High-frequency Force Balance Research Articles

Analysis of wind vibration characteristics of a slab‐type high‐rise residential building

AbstractA slab‐type high‐rise residential building with a depth‐to‐width ratio exceeding 6 was studied. Multi‐point surface pressure measurements and high‐frequency force balance (HFFB) wind tunnel tests were conducted to investigate the wind‐induced dynamic characteristics of the building. The dynamic responses, especially accelerations closely related to serviceability performance, were determined and examined in both time and frequency domains. The spectral properties of the time‐history acceleration responses were compared with frequency‐domain results. Analyses showed acceleration responses were amplified when along‐wind or across‐wind directions aligned with the shorter building axis. The slab‐type cross‐section reduced vortex shedding in the across‐wind direction. As a result, peak accelerations were often dominated by along‐wind excitations. Torsional accelerations approached those induced by along‐wind winds, likely due to both fundamental and higher modes.

Effects of corner modification on the Strouhal number of high-rise buildings under skewed wind

In recent years, the effectiveness of building corner modification measures in reducing wind loads on high-rise buildings has attracted great attention. The Strouhal number is a critical parameter in calculating the wind-induced response and equivalent static wind load of high-rise buildings in the across-wind direction. However, few studies have investigated the effects of corner modification on the Strouhal number of high-rise buildings under skewed wind conditions. Therefore, twenty-five models with different corner modifications were examined using high-frequency force balance (HFFB) wind tunnel tests in this study. Based on the experimental results, the base force coefficients and Strouhal number of the high-rise buildings with three types of corner modifications are discussed and compared with those of the square model. It was found that these three types of corner modifications are all effective in reducing the maximum mean and standard deviation of base force in the across-wind direction, with the reduction effect increasing as the corner modification rate increases. Three states of vortex shedding under specific corner modifications are identified by utilizing wavelet transformation. Formulas to evaluate the Strouhal number considering the effect of corner modification types and wind direction angles are fitted. The fitted formulas coincide well with the experiment data.

A comparison of high frequency force balance mode shape correction approaches for predicting wind effects on a high-rise building

A comparison of high frequency force balance mode shape correction approaches for predicting wind effects on a high-rise building

A corner protrusion strategy for the aerodynamic response mitigation of tall buildings with the aim of using non-structural components

Side protrusion, i.e., the addition of material on the side of a tall building, has been demonstrated as an effective strategy to mitigate the aerodynamic responses under a broad range of design wind speeds. This study investigates the possibility of producing similar behavior of side protrusion via a corner protrusion strategy. The proposed strategy is evaluated using two parameters, which are the gap ratio (GR) and the protrusion ratio (PR). High-frequency force balance (HFFB) wind tunnel testing was carried out to examine the aerodynamic performance of ten models under different wind angles. The results demonstrate that the corner protrusion strategy can achieve similar performance as the side protrusion strategy using nonstructural components (NCs). The reductions of base overturning moment (OTM) for model CP-14–86 are 33%, 39%, 33%, and 24% at the mean hourly design wind speeds (cities) of 42 m/s (San Francisco), 53 m/s (New York), 62 m/s (Houston), and 77 m/s (Miami), respectively. The effectiveness of the corner protrusion strategy occurs when the PR is larger than 6%. The benefits of using NCs to reduce wind responses of tall buildings are discussed, which is expected to be a competitive aerodynamic strategy not only for new buildings but also for retrofitting existing buildings.

Experimental study on the aerodynamic characteristics of combined angle transmission tower subject to skew wind

The transmission towers served as crucial carriers in the process of power transmission. The combined angle transmission tower of dual-angle steel with cruciform section are relatively a novel type of angle tower, which is being more widely used in transmission lines. The studies on aerodynamic characteristics of combined angle tower in skewed wind are limited. In this paper, the wind tunnel tests are conducted to investigate the aerodynamic characteristics of the combined angle tower, and the high frequency force balance (HFFB) test is introduced to obtain the transverse and longitudinal forces on the tower using the direct force measurement (DFM) method. The drag coefficients and wind load-distribution factors of the combined angle tower are obtained from the wind tunnel tests, and the effects of interference and tower leg spacing on the wind loads of the tower legs are fully explored. It can be found that the aerodynamic coefficients and drag coefficients of the combined angle steel tower leg tend to decrease with the increase of interference members and it also decreases with the decrease of the tower leg spacing. The test wind load-distribution factors of the isolated leg are generally larger than those defined by seven typical codes (e.g., US, EU, Japanese, and Chinese codes) and significantly exceed the code-specified values under partial wind angles. The observations obtained from this study may provide helpful guidance on the wind-resistant design of this relatively novel type of combined angle tower.

