Tornado Wind Field Research Articles

Tornado‐induced vibration assessment of construction elevator attached to high‐rise buildings

SummaryAs one of major construction facilities, the safety concern about elevator outer‐attached to slender high‐rise buildings during construction needs to be seriously addressed. To specifically identify the risk of such slender building–elevator systems in extreme tornado wind field, a safety assessment strategy using the critical velocity envelope is systematically developed. To overcome the difficulties in modeling such building–elevator system, a modified generalized flexural‐shear model developed previously was employed to reach an efficient estimate on the response of a case high‐rise building under simulated tornado wind excitations. To account for the effect of the flexibility of high‐rise building on the response of its attached construction elevator, the response of the elevator was obtained by finite element analysis with applied wind forces, as well as the displacement excitations at its supports with the building. To simulate the typical failure modes of elevator in all states, some critical reference velocity‐based envelopes were generated for the safety assessment where the factors of wind velocity, wind direction, and cage level are systematically addressed. The safety envelopes of the building–elevator system were proved to be instinctively efficient for the assessment of its wind resistance under various scenarios.

Study on wind load characteristics of stadium under tornado

The stadium is a wind-sensitive structure, and the tornado has a greater damage to the stadium. The Ward tornado generating device was used to simulate the wind load characteristics of the stadium during tornado action in order to investigate the wind load characteristics of the stadium. The results of this paper are compared with the experimental results to verify the accuracy of the simulation method. The wind load characteristics of the stadium and its most vulnerable location during a tornado are ascertained by examining the wind pressure coefficient and wind speed distribution law of the stadium’s upper and lower surfaces with different swirl ratio. The influence of uniform wind and wind profile on the wind load of the stadium is explored by comparing and analyzing the wind pressure coefficient of the stadium surface with and without wind profile at the entrance of the wind field. The impact of varying ground roughness on the stadium is compared in order to ascertain how it affects the wind speed, stadium wind pressure, and vortex core radius. The vorticity of the stadium in the tornado wind field with different swirl ratios was compared and analyzed, and the influence of the stadium on the vorticity development of the wind field was explored. This study reveals the effects of swirl ratio, ground roughness and wind profile on the wind pressure, wind speed and vorticity of the stadium under tornadoes, and determines the most vulnerable location and the most unfavorable factors. The above research is helpful to provide reference for disaster prevention and mitigation of stadium structure design.

An Approach for Assessing Misclassification of Tornado Characteristics Using Damage

Abstract Tornado characteristics (e.g., frequency and intensity) are challenging to capture. Assessment of tornado characteristics typically requires damage as a proxy. The lack of validation in the enhanced Fujita (EF) scale and the likelihood of rural tornadoes suggests that tornado characteristics are not accurately captured. This paper presents an approach to quantify the potential misclassification of tornado characteristics using Monte Carlo simulation for residential structures in rural areas. An analytical tornado wind field model coupled with fragility curves generates degrees of damage (i.e., DOD) from the EF scale in a wind speed–to-damage approach. The simulated DODs are then used to derive damage-to–wind speed relationships built from the National Weather Service Damage Assessment Toolkit (NWS DAT). Comparisons are then made between the simulated tornado characteristics and those derived from damage. Results from the simulations show a substantial proportion of tornadoes were “missed” and path width and pathlength on average are underestimated. An EF4 rating based on damage is favored for EF3–EF5 simulated tornadoes. A linear regression was utilized and determined damage-based wind speeds of different percentiles, damage length, damage width, and the number of structures rated at a particular DOD were important for prediction. The distribution of DODs was also used to predict wind speed and the associated intensity rating. These methods were tested on actual tornado cases. Tornadoes that have the same damage-based peak wind speed can be objectively assessed to determine differences in overall intensity. The results also raise questions about the level of confidence when assessing wind speed based on damage.

Tornado-affected safety assessment of tower cranes outer-attached to super high-rise buildings in construction

To fully address the safety concern of construction tower cranes outer-attached to super high-rise buildings in tornado wind fields, the safety envelopes of cranes under various scenarios are constructed. To more precisely obtain the time history of wind forces applied on the building-crane system in a tornado environment, the working conditions of cranes and the feature of the tornado field are considered in the development of various spatio-temporal computational fluid dynamics (CFD) models. To avoid sophisticated finite element analysis of the main building, the modified generalized flexural-shear model (FSM-MS) is used to estimate the along-wind and across-wind displacement response of main building and the response at the connection support of the tower crane. To reproduce possible failure modes of tower cranes as many as possible, the similar concept of incremental dynamic analysis was adopted to generate the safety envelopes of outer-attached tower cranes in various normal and typical abnormal scenarios. By this, whether a tower crane of concern is safe or not can be efficiently assessed by the ultimate wind velocity obtained from the envelope corresponding to certain working conditions. The feasibility of the safety assessment procedure has been systematically demonstrated on three case super high-rise buildings. The ultimate reference wind velocity of the tower crane was observed to be more sensitive to slewing angle. And, more attention should be paid to the issues of overloading and stiffness reduction of connection supports. The outcome from this concern has definitely enriched the framework of construction safety control management.

