High Frequency Force Balance Technique Research Articles

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-resistant structural optimization of supertall buildings based on high-frequency force balance wind tunnel experiment

The high-frequency force balance (HFFB) wind tunnel test is one of the most widely used techniques to estimate wind effects on supertall buildings. This paper introduces a wind-resistant optimal design method of supertall buildings based on HFFB wind tunnel experiments. Based on the HFFB wind tunnel test and wind-induced vibration analysis, it provides a reliable and efficient method for structural optimization of supertall buildings with different types of structural members and multiple constraints. Taking advantage of the classic linear mode shape assumption of the HFFB technique, this paper also presents an alternative routine that transforms all different types of constraints into a uniform frequency constraint, which significantly reduces the computational intensity while still offers acceptable accuracy of optimization results. Moreover, the sensitivity of the optimization results to the adopted basic wind pressure value is discussed. The total material cost of the optimized structure increases exponentially with the increase of the basic wind pressure, indicating that the structural costs of supertall buildings are quite sensitive to the adopted basic wind pressure for structural design and its value should be precisely determined by the wind climate analysis of the construction site.

An Aerodynamic Load Correction Method for HFFB Technique Based on Signal Decoupling and an Intelligent Optimization Algorithm

In high-frequency force balance (HFFB) wind tunnel tests, the aerodynamic wind loads at the base of the building model are usually amplified by the model-balance system. This paper proposes a new method for eliminating such an amplification effect. Firstly, the measured base bending moment signals are decoupled into independent components. Then, an optimization model is established to represent the problem of identifying the natural frequencies and damping ratios for the different modes of the model-balance system. Finally, the genetic algorithm (GA) is employed to seek the solution to the optimization problem, and the base bending moment is corrected through the identified dynamic parameters of the model-balance system. Compared to the conventionally used knocking method, the proposed method requires no extra knocking tests and can take the aerodynamic damping of the model-balance system into account. An engineering case, the Guangzhou East Tower (GZET), is taken as an example to show the effectiveness of the method.

Experimental study of wind effects on two neighboring super-tall buildings

With high-frequency force balance technique, wind tunnel tests have been carried out to study the wind effects on two iconic super-tall buildings located in South China, the Guangzhou East Tower and the Guangzhou West Tower. It was found that the normally used method, which calculates the background response and the resonant response separately and then combines them to get the total response, could give overestimated or underestimated response results. Neglecting the correlation between the background response and the resonant response is the very reason for the inaccuracy. Afterward, the wind-induced responses of Guangzhou East Tower and Guangzhou West Tower are computed by considering four different reference wind speeds obtained from different wind climate models, including the GB, GMB, ZL, and ARA models. It was found that different reference wind speeds from different wind climate models created significantly different structural response results. But, the computational maximum acceleration response...

Model shape correction method for high-frequency force balance technique

This study proposed the mode shape correction formulae for the high frequency force balance technique. The presented formulae respectively give the mode shape for translational modes and torsional modes, meaning that there is no need to distinguish the traditionally concerned along wind case and cross-wind case anymore. This treatment makes the mode shape correction much simpler especially when wind blows to a building obliquely. Comparison with some experimental results of other researchers shows the reliability of the proposed formulae. The mode coupling effect is also analyzed and discussed through an actual super-tall building, the Guangzhou East Tower. The results show that the mode shape coupling effect is significant and cannot be ignored.

Wind tunnel and full-scale study of wind effects on a super-tall building

This paper presents the analyzed results from a combined wind tunnel and full-scale study of the wind effects on a super-tall building with a height of 420m in Hong Kong. In wind tunnel tests, mean and fluctuating forces and pressures on the building models for the cases of an isolated building and the building with the existing surrounding condition are measured by the high-frequency force balance technique and synchronous multi-pressure sensing system under two typical boundary layer wind flow fields. Global and local wind force coefficients and structural responses are presented and discussed. A detailed study is conducted to investigate the influences of incident wind direction, upstream terrain conditions and interferences from the surroundings on the wind loads and responses of the high-rise structure. On the other hand, full-scale measurements of the wind effects on the super-tall building have been performed under typhoon conditions. The field data, such as wind speed, wind direction, structural acceleration and displacement responses have been simultaneously and continuously recorded during the passage of 12 typhoons since 2008. Analysis of the field measured data is carried out to investigate the typhoon effects on the super-tall building. Finally, the model test results are compared with the full-scale measurements for verification of the wind tunnel test techniques. The comparative study shows that the wind tunnel testing can provide reasonable predictions of the structural resonant responses. The resonant displacement responses are comparable to the background displacement responses so that the contribution of the background responses to the total displacement responses should not be underestimated. The outcome of the combined wind tunnel and full-scale study is expected to be useful to engineers and researchers involved in the wind-resistant design of super-tall buildings.