Prediction of along-wind loading on tall building based on two-dimensional aerodynamic admittance

The mismatch of simulated turbulence integral scale may occur in some wind tunnel model tests, especially in a short wind tunnel, which would lead to inaccurate prediction of the wind loading based on traditional aerodynamic admittance (traditional AAF) containing the three-dimensional effect (3D-effect). For this, a method for prediction of along-wind loading on tall building based on two-dimensional aerodynamic admittance (2D-AAF) is introduced. The key to the method is to identify the 2D-AAF which does not contain the 3D-effect through separating the 3D-effect from the traditional AAF. With this method, the 2D-AAFs of the along-wind loading of a CAARC building model in two turbulent flow fields with different turbulence integral scales are identified. As expected, the traditional AAF which contains 3D-effect changes with the turbulent flow field because 3D-effect is related to the turbulence integral scale. Different from the traditional AAF, the 2D-AAFs obtained in two different turbulent flow fields are almost consistent, confirming that the 2D-AAF is independent of the turbulence integral scale. An empirical expression for the 2D- AAF of along-wind loading of CAARC building is then proposed by fitting the experimental data. A comparison between the high-frequency-force-balance (HFFB) measurements conducted in a simulated boundary layer flow field and the prediction values shows that the reproduced spectrum of along-wind loading based on the 2D-AAF has high accuracy, verifying the validity of proposed prediction method.

Experimental Study of Wind Loads on Tubular Crossarms of Transmission Towers

The actions exerted by yawed wind on the crossarm substantially differ from those on the tower body due to the presence of the angle between the crossarm’s cross-section and the incoming flow, and this phenomenon deserves a better understanding. Focusing on the tubular crossarms with round-section members, this paper aims to determine insights into wind actions, as affected by various factors, such as the tower type, the mean width to mean height ratio, the solidity ratio, and the wind incidence angle, and thereby derive the wind loads for design. Accordingly, a succession of wind tunnel tests was conducted by using quadrilateral and triangular crossarm models with various solidity ratios and mean width to mean height ratios. The method of direct force measurement (DFM) is employed for the high frequency force balance (HFFB) test to attain the transverse and longitudinal forces on the crossarm simultaneously. It was found that the wind load coefficients increase with increases of the mean width to mean height ratio and decreases of the solidity ratio. Moreover, in the case of the incoming flow perpendicular to the crossarm face, it is concluded that compared to the quadrilateral crossarm, the triangular crossarm possesses a relatively smaller drag coefficient, provided a moderate mean width to mean height ratio of 1.19. Furthermore, an analysis of the experimental results shows that the skewed wind load factor specified in the relevant codes would overestimate the effects of the wind incidence angle and the variations in mean width to mean height ratio, which is evident in the tests but not considered in the codes. It is worth noting that the experimental results show that the crosswind force is pronounced, which is usually ignored in the skewed wind load factor-based approach. Comparatively, using wind load distribution factors seems to be more rational. Thus, the corresponding estimating equations are proposed for the wind load-distribution factors of the crossarms, which agree well with the experimental results. The developed equations would greatly facilitate the determination of wind loads on tubular crossarms.

An improved rigid rod method for wind-induced swing response prediction of heavily iced transmission lines

Wind-induced swing of overhead transmission lines may cause flashover and line tripping, which will greatly jeopardize the normal operation of power transmission system and give rise to great economy loss. This paper analyzed a swing flashover accident on a practical high-voltage (HV) transmission line and calculated the swing angle of the suspension insulator string using the rigid rod method recommended by the Chinese standard. The aerodynamic coefficients of the iced HV transmission lines were obtained through high frequency force balance wind tunnel tests. An improved rigid rod method was then developed for predicting wind-induced swing response of heavily iced transmission lines in mountainous terrain under severe icy weather conditions. Finally, the improved rigid rod method was also extended to deal with general transmission lines with light ice accretion or regular ice shapes.

