High-frequency Force Balance Research Articles

Investigation of wind load on 1,000 m-high super-tall buildings based on HFFB tests

This paper studies the wind load on 1,000 m-high super-tall buildings and provides basic reference for design, including the utilization of passive and active control devices. High-frequency force balance wind tunnel tests of super-tall buildings with different height are carried out to investigate the effects of building height and wind flow on the wind load. Both monsoon and typhoon climate wind flows are simulated based on target models suggested in literatures. The simulation of typhoon climate wind flows is carried out by a newly developed technique. The analysis of the experimental results confirms that the aerodynamic force is very sensitive to both building height and wind flow. In monsoon climate, the turbulence intensity decreases on increasing the height above ground. Thus, on increasing the building height, vortex shedding becomes increasingly intense and excites stronger structural vibrations in the across-wind direction, though the across-wind fluctuating overturning moment coefficient is almost the same. In typhoon climate, both the mean and the fluctuating overturning moment coefficients increase with the building height. This is mainly caused by the decreasing mean wind speed. The vortex excitation becomes weaker on increasing the building height, and this phenomenon is different from that observed in the monsoon climate. In order to better explain vortex-shedding excitation, a new parameter referred to as the characteristic turbulence intensity is defined herein as a weighted mean value of the turbulence intensity in the range of the building height. It provides a robust interpretation of the vortex excitation of super-tall buildings located in different wind flow and climate conditions.

Effects of aerodynamic modification mechanisms on interference from neighboring buildings

This paper investigates the effect of the single and multiple aerodynamic modification mechanisms on the dynamic behavior of the principal building that is interfered by a very closely located building. For the study, aeroelastic vibration tests and high-frequency force balance tests are conducted to compare responses and wind forces in a well-simulated turbulent boundary layer flow. The principal building is manufactured with three different building configurations to represent the single and multiple aerodynamic modification treatments; the neighboring building which produces interference effects is made into a square prism model. Results show that the multiple modification treatment is efficient in reducing wind forces in all interference location series. However, it is also found that in certain critical conditions, such treatment is sensitive to reduced velocity, and may amplify the interference effect and result in larger displacements.

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.

Effects of vertical ribs protruding from facades on the wind loads of super high-rise buildings

The auxiliary structures of a high-rise building, such as balconies, ribs, and grids, are usually much smaller than the whole building; therefore, it is difficult to simulate them on a scaled model during wind tunnel tests, and they are often ignored. However, they may have notable effects on the local or overall wind loads of the building. In the present study, a series of wind pressure wind tunnel tests and high-frequency force balance (HFFB) wind tunnel tests were conducted on rigid models of an actual super high-rise building with vertical ribs protruding from its facades. The effects of the depth and spacing of vertical ribs on the mean values, fluctuating values and the most unfavorable values of the local wind pressure coefficients were investigated by analyzing the distribution of wind pressure coefficients on the facades and the variations of the wind pressure coefficients at the cross section at 2/3 of the building height versus wind direction angle. In addition, the effects of the depth and spacing of vertical ribs on the mean values, fluctuating values and power spectra of the overall aerodynamic force coefficients were studied by analyzing the aerodynamic base moment coefficients. The results show that vertical ribs significantly decrease the most unfavorable suction coefficients in the corner recession regions and edge regions of facades and increase the mean and fluctuating along-wind overall aerodynamic forces.

Structural Improvements for Tall Buildings under Wind Loads: Comparative Study

The behavior of a very slender building is investigated under wind loads, to satisfy both strength and serviceability (comfort) design criteria. To evaluate the wind effects, wind tunnel testing and structural analysis were conducted, by two different procedures: (i) Pressure Integration Method (PIM), with finite element modeling, and (ii) High Frequency Force Balance (HFFB) technique. The results from both approaches are compared with those obtained from Eurocode 1 and the Italian design codes, emphasizing the need to further deepen the understanding of problems related to wind actions on such type of structure with high geometrical slenderness. In order to reduce wind induced effects, structural and damping solutions are proposed and discussed in a comparative study. These solutions include (1) height reduction, (2) steel belts, (3) tuned mass damper, (4) viscous dampers, and (5) orientation change. Each solution is studied in detail, along with its advantages and limitations, and the reductions in the design loads and structural displacements and acceleration are quantified. The study shows the potential of damping enhancement in the building to mitigate vibrations and reduce design loads and hence provide an optimal balance among resilience, serviceability, and sustainability requirements.

