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Downbursts belong to sudden, local, and strong convection weather, which present significant destruction for structures. At any given time, there are approximately 2000 thunderstorms occurring on the Earth. Many studies have investigated the effects of downbursts on different structures. However, the extensive range of varying wind field parameters and the diverse representations of wind speeds render the study of structural wind effects complex and challenging under downbursts. This study firstly reviews the research of wind field properties of downbursts according to four common approaches, and the major findings, advantages, and disadvantages of which are concluded. Then, failure analysis of transmission line systems under stationary and moving downbursts is explored. The article also reviews the wind pressure on the roof of different kinds of low-rise buildings, and some dominant parameters, namely roof slope, distance of building from downburst center, wind direction angle, and so on, are discussed. Moreover, the wind effects caused by downbursts on high-rise buildings and some specialized structures are also considered because more and more wind hazards are related to downbursts. Finally, the limitations of the current study are pointed out, and recommendations for further research are given for the accurate assessment of the effects of wind on buildings, with a view to providing safer and more economical wind-resistant design solutions for structures. [ABSTRACT FROM AUTHOR]

Thunderstorm downbursts can impose critical wind loads on long-span transmission line structural system. Existing research has predominantly focused on stationary downbursts, with limited attention given to the analysis of travelling downbursts influenced by environmental wind and parent cloud motion. This paper utilizes the impinging jet method to simulate a travelling downburst at a small Reynolds number, employing the Delayed Detached Eddy Simulation turbulence model for solving the wall-bounded turbulent flow. The radial wind speed distribution of the travelling downburst is subsequently validated against previous experimental, numerical, and observational data. A comparative study is then conducted to discern the differences in the radial wind speed distribution between stationary and travelling downbursts. Finally, a parametric study investigates the influence of jet inlet speed, environmental wind and parent cloud motion on the maximum radial wind speed distribution of the travelling downburst. The findings reveal a significant disparity in the radial wind speed distribution between stationary and travelling downbursts. Notably, parent cloud motion, environmental wind, or their combined effects can alter the height providing the maximum radial wind speed. This variation has implications for the wind load distribution of travelling downburst across the conductor span of the transmission line structural system. [ABSTRACT FROM AUTHOR]

As the region with the highest amount of solar radiation, Qinghai–Tibet Plateau has the highest frequency of thunderstorms in China. Downburst generated in thunderstorm can cause high‐intensive wind near the ground, which is a critical threat to heliostats. Herein, the impact of the wind loads on the heliostat response under downburst is investigated using a proven finite‐element model and the influence is also compared with that under atmospheric boundary layer (ABL) wind. The results demonstrate that the downburst development process, the movement of the storm center, and the heliostat's elevation angle are highly associated with the wind‐induced response characteristics of the heliostat under downburst. The maximum displacement and stress of the heliostat with different elevation angles under the downburst are significantly larger than those under ABL wind. Additionally, when the elevation angle of the heliostat is 30°, the maximum value of the mirror's first principal stress under downburst is 1.88 times greater than it is under ABL wind. Therefore, the impact of wind‐induced vibration caused by downburst is necessarily considered on the safety of heliostat in Qinghai‐Tibet Plateau with abundant solar energy resources and high incidence of downbursts. [ABSTRACT FROM AUTHOR]

So far, wind loads on structures are almost exclusively evaluated by Extended Pressure Systems (EPS) cyclone models according to design codes. However, there is evidence that downburst wind fields, originating from thunderstorms (TSs) events, may have caused or contributed to the collapse of transmission line towers around the world. In this context, this work proposes a complete methodology to simulate a wind field from combined EPS (synoptic) and TS (nonsynoptic) storms. The proposed combined wind field model is applied to a real transmission line segment and the results are compared with those of the system subjected to the equivalent static loading method proposed in the IEC 60826/2003 code. The finite central differences explicit method is adopted for dynamic integration, taking into account nonlinearities. The assessment confirmed the localized and transient nature of the TS phenomenon. The results indicated that, for most of the elements analyzed, the equivalent static method for synoptic winds of IEC 60826/2003 resulted in a lower response than that obtained through the proposed combined model, which considers a TS carried by an EPS background wind. Thus, the proposed method to combine synoptic and nonsynoptic wind fields might be a useful tool to assist in the design of structures, such as transmission lines, which are susceptible to the effects of combined EPS and TS wind fields. [ABSTRACT FROM AUTHOR]

The generation of wind fields is of interest in the study of the structural performance of wind turbines in critical events, such as thunderstorm downbursts. Various methods ranging from the use of empirical data to employing computational simulations are typically adopted to study the response of wind turbines in downburst flow fields. While the former approach is limited in the ability to account for accurate and spatially resolved details of the flow field, the latter is expensive and, therefore, has limitations in its use. As an alternative, in this work, we propose a paused downburst model in which a snapshot of a time-dependent computational fluid dynamics (CFD) simulation is used to generate "mean" wind fields during thunderstorm downbursts. The developed model for the mean wind field is validated against recorded downburst data in the literature. The turbulent component of the wind field is generated using computationally inexpensive techniques based on Fourier-based power spectral density functions and coherence functions. In an illustrative example, the combined mean and turbulence wind fields are generated and applied on a utility-scale wind turbine to study structural load characteristics during a downburst event. [ABSTRACT FROM AUTHOR]