The effect of wind characteristics on tall buildings with porous double-skin façades

Wind loads on tall buildings with porous double-skin façade (PDSF) systems were studied for various wind characteristics. Experiments were performed on a small-scale building model in a boundary layer wind tunnel. A single-skin building model was used as a reference case alongside three PDSF systems of 25%, 50% and 65% porosity. Two atmospheric boundary layer (ABL) simulations were created corresponding to category I (rural) and category III (suburban) ABLs of the EN-1991-1-4:2005 standard. Wind loads on the building model were studied based on high-frequency force balance measurements of integral moments, while pressure distribution on the building model surfaces was analyzed based on pressure measurements. Experiments were performed for ten flow incidence angles 0° < α < 45° with a 5° increment. The obtained results are considered highly relevant both for research purposes and engineering applications, especially considering the lack of similar studies currently available. In particular, the rural ABL simulation yields larger mean pressure coefficients. The effect of the ABL characteristics on the pressure coefficient standard deviation depends substantially on α, while the most notable difference is observed on the windward building model surface at α = 0°, where the suburban ABL causes a substantially higher standard deviation. The mean across-wind moment coefficient of the suburban ABL simulation is lower at all α points, while its standard deviation is lower for α > 0°. The along-wind moment coefficient in the suburban ABL simulation is ∼20% lower than in the rural ABL simulation, a trend which is observed for all α points. In contrast, the along-wind moment standard deviation is ∼25% larger in the suburban ABL simulation. The power spectral density of the along-wind moment is considerably lower in the suburban ABL simulation.

Experimental study of static wind force on typical substation post disconnect switchgear three-post structure under different wind angles

As a wind load-sensitive structure, the wind characteristics of typical post-disconnect switchgear in substations are of great significance to the design of the wind resistance of the structure. To study the static wind characteristics of the three-post structure of post-disconnect switchgear, this paper conducts high-frequency force balance (HFFB) wind tunnel tests on the three-post model of post-disconnect switchgear and obtains information on the variation law of the static wind force coefficient of the model with the wind angle and the most unfavorable wind angle. The results show that the model drag coefficient, combined force coefficient, and skewed wind load factor are M-shaped and symmetric at about 90° under different wind angles, and the most unfavorable wind angles are 60° and 120°; the lift coefficient and torsional moment coefficient are antisymmetric about 0°, and the overall values are small. In general, the model is mainly subjected to along-wind static wind force, and the static wind force distribution is influenced by the skewed wind and changes uniformly. The model mainly produces a bending effect under the action of static wind force, and the torsion effect is not obvious.

Effects of side and corner modification on the aerodynamic behavior of high-rise buildings considering serviceability and survivability

This study explores the complementary effects of side and corner modification on the aerodynamic behavior for high-rise buildings across representative design wind speeds. Twelve doubly-symmetric prismatic models were examined using high-frequency force balance (HFFB) wind tunnel testing at the University of Florida. The effectiveness of the aerodynamic strategies was quantified using roof drift and roof acceleration under different design wind speeds covering serviceability and survivability. The results show that both corner and side modifications can achieve promising aerodynamic performance under high design wind speeds. However, the effectiveness of the aerodynamic strategies is significantly reduced under low design wind speeds. With a corner modification strategy, the vortex shedding frequency is increased, leading to worse across-wind response at lower design wind speeds when compared to the square benchmark model. To address this issue, side modifications (i.e., side protrusions) can be used to preserve the vortex shedding frequency and achieve competitive aerodynamic performance while simultaneously maintaining the floor area and geometry. This research explores new aerodynamic modification options for owners, architects, and structural engineers with the aim of better aerodynamic performance for high-rise buildings without compromising other design objectives.

Refined prediction of crosswind forces on rectangular tall buildings with different side ratios

The crosswind responses of tall buildings are complicated due to numerous influencing factors. However, the relevant codes for estimating the crosswind responses have a limited application scope and the corresponding empirical formulas may cause inaccurate assessment of structural responses due to the fitting errors. This study aims to achieve more accurate and flexible predictions of the crosswind forces and wind-induced responses of rectangular tall buildings in comparison with using the conventional empirical formulas. A series of high-frequency force balance (HFFB) tests was conducted on rectangular tall buildings with side ratios from 0.2 to 5 to investigate the crosswind force spectra and the root mean square (RMS) base moment coefficients in two terrain categories. The crosswind force spectra and the crosswind responses of the structure were predicted by using the new proposed formulas and genetic algorithm-backpropagation (GA-BP) neural network, respectively. The results showed that the RMS crosswind base moment coefficients of rectangular tall buildings generally increase with the increase in the side ratio, and can be expressed as piecewise functions related to the side ratio. The proposed formulas can effectively predict the crosswind force spectra of rectangular tall buildings with different side ratios. Compared with the traditional nonlinear fitting method, the GA-BP neural network has higher fitting accuracy and stronger ability for extrapolation prediction. Combined with the random vibration theory, the GA-BP neural network can accurately predict the crosswind responses of rectangular tall buildings. By comparing with the relevant codes, it shows that the crosswind responses of rectangular tall buildings under different side ratios are overestimated to different degrees by GB 50009-2012.