High-frequency force balance technique for tall buildings: a critical review and some new insights

The high frequency force balance (HFFB) technique provides convenient measurements of integrated forces on rigid building models in terms of base bending moments and torque and/or base shear forces. These base moments or forces are then used to approximately estimate the generalized forces of building fundamental modes with mode shape corrections. This paper presents an analysis framework for coupled dynamic response of tall buildings with HFFB technique. The empirical mode shape corrections for generalized forces with coupled mode shapes are validated using measurements of synchronous pressures on a square building surface from a wind tunnel. An alternative approach for estimating the mean and background response components directly using HFFB measurements without mode shape corrections is introduced with a discussion on higher mode contributions. The uncertainty in the mode shape corrections and its influence on predicted responses of buildings with both uncoupled and coupled modal shapes are examined. Furthermore, this paper presents a comparison of aerodynamic base moment spectra with available data sets for various tall building configurations. Finally, e-technology aspects in conjunction with HFFB technique such as web-based on-line analysis framework for buildings with uncoupled mode shapes used in NALD (NatHaz Aerodynamic Loads Database) is discussed, which facilitates the use of HFFB data for preliminary design stages of tall buildings subject to wind loads.

HFFB technique and its validation studies

The high-frequency force-balance (HFFB) technique and its subsequent improvements are reviewed in this paper, including a discussion about nonlinear mode shape corrections, multi-force balance measurements, and using HFFB model to identify aeroelastic parameters. To apply the HFFB technique in engineering practice, various validation studies have been conducted. This paper presents the results from an analytical validation study for a simple building with nonlinear mode shapes, three experimental validation studies for more complicated buildings, and a field measurement comparison for a super-tall building in Hong Kong. The results of these validations confirm that the improved HFFB technique is generally adequate for engineering applications. Some technical limitations of HFFB are also discussed in this paper, especially for higher-order mode response that could be considerable for super tall buildings.

Aerodynamic force characteristics and galloping analysis of iced bundled conductors

Aerodynamic characteristics of crescent and D-shape bundled conductors were measured by high frequency force balance technique in the wind tunnel. The drag and lift coefficients of each sub-conductor and the whole bundled conductors were presented under various attack angles of wind. The galloping possibility of bundled conductors is discussed based on the Den Hartog criterion. The influence of icing thickness, initial ice accretion angle and sub-conductor on the aerodynamic properties were investigated. Based on the measured aerodynamic force coefficients, a computationally efficient finite element method is also implemented to analyze galloping of iced bundled conductors. The analysis results show that each sub-conductor of the bundled conductor has its own galloping feature due to the use of aerodynamic forces measured separately for every single sub-conductors.

A probabilistic approach for the full response estimation of tall buildings with 3D modes using the HFFB

The high frequency force balance technique for the assessment of wind actions on tall buildings is widely adopted mainly due to its relatively fast setup and capacity for providing estimates of the resultant wind action independently of building geometry. Nevertheless, some shortcomings represent a limit to its adoption as an experimental methodology. In particular, the lack of knowledge on the pressure field makes it necessary to make arbitrary assumptions on the floor loads that can consequently generate important errors – usually more relevant for buildings with non-linear and coupled modes – in the response estimation. Also, the adoption of the method is often hindered by the limitation of considering only the first three fundamental modes in the response assessment.The novel methodology which is proposed in this paper has the aim of overcoming the aforementioned problems. The reliability framework in which the method is set allows the consideration of the uncertainties arising from the lack of knowledge on the pressure field through the definition of probabilistic models and provides the possibility of estimating any response parameter of interest, for any kind of building, considering exclusively the base measurements. The application to two representative case study buildings with coupled and non-linear modes shows, through comparison with reference results obtained from synchronous pressure measurements, the effectiveness of the method in providing conservative estimates of the response parameters.