Reliability of electric power grids under tornadoes

Strong wind disasters, especially tornadoes with high damage scales, constantly cause huge damage to electric power grids (EPGs). In this paper, a concept is presented to quantify the connectivity reliability of EPGs under tornadoes. First, a tornado wind field is established using a modified Rankine vortex model, which is a representative 2D tornado wind field model. Second, fragility models of grid components, including transmission substations, transmission support structures, distribution poles, distribution conductors, and local circuits, are introduced. Third, a Monte Carlo simulation is presented to evaluate the connectivity reliability of EPGs under tornadoes. Finally, the connectivity reliability is verified under different scales and propagation angles of tornadoes by taking an actual EPG in China as a case study. Results show that the connectivity reliability directly correlates with the maximum wind speed and the propagation angle of the tornado, and the EPG experiences severe damage when the tornado exceeds the Enhanced Fujita 3 scale.

A three-dimensional six-degree-of-freedom windborne debris trajectory model for tornadoes

Windborne debris is a major cause of damage to the built environment during tornadoes. A fundamental step for quantifying this damage is to provide models that are able to describe the trajectory of the flying debris and thereby determine their impact location and associated momentum/energy. This goal can be achieved through the adoption of an appropriate three-dimensional (3D) six-degrees-of-freedom (6DOF) debris trajectory model. However, existing 3D 6DOF debris trajectory models are focused on describing the debris flight in straight line winds and therefore cannot be applied to tornado winds that are characterized by relatively small but rapidly rotating vortices. This paper presents a 3D 6DOF debris trajectory model for describing the flight of windborne debris in tornadoes. The proposed solution strategy is based on a predictor-corrector time-marching scheme which solves the equations of motion for each time step while updating the wind field from an appropriate tornado wind field model. A convergence analysis is carried out to determine the suitable time step that balances the accuracy of the numerical scheme with the computational run-time. The proposed model is then used to show the significant difference in modeling the debris trajectories in tornado wind fields as compared to straight line winds. • A six-degree-of-freedom windborne debris trajectory model for tornadoes is presented. • The proposed solution strategy is based on a predictor-corrector method. • The time step is determined by balancing accuracy and efficiency. • Limits of straight-line winds in modeling the flight of debris in tornadoes is shown.

Tornado wind hazard mapping and equivalent tornado design wind profile for Canada

Tornadoes cause property damage and fatalities. A systematic assessment of tornado wind velocity hazard is presented in the present study for Canada based on reported historical tornado catalogue. For the assessment, the Poisson model, zero-inflated Poisson model, and negative binomial model, as well as the adaptive Gaussian kernel smoothing technique, are used to model tornado occurrence. The modeling takes into account the tornado reporting bias due to population density and uses the cloud-to-ground lightning flash density and the thunderstorm days as the explanatory variables. Statistical analysis results indicate that the negative binomial model is preferable.The tornado wind velocity hazard maps for Canada, in terms of wind velocity at 10 m height above the ground surface and return period T, VT(10), are developed based on an available probabilistic tornado wind field model. Also, a simple equivalent tornado design wind profile is developed based on the return period value of tornado wind speed at different heights. The developed tornado design wind profile only depends on VT(10), which greatly facilitates its potential use for structural design subjected to tornado wind load and its possible implementation in structural design code and standards.

