shaped cross-arms in a transmission tower head under skew wind is presented. A wind tunnel test was conducted under skew wind by ...using a scaling model, and a numerical simulation was performed with a high-precision three-dimensional tower head model. The drag coefficient, skew wind load factor, and wind load distribution factor of the cross-arms obtained from the experiment and numerical simulation were identified and compared with those acquired from different codes. It is concluded that the indirect force measurement method was found to underestimate aerodynamic forces acting on windward cross-arms. For single cross-arm, the horizontal cross-arm showed stronger shielding effect than the inclined cross-arm. A fitting formula for the skew wind load factor of the cross-arm is proposed, and values obtained from it matched experimental and simulation data well. A comparison between calculated and measured wind load distribution factors showed that although longitudinal load distribution factors calculated with the Chinese standard (2018) were consistent with measured data, calculated transverse load distribution factors did not match measurements well. It is suggested that the effect of the transverse lift wind force should be considered in the calculation of wind load distribution factors.
•The aerodynamic characteristics of steel tubular cross-arms were investigated by wind tunnel test and CFD simulation.•The relationship between the divided horizontal and inclined cross-arms under skew wind was studied.•The prediction differences of aerodynamic coefficients from different codes was given.•The modified calculation formulas for skewed wind load factor and wind load distribution factor were given.
Structural analysis and wind load effects on parabolic trough solar collector (PTSC) plays a critical role in the efficient and reliable operation of solar thermal power plants. In this study, the ...likely effect of the different wind load conditions on the stability of the large aperture PTSC has been undertaken. The investigation is an extension of the coupled optical and thermal analysis of the large aperture of PTSC, considering the manufacturing standard following Euro Trough and the availability of the receiver size (70 mm–110 mm). Based on the optical and thermal analysis, it has been identified that 9 m is the largest achievable aperture size, with a 110 mm absorber having an intercept factor of 0.94. The analysis is performed for the various operating condition such as pitch angle (0°–180°), yaw angle (−90°–90°) and wind speeds (5 m/s to 25 m/s). Firstly, the wind load coefficients have been estimated to determine the pressure, forces and pitching moment on the PTSC. The maximum drag force is encountered for the pitch angle of 0°, i.e. 6888 N for the wind speed of 25 m/s. Based on the pressure and gravity force acting on the concentrator, the effort has also been made to estimate the deformation in the concentrator and same is compared with Euro Trough collector. The maximum variation in the deformation is observed as around 35% for the pitch angle of 30°.
AbstractA probabilistic approach is developed to estimate multivariate extreme wind loads by introducing copula theory. Three major concerns are addressed: (1) the characterization of pairwise ...dependence among extreme load coefficients, (2) the construction of a probability model of multivariate extreme load coefficients, and (3) the probabilistic estimation of multivariate extreme wind loads with randomness of the mean wind speed (i.e., wind climate change). Theoretical and numerical analyses are carried out with the aid of wind tunnel data. The results show that using rank dependence (Kendall’s tau and Spearman’s rho) is more appropriate than using Pearson correlation coefficient in defining dependence for extreme load coefficients. The Gaussian copula is convenient for deriving the joint distribution of multivariate extreme load coefficients but is not applicable for high-dimensional problems. In contrast, the vine copula is flexible and can provide a better estimate of the joint distribution function without dimension limitations. Multivariate annual maximum wind loads can be estimated via either first- or full-order methods. Dependence of the extreme load coefficients and randomness of the wind speed are both found having effects on the dependence of extreme wind loads. Moreover, the procedure of simulating multivariate annual maximum wind loads is presented to facilitate the use in practical problems.