Long-span roofs are vulnerable to wind damage, especially from downbursts. Previous studies on downburst-induced wind pressure on these structures have primarily focused on the mean component while neglecting the fluctuating wind pressure. This study aims to analyze the fluctuating wind pressure on a long-span dome roof subjected to stationary downburst-like winds through synchronous wind pressure measurement tests. The experiment considers the impact of the radial distance, which refers to the horizontal distance between the downburst center and the dome roof. A detailed analysis is performed on the distribution condition, correlation coefficient, and coherence of fluctuating wind pressure. The results reveal that the worst fluctuating wind pressure occurs at a radial distance of approximately 1.4 to 1.7 times the downburst jet diameter (Djet). The strongest correlation of fluctuating wind pressure is observed for measurement points at the windward roof edge, owing to airflow separation. The magnitude of the correlation coefficient generally increases with increasing radial distance from 0.8Djet to 1.2Djet, but then decreases with a further increased radial distance. These findings can improve our understanding of downburst-induced fluctuating wind pressure on long-span roofs and provide valuable insights for the downburst-resistant design of relevant engineering structures. [ABSTRACT FROM AUTHOR]

This paper investigates the effects induced by thunderstorm downbursts to steel building structures that have been previously damaged during strong directivity ground motion events. To achieve this objective, one four-story steel moment-resisting frame that was tested at the E-defense laboratory, Japan was analyzed in the nonlinear range using OpenSees. The seismic response was numerically simulated, obtaining a satisfactory agreement with the experimental evidence, revealing that the effects of such wind events and vertical ground motions were significant. These effects should be addressed during the design of low and medium buildings subjected to initial damage and subsequent thunderstorm downbursts and the ductility demands on structures subjected to multi-hazards can be quantified. The wind loads are applied as an externally applied dynamic load and the revised ductility demands are determined directly. The obtained results are compared to what is expected by experimental tests. [ABSTRACT FROM AUTHOR]

Long-span roof structures are vulnerable to being damaged by downbursts due to the sensitivity of such structures to wind loads. However, there is still no simple and practical method for the downburst-resistant design of such structures to date. Inspired by the wind-induced vibration coefficient (WVC) frequently used by engineers, this paper proposes a downburst-induced vibration coefficient (DVC) to evaluate the response of a roof structure subjected to downbursts. The proposed DVC is defined as the ratio of the total response to the mean response of the structure due to the action of downburst wind loads. The influences of different parameters on the downburst-induced vibration response of a long-span single-layer spherical reticulated structure are first analyzed in detail. Then, using a statistical approach, a design DVC, which can be practically adopted for civil engineers to evaluate the total structural response, is proposed to facilitate the structural downburst-resistant design. With the influences of different structural parameters fully taken into consideration, the design DVCs of displacement and internal force are studied through orthogonal analysis. Finally, regression models of the design DVCs are presented, and their reliability is verified via the application to a case study. The verification results indicate that the proposed regression models of the design DVCs are reliable and can be utilized to simply calculate the structural downburst-induced vibration response. The results obtained can serve as a reference for the downburst-resistant design of long-span roof structures. [ABSTRACT FROM AUTHOR]

In the numerical simulation of stochastic nonstationary wind loads, rapidly varying wind speed fluctuations about a deterministic mean value typically represent the sole source of random variability. Adopted in deterministic-stochastic methods, this assumption may not fully capture the aleatory nature of a hazard, especially for nonsynoptic outflows such as intense thunderstorms or microbursts with noticeable nose-like mean wind velocity profiles that vary with each occurrence. Such profiles depart from the theory of atmospheric boundary layers. To introduce additional randomness in the digital simulation of these nonsynoptic wind fields and loads, a numerical procedure was developed in this study to generate unique realizations of analytical functions that describe changes in mean wind velocities with elevation. The utility of the proposed procedure for performance-based wind engineering (PBWE) is demonstrated for the fragility analysis of two monopole towers subjected to loads from nonsynoptic thunderstorm outflows. [ABSTRACT FROM AUTHOR]

• Loading provisions are developed to simulate the critical effect of downbursts on wind turbines. • A parametric study is conducted using the numerical model, HIW-TUR, to determine the critical profiles associated with the moving mean component of the downburst for different wind turbines sizes and configurations, while accounting for the uncertainty in the downburst location. • Dynamic analyses under turbulence are conducted using the numerical code, FAST, to determine the downburst gust response factor, which is superimposed to the mean component profiles to present a complete set of design load provisions. • An example is presented to demonstrate the application of the developed loading provisions on a real wind turbine. Wind turbines are among the rapidly growing sources of sustainable energy. The review of design codes for this type of structure shows a lack of procedures for estimating the wind loads on wind turbines due to high-intensity wind events such as downbursts. There is a challenge in the analysis and design of wind turbines under downbursts because of the sudden and localized nature of such events, which makes the forces acting on the tower and blades dependent on the characteristics of the event. As such, the objective of the current study is to develop design load provisions that can simulate the critical effect of downbursts on wind turbines. These provisions are developed in two phases. Identifying the peak effects of the moving mean component of the downburst and determining the associated critical profiles are the aim of the first phase. To achieve this task, a comprehensive parametric study is conducted using the previously developed numerical model, HIW-TUR, on a wide range of wind turbine sizes and airfoil types in order to assess the critical response of wind turbines under the mean component of downbursts. The study takes into account the variations in the downburst size and its location relative to the tower center. Then, dynamic analyses under turbulence are conducted in the second phase using the open-source code, FAST, to determine a downburst gust response factor to magnify the effect of the mean component. The downburst gust response factor is then superimposed with the mean component profiles to present a complete set of design provisions. An example is presented to demonstrate the application of the developed loading provisions on a real wind turbine and to compare with the extreme synoptic wind load cases specified by the International Electro-technical Commission Loading Code. [ABSTRACT FROM AUTHOR]