Numerical model for analysis of wind turbines under tornadoes

According to the Global Wind Energy Council, the installation rate for wind turbines is accelerating around the world, leading to the introduction of new wind turbine farms in regions where wind is abundant. As a result, the number of wind turbine towers prone to severe wind loads is increasing, as is the rate of failure due to unexpectedly strong wind loading. Wind turbines are typically designed to resist the synoptic wind loads specified in current International Electrotechnical Commission (IEC) guidelines, but these standards do not account for High-Intensity Wind (HIW) events such as tornadoes or downbursts. Because of the localized nature of HIW events, identifying critical locations that result in peak forces acting on the tower and blades is a challenging task. For this reason, a built-in-house numerical model has been developed in this study for simulating a three-blade horizontal-axis wind turbine tower exposed to 3D tornado wind fields. An extensive study has been conducted with the goal of determining both the critical location of a tornado that will cause peak straining actions on the tower and blades, and the optimal pitch angle that will minimize the effects of that tornado. For the considered wind turbine, the minimum straining actions on the blades were found to occur when the pitch angles are 60°, while the minimum straining actions on the base of the tower takes place when the pitch angles are 15°. A comparison of the simulation results with the extreme wind load scenario, stated in the current IEC guidelines, revealed that the predicted straining actions on the blades arising from the conditions specified in the IEC recommendations are less than the straining actions due to the F2 tornado considered in the study.

A framework to compare wind loads on low-rise buildings in tornadoes and atmospheric boundary layers

This paper presents a framework to compare the building aerodynamics measured in a tornado-like vortex simulator and a typical boundary layer wind tunnel. The method relies on the hypothesis that the aerodynamic and static pressures can be separated and on simultaneous wind and pressure measurements. Test data from a tornado-vortex simulator and a boundary layer wind tunnel are presented to illustrate the method. The simulated tornado had a swirl ratio such that the core had multiple vortices. A building model was placed to the side of the translation path, at a position 0.8 core radius diameters from the tornado center. Surface pressures were measured on both a circular ground plate and the building model surfaces, synchronized with the velocities measured at four cobra probes placed near each corner of the building. A large set of nominally identical runs were used to obtain ensemble statistics of both pressures and velocities. The static pressure induced by the tornado was removed from the overall building pressures using a conditional average of the ground plane tornado pressure field. A quasi-steady (QS) vector model was developed in boundary layer results for the same building and then used to compare the aerodynamic results in the tornado, based on the velocities measured by the probes near the building in the tornado wind field. The QS model is found to predict the order of pressure variations but misses the local pressure distributions. The estimations are greatly improved when the upstream wind azimuth angles are shifted in the QS model to the direction of tornado rotation. The degree of the required azimuth shift for better predictions are shown to be governed by the curvature of the tornado wind field in the horizontal plane near the building. Furthermore, this curvature of the tornado wind field is observed to reduce the size of separation bubbles on the leeward walls, increasing the magnitude of suctions on these walls. The rotating multi-cell vortices confined in the tornado core also produce high pressure fluctuations that are not well captured by the QS model.

Damage Analysis of Three Long-Track Tornadoes Using High-Resolution Satellite Imagery

Remote sensing of tornado damage can provide valuable observations for post-event surveys and reconstructions. The tornadoes of 3 March 2019 in the southeastern United States are an ideal opportunity to relate high-resolution satellite imagery of damage with estimated wind speeds from post-event surveys, as well as with the Rankine vortex tornado wind field model. Of the spectral metrics tested, the strongest correlations with survey-estimated wind speeds are found using a Normalized Difference Vegetation Index (NDVI, used as a proxy for vegetation health) difference image and a principal components analysis emphasizing differences in red and blue band reflectance. NDVI-differenced values across the width of the EF-4 Beauregard-Smiths Station, Alabama, tornado path resemble the pattern of maximum ground-relative wind speeds across the width of the Rankine vortex model. Maximum damage sampled using these techniques occurred within 130 m of the tornado vortex center. The findings presented herein establish the utility of widely accessible Sentinel imagery, which is shown to have sufficient spatial resolution to make inferences about the intensity and dynamics of violent tornadoes occurring in vegetated areas.

NUMERICAL SIMULATION ON AERO-DYNAMICAL EFFECTS OF HIGH-RISE BUILDINGS IMPACTED BY TORNADOES

A tornado is a large scale swirling flow and is of extreme destructive power. It is one of the most critical targets of wind resistance and disaster reduction projects. Previous studies mostly focused on tornadoes attacking low-rise buildings and steel transmission towers, etc., and paid little attention to the flow characteristics and aero-dynamical responses of high-rise buildings in urban environment when attacked by tornadoes. This study conducted large-eddy simulations on tornado wind field and its interaction with building models in scales of laboratory simulator. The results were validated with experimental values, obtaining some wind load characteristics and flow mechanisms in laboratory scales. Subsequently, full-scale high-rise buildings with typical cross-section shapes attacked by a full-scale F2 tornado were simulated based on the validated methods and models. The flow field characteristics and aerodynamic load response characteristics in full-scale cases were evaluated. The flow field differences and aerodynamic load generation mechanism for full-scale tornado impacting different scales and shapes of high-rise buildings were obtained.