Dynamic response estimation of tall buildings with 3D modes: A probabilistic approach to the high frequency force balance method

The estimation of the wind-induced response of tall buildings is often carried out through the use of high frequency force balance (HFFB) tests. This technique is characterized by its versatility and rapidity of use, especially in comparison to other experimental methods. Nevertheless, the estimates of the structural response that can be obtained from its application are affected by uncertainties due to the lack of knowledge on the pressure field over the building surface. The importance of this is particularly relevant in the case of modern tall buildings with their complex geometric shapes and 3D and non-linear modes.This paper presents a novel approach for dealing with this possible source of error in the response estimation. In particular the uncertain spectral structure of the floor loads is modeled through a limited number of probabilistic parameters. This allows the definition of a local level fragility model that includes both the uncertainties inherent to the HFFB technique and the aleatory nature of the aerodynamic loads. The proposed model is applied to a case study building presenting non-linear 3D modes.

Interference effects of two and three super-tall buildings under wind action

Most previous investigations on interference effects of tall buildings under wind actions focused on the wind induced interference effects between two buildings, and the interference effects of three or more buildings have seldom been studied so far due to the huge workload involved in experiments and data processing. In this paper, mean and dynamic force/response interference effects and peak wind pressure interference effects of two and three tall buildings, especially the three-building configuration, are investigated through a series of wind tunnel tests on typical tall building models using high frequency force balance technique and wind pressure measurements. Furthermore, the present paper focuses on the effects of parameters, including breadth ratio and height ratio of the buildings and terrain category, on the interference factors and derives relevant regression results for the interference factors.

Economic perspectives of aerodynamic treatments of square tall buildings

This paper investigates the impact and value of aerodynamic modifications, based on wind engineering considerations, on the building economics of tall structures. Aerodynamically efficient plan shapes are shown to be an effective means of suppressing wind-induced loads, and hence construction cost, but may come at the cost of reducing both the size and value of saleable/rentable floor area. Therefore, it is not straightforward for engineers to decide on aerodynamic modifications, such as the size and type of corner modifications, with regard to both the structural responses and the effects on economic rewards. A series of wind tunnel tests were conducted at the CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, using the high frequency force balance technique for building models with different corner modifications. Empirical formulae are proposed to relate the cross-wind responses to the building dimensions and dynamic properties. Those formulations are subsequently employed to assess the financial profits for each building configuration. The construction costs and financial returns of tall buildings are also examined.

Wind Tunnel Model Tests of Large Billboards

Two series of wind tunnel tests on 1: 200 scale models of large billboard structures were carried out in turbulent boundary layer flow. The tested configurations in the first series are single-panel rectangular billboards with width to depth ratio varying from 1 to 3 and depth to height ratio varying from ⅓ to ⅔, and in second series are two-panel billboards with the angle between two panels (ϕ) varying from 0° to 30°. The high-frequency force balance technique was employed. The test results show that the mean drag force coefficient CD of single-panel billboards attains its maximum value of about 1.3 to 1.5 when the wind attack angle θ lies between −45° and +45°. The maximum value of mean forces on two-panel billboards is slightly higher than that of the corresponding single-panel billboard. Wind-induced torsion about the vertical axis of single-panel billboards increases as θ increases, and attains its maximum value at θ of about ±45°. The maximum torsion can be estimated by multiplying the maximum drag force with the peak horizontal eccentricity of about 15% of the billboard width. For two-panel billboards, the eccentricity increases as ϕ increases, and reaches 24% at ϕ = 30°. The aerodynamic admittance function for all billboard configurations agrees reasonably well with the well-known Vickery's admittance formula.

Mode shape correction for wind-induced dynamic responses of tall buildings using time-domain computation and wind tunnel tests

The high-frequency force-balance (HFFB) technique is a common and effective wind tunnel testing method for the assessment of wind-induced dynamic responses of a tall building. The technique works in the frequency domain with the inherent assumption of ideal mode shapes so that the base moments can approximate the generalized forces. In this study, time histories of wind forces at several levels of a tall building model are obtained in the wind tunnel with a multi-channel pressure scanning system. This enables the building responses to be computed directly in the time domain for buildings with non-ideal mode shapes. Mode shape correction factors for the HFFB technique are thus obtained for the dynamic deflections and moments. The dependence of the mode shape correction factors on mode shapes and vertical distributions of wind excitation and building masses is investigated systematically. These correction factors are found to agree generally with the predictions proposed in previous studies but some differences are noted on the commonly assumed profiles of wind excitation and correction for torsion.

Simplified formulas for evaluation of wind-induced interference effects among three tall buildings

The base-bending moment (BBM) response and the mean BBM of grouped high-rise buildings are studied by a series of wind tunnel tests on typical tall building models using the high-frequency force balance technique. Interference excitations of two upwind buildings with various heights in different upwind terrains are considered. An effective method is proposed to represent the distribution of the envelope interference factor (EIF) among three tall buildings. The results show that two upstream buildings cause more adverse dynamic effects on the downstream building than a single upstream building does. Significant correlations are found in the distributions of the interference factors of different configurations and upwind terrains. Relevant regression equations are proposed to simplify the complexity of the multi-parameter wind-induced mean and dynamic interference effects among three tall buildings. Finally, an example of how to use the data provided in this paper and the proposed methodology is presented.