Outages of power due to transmission tower failures can cause social and economic disasters. Investigations of transmission line failures around the world have recorded that they are generally from high intensity winds from downbursts and tornadoes. Downbursts represent the greatest threat due to the extreme and extended wind events that they generate. Many studies have investigated the applications of these events on transmission line systems (TLS). However, the wide ranges for the different downburst parameters and the varying representations of downburst wind speeds, which are different from boundary layer wind profiles, have complicated the investigation of transmission line failures under these types of loads. This study reviews the research to date on TLS under downburst wind loads. It explores downburst wind loads, their simulation models and the structural behaviour of TLS under downburst wind loads. Modelling of TLS, static and dynamic analysis are all reviewed. Failure analysis, critical downburst parameters, ideal retrofitting methods to avoid such catastrophic failures, and optimization criteria of TLS are also discussed. Finally, recommendations for future research are made. [ABSTRACT FROM AUTHOR]

Collapse of transmission towers due to downbursts is often initiated by local failure of key structural members, while the local failure of key structural members is related to local material and geometrical nonlinearities. This paper presents a multi-scale finite element (FE) model for the failure analysis of transmission towers under downburst-induced wind loading. The potential local failure areas of the tower are modeled by shell or solid elements, and the remaining parts by beam elements. In this way, the failure of the tower can be accurately simulated on the one hand and the computational effort can be reduced on the other hand. This paper first introduces how to determine the downburst-induced wind loading on transmission towers. Both the conventional beam and multi-scale FE models of the transmission tower are then developed and used in the failure analysis. A comparison of the failure results obtained by the two FE models show that the multi-scale FE model can effectively simulate the stress concentration of angle members around the bolt connections and the cross-section plastic collapse of key structural members, leading to a different failure pattern for the tower from the conventional FE method. It is suggested that the multi-scale FE model should be used for better accuracy in the failure analysis of transmission towers under downburst loading. [ABSTRACT FROM AUTHOR]

Electrical transmission tower-line systems transport the needed electrical energy. These systems are spatially distributed and subjected to extreme natural environmental events such as downbursts. The capacity curve of a single tower has been reported under the downbursts but neglecting the turbulent wind component and potential dynamic tower-wire interaction. This study is focused on the assessment of capacity curve of a tower within the tower-line system considering the turbulent winds and the tower-wire interaction. The incremental dynamic analysis and the nonlinear static pushover analysis are employed to estimate the capacity curve. Moreover, comparison of such a capacity curve to that of a single tower is also presented. It was shown that the turbulent winds introduce variability in the capacity curve. The estimated capacity curve by using the nonlinear static pushover analysis is in close agreement to the average capacity curve obtained based on the curves estimated using the incremental dynamic analysis. Most importantly, it is shown that the capacity curve of a single tower represents adequately that of a tower within a tower-line system, and the effect of the dynamic interaction between the tower and the wires on the capacity curve is negligible. [ABSTRACT FROM AUTHOR]

A numerical investigation on the microburst-like wind characteristics in block array configurations has been performed using Computational Fluid Dynamics (CFD). The CFD modelling of impinging jet mimics a microburst wind shear. Effects of plan and frontal area densities on the drag and lift force acting on the arrays are studied by investigating the wall shear stress and pressure distributions. A semi-empirical model based on Poreh et al. (1967) is derived to estimate the spatially-averaged wall shear stress of the finite urban array located near the microburst storm centre. Moreover, the pressure and viscous drag force acting on obstacles in the arrays with different plan and frontal area densities are discussed and compared with the published results regarding the arrays placed in a neutrally stratified Atmospheric Boundary Layer (ABL) flow. The present results show that the viscous drag is insignificant relative to the total drag force for all the cases with different frontal and plan area densities (i.e. roughness packing densities). The mean vertical lift force acting on the arrays for various packing densities is discussed, and the lift force is compared with drag and resultant forces. The averaged lift force acting on a block in the array is 0.3–0.6 times of the magnitude of the resultant force. Therefore, it should be taken into account for the design and maintenance of high-rise buildings in cities. [ABSTRACT FROM AUTHOR]

Transient winds, such as thunderstorm downbursts, are the cause of design-load wind speeds in many countries. An understanding of the loading experienced by buildings during a downburst is therefore important to allow well designed and engineered buildings to be constructed. In contrast to boundary layer winds, the maximum wind speed in thunderstorm downbursts occurs as low as z m =30 m above the ground, within the range of heights of man-made structures, suggesting that the wind loading will be dependent on the building eaves height relative to z m . In a novel set of experiments, the University of Birmingham Transient Wind Simulator (a 1 m diameter impinging jet with aperture control) has been used to simulate a downburst striking buildings of different heights, ranging from below to above z m . Two forms of building have been used – a square-plan, flat-roofed structure, and a rectangular, portal-frame – at three angles (0°, 45° and 90°) relative to the radial wind direction. Pressure coefficients have been calculated (using eaves height velocity) over the roofs of these buildings, and are shown to be of greatest magnitude when the roof is above the region of maximum outflow velocity, with the exception of windward edges perpendicular to the flow, when they are generally greatest for the lowest building heights. [ABSTRACT FROM AUTHOR]