HFFB Test and Wind-Induced Vibration Analysis on 1 000 kV Transformer Frame

In order to propose a complete, wind-resistant design method for ultra-high voltage (UHV) transformer frames, the wind-induced vibration characteristics of a 1 000 kV transformer frame (TF1000) were studied using a high-frequency force balance (HFFB) test. Five section models and one whole model of the TF1000 were designed and constructed using 3D printing, and these were evaluated in a wind tunnel by means of HFFB tests for multiple loading scenarios. The finite element method (FEM) was used on the test data to analyze the wind-induced vibration on the TF1000. The results demonstrate that the shape factor of the TF1000 is significantly affected by the flow field type and solidity ratio; the minimum value occurs when the wind direction is between 30 and 45°. Moreover, all the shape factor values obtained by the test are larger than those established by the Chinese code. The wind-induced vibration analysis indicates that the most unfavorable wind direction for the TF1000 is approximately 60°, with a wind-induced vibration coefficient between 1,7 and 3,9.

The Interference Effects of Wind Load and Wind-Induced Dynamic Response of Quayside Container Cranes

Strong wind has caused damage to group-arranged quayside container cranes in terminals and ports in recent years. Interference may amplify the wind loads in some cases. However, the interference effect among cranes has rarely been studied. In this study, high-frequency force balance tests were conducted to obtain the wind load of isolated and group-arranged container cranes. The results of the computational fluid dynamics simulation were validated by wind tunnel tests and provided the mean wind loads of all 15 types of member cranes. According to the results from wind tunnel tests, the fluctuating wind loads of each member were generated using the weighted amplitude wave superposition method. Based on dynamic finite element methods, the wind-induced responses were obtained considering the interference effect. It was found that the interference effect is the combined effects of both the shielding effect and the amplification of turbulence. Although in some cases the fluctuating and peak wind loads can increase by up to 16% and 6%, respectively, those in the most unfavorable cases are reduced by the interference effect. The interference factor for extreme nodal deformation is 0.56 and 0.69. The interference effect in container cranes mainly appears as a shielding effect, reducing the wind loads and response of the structures in unfavorable cases.

Effects of Corner Modification on the Wind-Induced Responses of High-Rise Buildings

Aerodynamic optimization of building geometry has received significant attention in the design community. In this paper, a process with the high-frequency force balance (HFFB) technique to determine the most effective mitigation measure and the synchronized pressure integration (SPI) technique to verify the effect is developed for the aerodynamic optimization of high-rise buildings. Then, the process is applied to a 318 m-tall high-rise building. Tests show that the wind force on the building will not be symmetrical about the wind azimuth due to the interfering effect. The standard deviation of the base bending moment in the cross-wind direction is much larger than that in the along-wind direction. It indicates that the cross-wind loads will be dominated, providing a remarkable building height. The aerodynamic treatment of corner modifications has a considerable benefit in reducing the cross-wind loads and responses. Among the four corner modifications, the model with a 10% roundness radius to width ratio has the best mitigation effect in the along wind and cross-wind direction. Furthermore, the mean and extreme base overturning moments obtained by the SPI and the HFFB tests almost coincided with wind azimuth with acceptable discrepancy.