Equivalent static wind loads analysis of tall television towers considering terrain factors of hilltops based on force measurement experiment

Wind field in mountainous regions demonstrates unique distribution characteristic as compared with the wind field of the flat area, wind load and wind effect are the key considerations in structural design of television towers situated in mountainous regions. The television tower to be constructed is located at the top of Xiushan Mountain in Nanjing, China. In order to investigate the impact of terrain factors of hilltops on wind loads, firstly a wind tunnel test was performed for the mountainous area within 800m from the television tower. Then the tower basal forces such as bending moments and shear strength were obtained based on high frequency force balance (HFFB) test. Based on the experiments, the improved method for determining the load combinations was applied to extract the response distribution patterns of foundation internal force and peak acceleration of the tower top, then the equivalent static wind loads were computed under different wind angles, load conditions and equivalent goals. The impact of terrain factors, damping ratio and equivalent goals on the wind load distribution of a television tower was discussed. Finally the equivalent static wind loads of the television tower under the 5 most adverse wind angles and 5 most adverse load conditions were computed. The experimental method, computations and research findings provide important references for the anti-wind design of high-rise structure built on hilltops.

Aerodynamic Admittance Research on Wide-body Flat Steel Box Girder

Aerodynamic admittance is a key parameter affecting the analysis accuracy of the bridge buffeting response. However, few articles have covered the issue of the wide-body flat box girder with much smaller depth-width ratio (such as 1/12 studied in this paper). Therefore, to explore the real buffeting force of the wide-body flat box girder, experiments of the static force coefficients and aerodynamic admittance are carried out in wind tunnel. Wooden segmental model has a scale ratio of 1/60 to an actual 42 m wide suspension bridge girder. Using the high-frequency-force-balance (HFFB) equipment fixed with the segmental model under the conditions of different wind speeds and different wind attack angles, wind power spectrums of the buffeting force are measured and the variation of aerodynamic admittance parameters is analyzed. The results show that the aerodynamic admittance of the box girder measured in the experiment with this so much smaller depth-width ratio differs from the corresponding classical Sears expression. Some inspiration can be presented for the future study of the buffeting response for this kind of much wider bridge with the similar ratio in this paper.

Downstream interference effect of high-rise buildings under turbulent boundary layer flow

This paper addresses the interference effects of high-rise building caused by either an upstream or downstream adjacent building. During the study, aeroelastic vibration tests and high-frequency force balance tests are conducted for two identical square prism models of buildings with an aspect ratio of 8 in a turbulent boundary layer flow. Later, critical locations for interfering buildings are identified by examining interfered force spectral characteristics and various patterns of response trajectories. Finally, an idealized 2D numerical simulation of CFD is tried for the simulations of some critical cases. Although the absolute values from CFD results cannot be perfectly compared with the experimental results, the simulated vorticity movement generated from the existence of both buildings provides an intuitive way for the enhancement of the downstream interference mechanism.

Wind-induced response of large offshore oil platform

In connection with wind sensitivity for the towering and hollowed-out structures (drilling derrick, crane, etc) of large offshore oil platform, the wind-induced response of towering structure was studied. By the similarity criteria, high frequency force balance tests for the large offshore oil platform under 0–360° wind directions were carried out, and the spatial distribution model of fluctuating wind load acting on platform was presented. Also, the characteristics of wind-induced vibration and the changing rule of gust loading factors were obtained precisely through wind-induced assessment in all directions. The results show that: the RMS (Root Mean Square) of the fluctuating across-wind load is about 10% of the fluctuating along-wind load on the platform; the vibration is mainly focused on the towering and hollowed-out structure like derrick, and the RMS of the across-wind acceleration is about 55% of the along-wind acceleration; the towering derrick has a big dynamic magnification of fluctuating across-wind load. The across-wind load can not be neglected in wind resistance design of large offshore oil platform, also the wind-induced response on the top/bottom of derrick and the magnification of fluctuating across-wind load of towering structure should be mainly considered.

Experimental and numerical investigation of a large tuned liquid column damper

During the early design stages of a relatively slender 63-storey high-end residential tower located on the East Coast of the USA, a series of high-frequency force balance and simultaneous pressure integration wind tunnel studies highlighted that the highest occupied levels of the tall building could experience wind-induced motion that had the potential to exceed standard industry occupant-comfort criteria. In order to mitigate these excessive vibrations, additional damping was needed. Among the different types of auxiliary damping devices that can be introduced in a tall building to control wind-induced sway, the so-called tuned liquid column damper was chosen for this specific project primarily because of its simplicity in construction, low initial cost and nearly maintenance-free operation.

Mitigation of wind-induced accelerations using Tuned Liquid Column Dampers: Experimental and numerical studies

During the early design stages of a relatively slender 42-storey high-end residential building located in the Middle East, a series of High Frequency Force Balance wind tunnel tests highlighted that the highest occupied floors could experience wind-induced motion which – depending on the inherent damping of the finished structure – had the potential to exceed standard industry occupant comfort criteria. In order to mitigate these excessive vibrations, a Tuned Liquid Column Damper solution was proposed for this building. The performance prediction and validation of the behaviour of such device involved: an initial campaign of full scale measurements to validate frequencies and inherent damping of the structure near completion; a series of shake table tests employing a 1:20 scale physical model; and a final full scale extrapolation study using Computational Fluid Dynamics. The damper study, which this technical paper is focused on, was part of a wider range of wind engineering consultancy services which included: wind climate study; pedestrian and terrace / balcony level wind microclimate study; overall wind loading study and façade pressure study.