An experimental investigation is performed to assess the characteristics of the fluid–structure interactions and microburst-induced wind loads acting on a wind turbine model sited in microburst-liked winds. The experiment study was conducted with a scaled wind turbine model placed in microburst-like winds generated by using an impinging-jet-typed microburst simulator. In addition to quantifying complex flow features of microburst-like winds, the resultant wind loads acting on the turbine model were measured by using a high-sensitive force–moment sensor as the turbine model was mounted at different radial locations and with different orientation angles with respect to the oncoming microburst-like winds. The measurement results reveal clearly that, the microburst-induced wind loads acting on the turbine model were distinctly different from those in a conventional atmospheric boundary layer (ABL) wind. With the scales of the wind turbine model and the microburst-like wind used in the present study, the dynamic wind loadings acting on the turbine model were found to be significantly higher (i.e., up to 4 times higher for the mean loads, and up to 10 times higher for the fluctuation amplitudes) than those with the same turbine model sited in ABL winds. Both the mean values and fluctuation amplitudes of the microburst-induced wind loads were found to vary significantly with the changes of the mounted site of the turbine model, the operating status (i.e., with the turbine blades stationary or freely rotating), and the orientation angle of the turbine model with respect to the oncoming microburst-like wind. The dynamic wind load measurements were correlated to the flow characteristics of the microburst-like winds to elucidate underlying physics. The findings of the present study are helpful to gain further insight into the potential damage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of the wind turbines in thunderstorm-prone areas. [ABSTRACT FROM AUTHOR]

Having been identified as the cause of design load winds in many parts of the world, transient winds such as gust fronts and thunderstorm downbursts have been increasingly researched over recent years. The difficulties in simulating the flow structure of downbursts in the laboratory, particularly their rapid radial acceleration and associated ring vortices, have complicated measuring wind loads on structures subject to these conditions. The University of Birmingham Transient Wind Simulator (UoB-TWS, a 1 m diameter impinging jet with aperture control) has been used to simulate the transient aspects of downburst-like flow, allowing the pressure distributions they create over cube and portal framed structures to be measured for the first time, at model-scale (1:1600). Analysis of the velocity and pressure fields show that the simulator is capable of creating velocity fields which are similar to those observed in nature. Development of the ring vortex is demonstrated through phase-plot analysis. Two methods of calculating the turbulence intensity of the unsteady flow field have been used, giving mean values of between 3% and 10% depending on the method. Force coefficient time series have been estimated with the buildings angled at 0°, 45° and 90° to the radial wind direction. These are presented along with the instantaneous pressure coefficient distribution at the time of maximum roof suction. This novel research also highlights the difficulties of undertaking transient flow at model scale and drawing conclusions which are applicable to full-scale, i.e., where no two events are the same. [ABSTRACT FROM AUTHOR]

In the present study, an experimental investigation is conducted to quantify the characteristics of the microburst-induced wind loads (i.e., both static and dynamic wind loads) acting on a high-rise building model, compared to those with the test model placed in conventional atmospheric boundary layer (ABL) winds. The experimental study is performed by using an impinging-jet-based microburst simulator available at Iowa State University. In additional to conducting flow field measurements to quantify the flow characteristics of the microburst-like wind, both mean and dynamic wind loads acting on the test model induced by the microburst-like wind are assessed in detail based on the quantitative measurements of the surface pressure distributions around the test model and the resultant aerodynamic forces. It is found that the microburst-induced wind loads acting on high-rise buildings would be significantly different from their counterparts in conventional ABL winds. Both the static and dynamic wind loads acting on the high-rise building model were found to change significantly depending on the radial locations and the orientation angles of the test model in respect to the oncoming microburst-like wind. The dynamic wind loads acting on the test model were found to be mainly influenced by the periodical shedding of the primary vortices and the high turbulence levels in the microburst-like wind. The findings derived from the present study are believed to be useful to gain further insight into the underlying physics of the flow–structure interactions of high-rise buildings in violent microburst winds for a better understanding of the damage potential of microburst winds to high-rise buildings. [ABSTRACT FROM AUTHOR]

Abstract: Recognition of the significance of wind loading on low-rise buildings and transmission line structures during the passage of thunderstorm downbursts has prompted research aimed at modeling downburst wind fields and simulating wind flows. The non-uniformity and non-stationarity of thunderstorms in both time and space pose considerable challenges for effective simulation. In this paper, the evolutionary behavior of downburst winds is examined in the time–frequency domain using both stationary wavelet transform and Hilbert transform. Stationary wavelet transform first decomposes a sample of a multi-component non-stationary random process into a set of mono-component signals. These signals are subsequently transformed into analytic signals with the Hilbert transform, which yields the instantaneous amplitudes and frequencies. An efficient simulation approach is then proposed for thunderstorm downburst winds using the instantaneous properties as the basis. Simulation is performed based on a sample realization of the process without the customary assumptions of piecewise stationarity or parametric models. The method is extended to the simulation of multivariate random processes utilizing proper orthogonal decomposition. Analytical expressions for the statistical properties of the underlying random processes are formulated. Example simulations of measured full-scale downburst wind data are presented to demonstrate the efficacy of the proposed method. [Copyright &y& Elsevier]