Effects of Surface-Attached Vertical Ribs on Wind Loads and Wind-Induced Responses of High-Rise Buildings

Façade design tends to be diverse with increasing requirements for architectural functions in modern high-rise buildings, leading to various aerodynamic roughness conditions of the building surface. A typical practice is setting vertical ribs on the building façade. This study aims to clarify the effects of vertical ribs on the wind loads and wind-induced responses of high-rise buildings. The models with four vertical rib configurations were tested in the open and suburban exposures using the High-Frequency Force Balance (HFFB) technique in a wind tunnel. The base overturning moments and corresponding responses are demonstrated and compared between models and exposures. Results show that the vertical ribs with a relative width of 4% can not only reduce the mean force coefficients in the along-wind direction but also attenuate the cross-wind vibration by disrupting the regular vortex shedding. Moreover, the half-distributed and full-distributed rib configurations have almost indistinguishable benefit, indicating that the ribs in the corner region of the building façade play a dominant role in reducing the wind-induced responses. Although the cross-wind responses of the building would be less severe in the suburban exposure than those in the open exposure, the reduction rate of the wind-induced responses by the vertical ribs remains almost unchanged.

Aerodynamic loads correction method based on wavelet packet transform in high-frequency force balance tests

High-frequency force balance (HFFB) technique is a commonly used method for acquiring wind-induced response of tall buildings in wind tunnel tests. However, the resonance buffeting of the balance-model system (BMS) results in the distortion of measured aerodynamic loads. When the signal distortion is complicated due to additional vibration modes and noise interference, the existing methods fail to obtain correction results. Thus, a new correction method based on wavelet packet transform (WPT) is proposed in this paper to address the issue. The distorted signals are decomposed by WPT, and the relationship between wavelet packet coefficients and power spectral density (PSD) estimation is established. Then, the reference line that conforms to the aerodynamic attenuation characteristics is extracted by a single-degree-of-freedom (SDOF) correction method combined with a parameter identification method. Subsequently, the wavelet packet coefficients of SDOF correction results are adjusted according to the reference line. Finally, the time history of the aerodynamic load after correction is reconstructed by inverse WPT (IWPT). The proposed method is validated by a numerical example and applied to a HFFB test. Compared with the existing methods, the proposed method can address more complex distortion scenes and extend the correction frequency range to the Nyquist frequency.

A practical combination method for wind-induced responses of high-rise buildings: Based on the correlation of extremes

This study proposes a practical three-dimensional combination method for wind-induced responses of high-rise buildings. Five high-rise buildings with different side ratios are considered, where the base shears in two translational directions and base torques of each model are measured by the high-frequency force balance technique. The wind-induced dynamic responses are then calculated based on the measurement. The wind-induced extreme response along one direction and the simultaneously observed wind-induced responses along the other two directions are systematically analyzed, through which the combination factors are determined from a probability view. The factors are then validated by comparing with those specified in several proceeding international codes and standards. The effects of the resonant component, building side ratio, and building surroundings on the combination of wind-induced responses are investigated. The results show that the correlation of the extremes is considerably different from the correlation of time histories. Generally speaking, neglecting the resonant component will underestimate the combination factors. The building side ratio has substantial influence on the combination factor. In the end, the proposed method is employed to calculate combination factors of an actual high-rise building to verify its application in engineering practice.

Wind load investigation of self-supported lattice transmission tower based on wind tunnel tests

Through a series of wind tunnel tests on a self-supported lattice transmission tower model, the wind loads on the tower were investigated in this paper. According to similarity laws, the scaled tower model was designed and fabricated, and the atmospheric boundary layer wind field of Category-B specified in Chinese load code was simulated in the wind tunnel laboratory. Based on the aeroelastic wind tunnel tests and the high frequency force balance tests, the dynamic vibrations, the nondimensional wind force coefficients and the generalized power spectral densities (PSDs) of dynamic wind forces were analyzed and discussed. For the convenient application, the empirical formulas of the generalized PSDs of dynamic wind forces were established and the nonlinear mode shape correction was introduced, which were verified via comparison between the aeroelastic tests and the calculated dynamic responses of tower top. It is indicated that the unfavorable wind attack angles may happen around θ = [15° 30°] and θ = [60° 75°] for this type of tower; the generalized PSDs of wind forces are unimodal in shape, and the corresponding effective bandwidths and the nondimensional frequencies of peaks vary with the wind attack angle θ, especially those of the cross-wind forces are larger than those of the along-wind forces obviously; the established empirical formulas of the generalized PSDs of dynamic wind forces and the derived nonlinear mode shape correction factor are applicable.

Time-Domain Method for Correction of Aerodynamic Loads in High-Frequency Force Balance Tests

AbstractThe high-frequency force balance (HFFB) technique is commonly used in wind tunnels to investigate the wind loads and wind-induced responses of high-rise buildings. This paper presents a tim...