Computer modelling of close-to-ground tornado wind-fields for different tornado widths

Tangential velocity (Vt) of tornadoes is the major parameter that causes building damage. In-field tornado measurements are less reliable at less than 20 m above ground level (AGL). Laboratory tornado simulators suggest that swirl ratio (S) and radius (ro) are the major tornado parameters that influence the Vt. However, due to scaling problems, the laboratory simulators also report the Vt at greater than 20 m AGL. Well-refined computational fluid dynamics (CFD) models can evaluate the Vt at less than 10 m AGL. However, the CFD models are limited to ro = 1.0 km, and the effect of ro on Vt is not investigated. The aim of this study is to investigate the maximum Vt for different ro close to ground. Simulation results show that increasing ro decreases the maximum Vt with respect to Vro. Moreover, by increasing ro, the corresponding elevation of occurrence of maximum Vt (zmax) will increase. However, for all tornado radii, the zmax is between 20 m and 64 m AGL. In addition, results show that for all ro, the radial Vt profile has two peaks at z < 10 m AGL due to strong shear force close to the ground and at higher elevation the profile transits to Rankine Combined Vortex Model (RCVM).

The calculation of train stability in tornado winds

This paper presents a novel methodology for calculating the risk of a train overturning accident due to tornadoes. It applies a recently developed model of tornado wind fields to the complex case of a moving vehicle passing at different distances from the centre of a moving tornado. The wind speed and direction relative to the vehicle can thus be calculated. Through the use of quasi-steady force coefficients and an allowance for dynamic effects, this allows aerodynamic forces and moment time histories on the vehicle to be calculated. A parametric investigation of the effects of tornado size, strength and translational speed, and vehicle speed is then presented. A stochastic analysis methodology is then set out that allows the probability of a train overturning accident to be determined for specified statistical distributions of tornado parameters and vehicle operational parameters. It is shown that the reduction of train speed at times when tornadoes are expected would lead to a very significant reduction in accident risk. Finally the requirements for further work to refine the methodology are set out – specifically the need for statistical distributions of tornado parameters and for force and moment coefficients obtained from proper physical or numerical simulations of tornado characteristics.

A conceptual model for wind and debris impact loading of structures due to tornadoes

This paper presents a novel conceptual design framework which takes into account the direct wind loads and pressure loads acting on a structure due to the passing of a tornado. Furthermore, for the first time, the potential damage due to debris impact has been incorporated enabling a holistic assessment of structural loading to be considered. The model is built on a recently developed wind and pressure field model that captures the main features of tornadoes, which is used to generate a large number of tornado wind and pressure field realisations from which values of particular load effects can be determined. A cumulative distribution function of load effect is thus derived, which can be combined with tornado climatology probabilities to determine load effects at a particular risk level. This use of this framework is illustrated through two examples – the direct wind and pressure loads on a low-rise portal frame structure, and the debris loads on a medium rise rectangular structure.

Doppler radar-derived wind field of five tornado events with application to engineering simulations

Doppler radar data corresponding to five tornado events are analyzed using the Ground-Based Velocity Track Display method and the three-dimensional velocity field of nine volumetric samples is extracted. These samples are selected to cover a range of wind speeds (between 36m/s and 64m/s) and vortex structures representative of EF0 to EF3 tornadoes in a first attempt to generate a tornado wind field database. Tangential velocity profiles, swirl ratios and vortex structures, i.e. single-celled or two-celled vortex, are determined for each of these volumetric samples.Among the nine volumetric samples, two show single-celled characteristics, vortex breakdown bubble is evident in one and four demonstrate two-celled vortex characteristics. The radial profiles of the tangential velocity are in good agreement with a modified Rankine vortex model. The variation of maximum tangential velocities with height is very different when compared to the velocity variation in typical atmospheric boundary layer flows. The swirl ratios of the tornado volumetric samples are computed using the flow rate through the updrafts and the maximum circulation in the flows.

Modelling wind fields and debris flight in tornadoes

This paper describes the derivation of a simple yet realistic engineering model of tornado wind and pressure fields. This novel model is shown to be capable of providing a method for predicting wind speed and pressure time histories and debris impact energies that can ultimately be used in the development of a rational risk-based design methodology for tornado wind loads on buildings. A stationary one-cell tornado vortex is first considered, and the circumferential and vertical velocities and pressure profiles derived from a simple assumption for radial velocity (that is bounded in the radial and vertical directions) and the use of the Euler equations. The generalisation of this model to a two-cell tornado form is then set out. This model is then used to investigate the trajectories of wind borne debris in tornado wind fields, and for the first time, this analysis reveals the important dimensionless parameters of the problem and the parameter boundary between falling and flying debris. An asymptotic long time solution for debris paths is also derived.