Abstract: Downburst wind loads pose great threats to many structural systems and industrial aviation, especially large structures. Previous investigations on downburst wind events focused on steady state simulation and were then developed for unsteady state simulation. The total age of downburst events is rarely investigated, but the importance of the event period arises during any study of large extended structural systems such as transmission line systems, where they can experience most of the downburst period. In addition, the accumulated temporal profiles of downburst wind speeds are needed for dynamic analysis and for developing the intensity decay function that can simulate the temporal profile of the event. In this paper a developed parametric study is coupled with numerical CFD simulations to investigate several observed downburst events. The different parameters of the downburst events such as the total age, intensity period, decay period, downburst diameter, initial location, path direction and parent storm translation speed have been estimated by relating the recorded data to the coupled parametric-CFD study. Then an accurate period for depicting the actual age of downburst events is presented and a relationship between downburst sizes and their ages is developed. [Copyright &y& Elsevier]

Highlights: [•] Nonlinear inelastic response of a transmission tower under different wind events. [•] Nonlinear static pushover analysis is used to obtain capacity curves. [•] Capacity curves for different wind directions are used to construct a capacity surface. [•] Capacity under ABL and uniform wind loads provide approximate envelope for downburst events. [•] Findings could assist in evaluation of adequacy of existing towers under downburst loading. [Copyright &y& Elsevier]

An experimental study was conducted to quantify the flow characteristics of microburst-like wind and to assess the resultant wind loads acting on low-rise, gable-roof buildings induced by violent microburst-like winds compared with those in conventional atmospheric boundary layer winds. The experimental work was conducted by using an impinging-jet-based microburst simulator in the Department of Aerospace Engineering, Iowa State University. Two gable-roof building models with the same base plan and mean roof height, but different roof angle, were mounted over a homogenous flat surface for a comparative study. In addition to measuring the surface pressure distributions to determine the resultant wind loads acting on the building models, a digital particle image velocimetry system was used to conduct flow field measurements to reveal the wake vortex and turbulence flow structures around the building models placed in the microburst-like wind. The effects of important parameters, such as the distance of the building from the center of the microburst, the roof angle of the building, and the orientation of the building with respect to radial outflow of the oncoming microburst-like wind, on the flow features such as the vortex structures and the surface pressure distributions around the building models as well as the resultant wind loads acting on the test models were assessed quantitatively. The measurement results reveal clearly that when the building models were mounted within the core region of the microburst-like wind, the surface pressure distributions on the building models were significantly higher than those predicted by ASCE 7-05 standard, thereby induced considerably greater downward aerodynamic forces acting on the building models. When the building models were mounted in the outflow region of the microburst-like wind, the measured pressure distributions around the building models were found to reach a good correlation with ASCE 7-05 standard gradually as the test models were moved far away from the center of the microburst-like wind. It was also found that both the radial and vertical components of the aerodynamic forces acting on the building models would reach their maximum values when the models were mounted approximately one jet diameter away from the center of the microburst-like wind, while the maximum pressure fluctuations on the test models were found to occur at further downstream locations. Roof angles of the building models were found to play an important role in determining the flow features around the building models and resultant wind loads acting on the test models. The flow field measurements were found to correlate with the measured surface pressure distributions and the resultant wind loads (i.e., aerodynamic forces) acting on the building models well to elucidate the underlying physics of flow-structure interactions between the microburst-like winds and the gable-roof buildings in order to provide more accurate prediction of the damage potentials of the microburst wind. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a unified framework of a generalized gust-front factor for modeling winds in gust-fronts and their attendant load effects on structures. This is analogous to the gust loading factor, a widely recognized format used world-wide in codes and standards, for the treatment of gusts in conventional boundary layer winds. The generalized gust-front factor encapsulates dynamic features inherent in wind load effects in gust-fronts originating from winds in downbursts/thunderstorms. In view of the computational complexity of the formulation and the need to promote the usage of such a framework, a web-based portal is provided. The portal at http://gff.ce.nd.edu can be conveniently utilized to evaluate the generalized gust-front factor in conjunction with international standards. The proposed generalized gust-front factor reduces to conventional gust loading factor in the case of boundary layer winds, thus, it represents a most generalized dynamic wind load effects modeling framework on structures that is compatible with the format used in ASCE 7 or any other international standard. [Copyright &y& Elsevier]

Abstract: With an increasing focus on the issues of extreme wind events, particularly thunderstorms and downbursts, it is important not to lose sight of their impact with regards to other industrial structures, which are themselves sensitive even to typical flow fields. For the treatment of conventional boundary layer winds, a gust loading factor approach has been widely adapted in most international codes and standards for the design of buildings. Wind turbine towers, though fundamentally different from buildings when viewed as a multi-body system featuring a tower/nacelle and blades, may be treated much in the same way with modified gust loading factors. This paper introduces a possible approach to mesh conventional treatments of gust loading factors with those of newly introduced gust-front factors on wind turbine towers. Much like gust-front factors for buildings, this approach encapsulates many of the varied kinematic and dynamic changes associated with nonstationary characteristics of gust-fronts originating from thunderstorms/downbursts and the changing aerodynamics on wind turbine towers. From an example of a wind turbine tower system, gust-front loading effects as compared to those of boundary layer winds are investigated. [Copyright &y& Elsevier]