Application of tuned liquid dampers in controlling the torsional vibration of high rise buildings

Excessive motions in buildings cause occupants to become uncomfortable and nervous. This is particularly detrimental to the tenants and ultimately the owner of the building, with respect to financial considerations. Serviceability issues, such as excessive accelerations and inter-story drifts, are more prevalent today due to advancements in the structural systems, strength of materials, and design practices. These factors allow buildings to be taller, lighter, and more flexible, thereby exacerbating the impact of dynamic responses. There is a growing need for innovative and effective techniques to reduce the serviceability responses of these tall buildings. The current study considers a case study of a real building to show the effectiveness and robustness of the TLD in reducing the coupled lateral-torsional motion of this high-rise building under wind loading. Three unique multi-modal TLD systems are designed specifically to mitigate the torsional response of the building. A procedure is developed to analyze a structure-TLD system using High Frequency Force Balance (HFFB) test data from the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario. The effectiveness of the unique TLD systems is investigated. In addition, a parametric study is conducted to determine the robustness of the systems in reducing the serviceability responses. Three practical parameters are varied to investigate the robustness of the TLD system: the height of water inside the tanks, the amplitude modification factor, and the structural modal frequencies.

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.

Physics-Based Method for the Removal of Spurious Resonant Frequencies in High-Frequency Force Balance Tests

AbstractThe high-frequency force balance (HFFB) test is a widely used method for estimating lateral turbulent wind loads on tall buildings in a wind tunnel. The method relies on use of a highly accurate force sensor placed at the base of the tall building model to measure base forces and overturning moments. Because the turbulent wind load excitation is broadband, it is important to adequately design the connection detail between the model and the sensor. Accurate design is needed to avoid contaminating the measurement of the lateral wind loads by the potential spurious resonant effect, which is induced by flexibility in the connection detail. This design often relies on the experience of the wind tunnel modeler. On occasion, because of the limitations of test setup and environment, the connection detail between the building model and the HFFB force sensor may not be sufficiently stiff. Consequently, resonant response caused by the flexible connection may affect the power spectral density function of the ...

Large-eddy simulation of wind effects on a super-tall building in urban environment conditions

A combined study of large-eddy simulation (LES), wind tunnel testing and full-scale measurement is conducted for the evaluation of wind effects on a super-tall building in a complex urban area. To validate the numerical simulations, the wind tunnel experiments including synchronous multi-pressure and high-frequency force balance model tests are conducted in a boundary layer wind tunnel laboratory. The numerical predictions are then compared with the experimental results, demonstrating that the LES can provide comparable predictions of the wind effects on the super-tall building. Furthermore, the cross-validation of the predicted displacement responses by the LES against the wind tunnel and full-scale measurements are presented and the agreement among them is reasonably good. The main objective of this study is to explore an effective numerical approach for the accurate estimation of wind effects on tall buildings in urban environment conditions and promote the practical use of the LES in the wind-resistant design of complex structures.

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.

The past, present and future of high-frequency balance testing

Less than 30 years ago a new method was introduced in wind-tunnel testing of tall buildings, known variously as the High-Frequency Base Balance or High-Frequency Force Balance, which revolutionized the determination of design wind loads using model studies. The method is reviewed in hindsight, in the perspective of the present, and with a crystal ball to speculate on future developments. These viewpoints focus on various technical issues that have been solved, are being solved, and need to be solved. The intent is to assist the uninitiated develop appreciation for the technology involved, to identify various pitfalls awaiting those who embark in the method, and to identify areas of need so that practicing design engineers-the users of such studies-can appreciate the limitations and collaborate on future advances while promoting improved communication between executor and user.

Wind tunnel investigation of correlation and coherence of wind loading on generic tall twin buildings in close proximity

A popular modern architectural form for tall buildings is two (or more) towers which are structurally linked through such features as a shared podium or sky-bridges. The fundamental features of the wind loading and the structural links of such buildings can be studied by measuring load components on the individual unlinked towers along with their correlations. This paper describes application of dual high frequency force balance (DHFFB) in a wind tunnel study of the base wind loading exerted on generic tall twin buildings in close proximity. Light models of two identical generic tall buildings of square plan were mounted on DHFFB and the base wind loading exerted on the buildings was simultaneously acquired. The effects of the relative positions of the buildings on the correlations and coherences involving loading components on each building and on the two buildings were investigated. For some relative positions, the effects of the building proximity on the wind loading were significant and the loading was markedly different from that exerted on single buildings. In addition, the correlations between the loadings on the two buildings were high. These effects have potential to significantly impact, for example, the modally-coupled resonant responses of the buildings to the aerodynamic excitations. The presented results were not meant to be recommended for direct application in wind resistant design of tall twin buildings. They were intended to show that wind loading on tall buildings in close proximity is significantly different from that on single buildings and that it can be conveniently mapped using DHFFB.

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.