Empirical Approach to Evaluating the Tornado Fragility of Residential Structures

Tornado-induced wind load modeling has advanced significantly in recent years, but comparison of the experimental or numerical models to observed tornado damage is limited. This paper describes the development of empirically derived tornado fragility functions for residential structures following the 22 May 2011 Joplin, Missouri tornado. The fragility functions were created by combining a residential structure damage assessment of the Joplin tornado using the enhanced Fujita (EF) scale and a tornado wind field model conditioned to tree-fall patterns. The damage states for the fragility functions used the degrees of damage (DOD) for one- and two-story residential structures from the EF scale. The empirical tornado fragility functions were compared to analytically derived fragility functions for straightline winds from the FEMA Hazus hurricane model to provide a first attempt at ascertaining how different or similar failure wind speeds are for residential structures under tornado-induced and straightline wind loads. Median failure wind speeds from the empirical tornado fragility functions increased monotonically with DOD, from 33.4 m/s for DOD1 to 85.6 m/s for DOD9. Median failure wind speeds of the empirical tornado fragility functions and Hazus hurricane straightline wind fragility functions differed by 5% or less when suburban terrain was assumed throughout the damage path, and 25% or less when open terrain was assumed. The results suggest that tornado load amplification factors are at worst no more than 55% for residential structures, and may be lower than 10%.

Analysis of load characteristics and responses of low-rise building under tornado

Abstract In this paper, the numerical model of the tornado generator is established by using the computational fluid dynamics (CFD) method. The radar observed wind profiles of Spencer tornado (1998) are used as inlet boundary condition to form tornado wind field, and the influence of surface roughness is taken into account. This method is used to generate the tornado wind field with the same intensity as recorded in our country, and the wind load characteristics of low-rise building located in different positions in the tornado wind field are analyzed. Then the finite element model of the building is established by ANSYS software, and the building is considered as steel frame-steel plate composite wall structure, which rationality of the finite element model is determined by the modal analysis. The one-way fluid structure interaction method is adopted to transfer the tornado load to the surface of the finite element model, and the displacement response, deformation and damage state of the wall surface and roof surface of the building in different locations of the tornado wind field are analyzed in detail.

Behaviour of transmission line conductors under tornado wind

Electricity is transmitted by transmission lines from the source of production to the distribution system and then to the end users. Failure of a transmission line can lead to devastating economic losses and to negative social consequences resulting from the interruption of electricity. A comprehensive in-house numerical model that combines the data of computational fluid dynamic simulations of tornado wind fields with three dimensional nonlinear structural analysis modelling of the transmission lines (conductors and ground-wire) is used in the current study. Many codes of practice recommend neglecting the tornado forces acting on the conductors and ground-wires because of the complexity in predicting the conductors' response to such loads. As such, real transmission line systems are numerically simulated and then analyzed with and without the inclusion of the lines to assess the effect of tornado loads acting on conductors on the overall response of transmission towers. In addition, the behaviour of the conductors under the most critical tornado configuration is described. The sensitivity of the lines' behaviour to the magnitude of tornado loading, the level of initial sag, the insulator's length, and lines self-weight is investigated. Based on the current study results, a recommendation is made to consider conductors and ground-wires in the analysis and design of transmission towers under the effect of tornado wind loads.

Reconstruction of a near-surface tornado wind field from observed building damage

In this study, residential building damage states observed from a post-tornado damage survey in Joplin after a 2011 EF 5 tornado were used to reconstruct the near-surface wind field. It was based on well-studied relationships between Degrees of Damage (DOD) of building and wind speeds in the Enhanced Fujita (EF) scale. A total of 4,166 one- or two-family residences (FR12) located in the study area were selected and their DODs were recorded. Then, the wind speeds were estimated with the EF scale. The peak wind speed profile estimated from damage of buildings was used to fit a translating analytical vortex model. Agreement between simulated peak wind speeds and observed damages confirms the feasibility of using post-tornado damage surveys for reconstructing the near-surface wind field. In addition to peak wind speeds, the model can create the time history of wind speed and direction at any given point, offering opportunity to better understand tornado parameters and wind field structures. Future work could extend the method to tornadoes of different characteristics and therefore improve model's generalizability.