Two downburst wind fields with scales of 1 : 300 and 1 : 600 were simulated using adjustable guide vane, baffler and roughness elements in an atmospheric boundary layer wind tunnel. The simulated wind velocity profiles were compared with the empirical models and the structure of the wind fields were measured and investigated in detail. The wind pressure coefficient distributions on a large span cylindrical roof structure were tested in the simulated downburst wind field and compared with the results tested in field of exposure category B defined in GB 50009-2001. The results show that the horizontal velocities profiles of simulated downburst agree well with the empirical curves. The simulated profiles of vertical and horizontal velocities have the similar tendency and have good stability within the building model region. The tested mean wind pressure coefficient on the cylindrical roof in downburst is very close to that in exposure category- B. However, the extreme wind pressure coefficient in downburst is less than that in exposure category B. [ABSTRACT FROM AUTHOR]

During the past decade, many electrical transmission tower structures have failed during downburst events. This study is a part of a research program aimed to understand the behaviour of transmission lines under such localized wind events. The present study focuses on assessing the behaviour of self supported transmission line towers under downburst loading. A parametric study is performed to determine the critical downburst configurations causing maximum axial forces for various members of a tower. The sensitivity of the internal forces developing in the tower's members to changes in the downburst size and location was studied. The structural behaviour associated with the critical downburst configurations is described and compared to the behaviour under 'normal' wind loads. [ABSTRACT FROM AUTHOR]

Abstract: Localized severe wind events, in the form of downbursts and tornadoes, are responsible of the majority of transmission line structures failures in many regions around the globe. The wind profiles associated with these events are different than the conventional boundary layer wind profile that is typically used to design the supporting towers. A comprehensive study is presented in this paper to assess and understand the performance of transmission line structures under tornado loading. The study is conducted numerically using a three-dimensional finite-element model and incorporates velocity fields for various tornado scales, which are based on a computational fluid dynamics analysis. The numerical model accounts for the nonlinear behaviour of the conductors, ground-wires, and supporting guys. An extensive parametric study is conducted by varying the location of F4 and F2 tornadoes relative to the transmission line system. The study investigates the variation of the tower members’ internal forces with the tornado locations. It also provides an insight about the structural response of the towers under tornado wind loads. The dynamic effect associated with the translation motion of the tornado is assessed. Finally, the results of the parametric study are used to assess the sensitivity of the members’ peak forces with the parameters defining the location of the tornado relative to the line. [Copyright &y& Elsevier]

The majority of weather-related failures of transmission line structures that have occurred in the past have been attributed to high intensity localized wind events, in the form of tornadoes and downbursts. A numerical scheme is developed in the current study to assess the performance of transmission lines under tornado wind load events. The tornado wind field is based on a model scale Computational Fluid Dynamic (CFD) analysis that was conducted and validated in a previous study Using field measurements and code specifications, the CFD model data is used to estimate the wind field for F4 and F2 full scale tornadoes. The wind forces associated with these tornado fields are evaluated and later incorporated into a nonlinear finite element three-dimensional model for the transmission line system which includes a simulation for the towers and the conductors. A comparison is carried between the forces in the members resulting from the tornadoes, and those obtained using the conventional design wind loads. The study reveals the importance of considering tornadoes when designing transmission line structures. [ABSTRACT FROM AUTHOR]

Abstract: Thunderstorm generated gust fronts are responsible for various degrees of structural damage in many areas of the world. However, the resulting impact of gust front winds is not fully understood to such a level that their flow kinematics, dynamics and impact on structures can be quantified with some certainty. Gust front winds are transient in nature and have a flow profile which differs significantly from a typical boundary layer flow field. This study focuses on investigating the effects of this flow profile and its transient nature on the aerodynamics of bluff, prismatic bodies. A gust front type flow field is generated using a multiple fan wind tunnel and the resulting surface pressures are captured on a suite of prismatic models, which vary in size in relationship to the oncoming wind profile. The temporal variations in surface pressures are analyzed using traditional time, frequency and time-frequency domain schemes. Results indicate the changing nature of the surface pressure field in time, highlighting both qualitative and quantitative differences between local and area-averaged pressures under a host of flow profiles. [Copyright &y& Elsevier]

• A numerical model is developed to study the cascaded failure of transmission lines under downbursts. • An extensible catenary approach is utilized to predict the tension forces developed in the conductors. • The post failure geometry of the towers within the line is predicted. • A segment of an existing transmission line is considered as a case study. Transmission line's cascades have played a major role in several serious accidents around the world, as they are particularly prone to occurring in the presence of strong winds. The failure of a line component overloads the system, inducing a progressive failure that can involve many transmission line (TL) supports. Due to their localized nature, downbursts, in particular, cause unequal wind forces on various towers of a transmission line; the collapse of one tower can affect the adjacent towers through the unbalanced forces that develop in the conductors, which can lead to a cascaded-type of failure along the entire line. In this study, a numerical model is developed in-house to predict this type of line failure. A major challenge in this numerical model is the prediction of the geometric configuration of the tower and of the conductor forces after the tower collapses. Details of this unique numerical model are presented in this paper along with a case study for illustration. [ABSTRACT FROM AUTHOR]