Wind tunnel and full-scale study of wind effects on China’s tallest building

The Jin Mao Building with a height of 420.5 m and 88-storeys, located in Shanghai, is the highest tall building in Mainland China. This paper presents selected results from a combined wind tunnel and full-scale study of wind effects on the super tall building. In the wind tunnel test, mean and fluctuating force components on the building model for the cases of an isolated Jin Mao Building and the building with existing surrounding condition were measured by a high-frequency force balance technique under suburban and urban boundary layer wind flow configurations, respectively. Force coefficients, power spectral densities, displacement and acceleration responses were then presented and discussed. A detailed comparative study was conducted to investigate the influences of incident wind direction, upstream terrain conditions and interferences from the surroundings on the wind effects on the building. Serviceability of the super tall building under strong wind action was analyzed on the basis of the experimental results. On the other hand, full-scale measurements of wind effects on the Jin Mao Building were conducted under typhoon conditions. The field data, such as wind speed, wind direction and acceleration responses were simultaneously and continuously recorded during the passage of Typhoon Rananim in August, 2004. Analysis of the field data was carried out to investigate typhoon effects on the super tall building. Finally, the wind tunnel test results were found to be in good agreement with the full-scale measurements, illustrating that the wind tunnel tests can provide satisfactory predictions of wind-induced vibrations of the super tall building under typhoon conditions.

Along-Wind Dynamic Interference Effects of Tall Buildings

Most previous investigations have focused on the wind induced interference effects between two buildings. The interference effects of three or more buildings have seldom been studied so far due to the huge workload of experiments and data processing. In this paper, the along-wind dynamic interference effects of two and three tall buildings are investigated through a series of wind tunnel tests on typical tall building models using the high frequency force balance technique. The testing cases are totally over 8000. In order to conveniently and rapidly process the huge amount of testing data, a Windows-based software platform integrating artificial neural network, statistics and spectral computations is developed. Database technique is also employed in the platform to manage the experimental results. Because of a vast amount of experimental results, this paper presents only detailed results of the along-wind dynamic interference effects but not the across-wind effects. Some selected results of the twin-building configuration from the present tests are compared with the previous comparable results in literature. Effects of upstream terrain, height ratio and breadth ratio of the tested models on the interference factor are discussed in detail.

Across-wind loads of typical tall buildings

Previous studies have indicated that the across-wind dynamic responses of super-tall buildings are usually larger than the along-wind ones. With the increase of heights, the across-wind dynamic response of super-tall buildings has been a problem of great concern. In this paper, 15 typical tall building models are tested with high-frequency force balance technique in a wind tunnel to obtain the first-mode generalized across-wind dynamic forces. New formulas for the power spectra of the across-wind dynamic forces, the coefficients of base moment and shear force are then derived. Parametric analyses of the effects of factors on the across-wind loads of the buildings are performed. Besides, a SDOF aeroelastic model of a square tall building with an aspect ratio of 6 is selected from the above buildings and is tested to investigate its across-wind dynamic response and aerodynamic damping characteristics. The power spectrum of the across-wind force of the square building is employed to compute its across-wind dynamic responses with and without considering the effect of the aerodynamic damping. The computed responses are then compared with the corresponding responses from the aeroelastic model test to verify the present formulas of the across-wind loads of buildings.

Experimental test on bridge jointed twin-towered buildings to stochastic wind loads

This paper presents results of a study on wind loads and wind induced dynamic response of bridge jointed twin-towered buildings. Utilizing the high-frequency force balance technique, the drag and moment coefficients measured in wind tunnel tests, and the maximum acceleration rms values on the top floor of towers, are analyzed to examine the influence of building's plan shapes and of intervals between towers. The alongwind, acrosswind and torsional modal force spectra are investigated for generic bridge jointed twin-towered building models which cover twin squares, twin rhombuses, twin triangles, twin triangles with sharp corners cut off, twin rectangles and individual rectangle with the same outline aspect ratio as the twin rectangles. The analysis of the statistical correlation among three components of the aerodynamic force corroborated that the correlation between acrosswind and torsional forces is significant for bridge jointed twin-towered buildings.