Disaster investigation results have shown that most wind-induced damage to transmission towers is related to downbursts. To clarify the effects of downbursts' parameters on transmission towers with different heights, studies were conducted on five transmission towers with different diameters under static and moving downburst wind conditions. As a comparison, the responses of the towers under normal wind conditions were studied. The results showed that the effect of downbursts on the response of the transmission tower increased with the distance between the downburst center and the tower (r) when r < 1.0 Djet (Djet is the jet diameter of downburst) and then decreased when r > 1.0 Djet. The effects of jet diameter on the response of transmission towers with different tower heights were similar. As the jet diameter increased, the response of the tower continued growing until it reached a peak value and then steadily decreased soon thereafter. When the tower height was below 81.5 m, the wind load of the downburst on the transmission tower was significantly greater than that of the normal wind. As the tower height increased, the ratio of the transmission tower's response under the two types of wind fields rapidly declined to about 0.91–1.01. [ABSTRACT FROM AUTHOR]

Downburst is one of the high-intensity winds that cause transmission tower failures. The regulations of transmission tower-line systems under downburst wind loads cannot meet the design requirements at present. In this paper, the calculation formulas of the background and resonant components of transmission tower under downburst wind loads are obtained, based on the modal analysis theory of non-stationary wind for the single-degree-of-freedom system in the frequency domain. The effects of structural dynamic characteristics, damping ratio, and mean wind speed vertical profile on dynamic effect on structural response are discussed. Then the equivalent static wind load (ESWL) is obtained according to the maximum response and compared with the finite element method (FEM) in the time domain. Applications of these formulas are addressed to the cases from the empirical model of Holmes and field record of a rear flank downdraft (RFD). The results show that the maximum responses obtained by the current formulas match well with those from the modal decomposition method and dynamic analysis with FEM. The internal forces of tower members calculated by ESWL based on maximum response are closer to the results from FEM than those calculated by downburst loads recommended in ASCE guidelines. The presented framework can be used to assist the wind-resistant design of transmission towers considering downburst wind load. [ABSTRACT FROM AUTHOR]

A pulsed impinging jet is used to simulate the gust front of a thunderstorm downburst. This work concentrates on investigating the peak transient loading conditions on a 30 mm cubic model submerged in the simulated downburst flow. The outflow induced pressures are recorded and compared to those from boundary layer and steady wall jet flow. Given that peak winds associated with downburst events are often located in the transient frontal region, the importance of using a non-stationary modelling technique for assessing peak downburst wind loads is highlighted with comparisons. [ABSTRACT FROM AUTHOR]

This paper focuses on assessing the failure of one of the transmission towers that collapsed in Winnipeg, Canada, as a result of a microburst event. The study is conducted using a fluid-structure numerical model that was developed in-house. A major challenge in microburst-related problems is that the forces acting on a structure vary with the microburst parameters including the descending jet velocity, the diameter of the event and the relative location between the structure and the jet. The numerical model, which combines wind field data for microbursts together with a non-linear finite element formulation, is capable of predicting the progressive failure of a tower that initiates after one of its member reaches its capacity. The model is employed first to determine the microburst parameters that are likely to initiate failure of a number of critical members of the tower. Progressive failure analysis of the tower is then conducted by applying the loads associated with those critical configurations. The analysis predicts a collapse of the conductors cross-arm under a microburst reference velocity that is almost equal to the corresponding value for normal wind load that was used in the design of the structure. A similarity between the predicted modes of failure and the post event field observations was shown. [ABSTRACT FROM AUTHOR]

Downbursts are transient phenomena that produce wind profiles that are distinctly different from synoptic boundary layers. Wind field data from Computational Fluid Dynamics (CFD) simulations of isolated downburst-like impinging jets, are used to investigate structural loads of tall buildings due to these high intensity winds. The base shear forces and base moments of tall buildings of heights between 120 and 250 m produced by downburst winds of various scales are compared with the forces from the equivalent boundary layer gust winds, with matched 10-metre wind velocity. The wind profiles are mainly functions of the size of the downburst and the radial distance from the centre of the storm. Wind forces due to various downburst profiles are investigated by placing the building at different locations relative to the storm center as well as varying the size of the downburst. Overall it is found that downbursts larger than approx. 2,000m in diameter might produce governing design wind loads above those from corresponding boundary layer winds for tall buildings. [ABSTRACT FROM AUTHOR]

Past experience indicates that the majority of failures of electrical transmission tower structures occurred during high intensity wind events, such as downbursts. The wind load distribution associated with these localized events is different than the boundary layer wind profile that is typically used in the design of structures. To the best of the authors' knowledge, this study represents the first comprehensive investigation that assesses the effect of varying the downburst parameters on the structural performance of a transmission line structure. The study focuses on a guyed tower structure and is conducted numerically using, as a case study, one of the towers that failed in Manitoba, Canada, during a downburst event in 1996. The study provides an insight about the spatial and time variation of the downburst wind field. It also assesses the variation of the tower members' internal forces with the downburst parameters. Finally, the structural behaviour of the tower under critical downburst configurations is described and is compared to that resulting from the boundary layer normal wind load conditions. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations are employed to investigate the macro-flow dynamics and scale (Reynolds number) dependency of impinging jets with applications to downburst related high intensity winds. The flow is quasi-periodic with vortex rings, generated by the initial jet instability, impinging on the surface where they determine an unsteady separation-reattachment of the boundary layer. The maximum velocities are encountered at less than 0.05 jet diameters from the surface immediately after the touch down of the main ring-vortex. Due to the unstable separation-reattachment of the boundary layer the flow is Reynolds number (scale) dependent. The maximum in the mean velocity profiles increases and tends towards the wall as the Reynolds number increases. The unsteady Reynolds Averaged Navier–Stokes simulations provide time series which when fitted to the RFD full-scale data, (Gast, K.D., Schroeder, J.L., 2005. Extreme wind events observed in the 2002 thunderstorm outflow experiment. In: Proceedings of the 10th Americas Conference on Wind Engineering (10 ACWE), Baton Rouge, LA, USA), are used to estimate both the length and the velocity scales of the full-scale event. Moreover, these simulations provide a first spatio-temporal flow model for wind loading on structures due to downbursts. [Copyright &y& Elsevier]

• Structural effects of nonstationary wind loads (tornadoes, thunderstorms) are studied. • A general framework to assess performance capacity of tall structures is proposed. • Various model of tornado and thunderstorm wind fields are implemented. • New model for assessing the thunderstorm arrival rate is proposed. • Systematic comparison of life-cycle cost results between the two hazards is conducted. The continued development of performance-based wind engineering (PBWE) has led to the application of life-cycle cost assessment (LCCA) as a means to evaluate the risks and consequential losses over a structure's lifetime from damages caused by large-scale wind phenomena, such as synoptic storms and hurricanes. However, the advancement of this analytical tool with regard to non-synoptic, nonstationary windstorms such as downbursts and tornadoes has so far been lacking due to incomplete knowledge of either the wind fields or their induced fluid-structure forces. In this study, a methodology is proposed that extends PBWE and LCCA to characterize structural damages with respect to hazard parameters, describing these wind phenomena, and subsequently to quantify and compare the relative monetary losses that are incurred. The dynamics of two vertical structures subjected to wind loads of various intensities using multiple downburst and tornado wind field models and data from wind tunnel experiments will be analyzed. Key components of the methodology are tailored in the holistic framework to consider the risks and consequences posed by these nonstationary windstorms. [ABSTRACT FROM AUTHOR]

In many non-cyclone areas around the world, severe thunderstorm downbursts can produce annual gust peaks that are the design wind speeds of general structures, and the resultant wind loads pose a major potential threat to critical infrastructure, such as transmission line structural system. The flow fields generated by downbursts are different from those by the synoptic winds in the atmospheric boundary layer. In order to obtain a wind load model of downburst, scholars have made many attempts to study the flow characteristics of downburst-like events. However, in previous studies, the effects of different experimental setup parameters on distribution of downburst wind speed have not been fully discussed. In this paper, an experimental study is conducted using a pulsed wall jet experimental simulator located at Tohoku University, Japan, to analyse the flow fields of stationary and travelling downbursts, and the resultant wind load effects on transmission line structural system. The comparison results show that different experimental settings will affect the maximum wind speed obtained and the height providing the maximum wind speed. The translational speed ratio defined by the ratio of moving speed to jet inlet speed will affect the wind speed distribution in the longitudinal direction of the generated flow field of the travelling downburst. In addition, the comparative studies show that the wind speed distribution of travelling downburst cannot be simply expressed as the vector sum of stationary downburst wind speed distribution and the jet moving speed, and that speed of the travelling downburst can intensify the non-uniform wind loading relaying to the transmission line structural system. For the analysis of the downburst flow fields, a multi-degree-of-freedom analysis is needed to consider its three-dimensional effect, that is, the distributions of wind speeds along the three coordinate axes, and the comprehensive effects of influencing parameters. • Travelling speed can affect wind speed distribution of generated travelling downburst. • Wind speed distribution of travelling downburst cannot be simply expressed as vector sum of stationary downburst and jet moving speed. • Travelling speed can intensify non-uniform wind loading of travelling downburst relaying to transmission line structural system. • A multi-degree-of-freedom analysis is needed to consider three-dimensional effect of downburst wind fields. [ABSTRACT FROM AUTHOR]

Severe winds produced by thunderstorm downbursts pose a serious risk to the structural integrity of wind turbines. However, guidelines for wind turbine design (such as the International Electrotechnical Commission Standard, IEC 61400-1) do not describe the key physical characteristics of such events realistically. In this study, a large-eddy simulation model is employed to generate several idealized downburst events during contrasting atmospheric stability conditions that range from convective through neutral to stable. Wind and turbulence fields generated from this dataset are then used as inflow for a 5-MW land-based wind turbine model; associated turbine loads are estimated and compared for the different inflow conditions. We first discuss time-varying characteristics of the turbine-scale flow fields during the downbursts; next, we investigate the relationship between the velocity time series and turbine loads as well as the influence and effectiveness of turbine control systems (for blade pitch and nacelle yaw). Finally, a statistical analysis is conducted to assess the distinct influences of the contrasting stability regimes on extreme and fatigue loads on the wind turbine. [ABSTRACT FROM AUTHOR]