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CONTENTS
Volume 18, Number 4, April 2014
 

Abstract
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Key Words
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Address
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Abstract
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.

Key Words
tall buildings; wind tunnel; high-frequency balance; dynamic; aerodynamic; PSD; structural

Address
Daryl W. Boggs: CPP Inc, 1415 Blue Spruce Drive, Fort Collins, Colorado 80524, USA

Abstract
Since its development in the early 1980\'s the force balance technique has become a standard method in the efficient determination of structural loads and responses. Its usefulness lies in the simplicity of the physical model, the relatively short records required from the wind tunnel testing and its versatility in the use of the data for different sets of dynamic properties. Its major advantage has been the ability to provide results in a timely manner, assisting the structural engineer to fine-tune their building at an early stage of the structural development. The analysis of the wind tunnel data has evolved from the simple un-coupled system to sophisticated methods that include the correction for non-linear mode shapes, the handling of complex geometry and the handling of simultaneous measurements on multiple force balances for a building group. This paper will review some of the components in the force balance data analysis both in historical perspective and in its current advancement. The basic formulation of the force balance methodology in both frequency and time domains will be presented. This includes all coupling effects and allows the determination of the resultant quantities such as resultant accelerations, as well as various load effects that generally were not considered in earlier force balance analyses. Using a building model test carried out in the wind tunnel as an example case study, the effects of various simplifications and omissions are discussed.

Key Words
wind tunnel test; tall buildings; force balance; time domain analysis; frequency domain analysis

Address
T.C. Eric Ho and Peter Case: The Boundary Layer Wind Tunnel Laboratory, The University Of Western Ontario, London, Ontario, Canada
Un Yong Jeong: Gradient Wind Engineering Inc., Ottawa, Ontario, Canada

Abstract
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.

Key Words
wind tunnel validations; high-frequency force-balance model; high-frequency pressure integration model; multi-force balance system; HFFB/aeroelastic hybrid model; field measurement; wind tunnel tests; nonlinear mode shapes; aeroelastic effects

Address
Jiming Xie: Zhejiang University, Hangzhou, Zhejiang, China
Jason Garber: RWDI Inc., 650 Woodlawn Road West, Guelph, Ontario, N1K 1B8, Canada

Abstract
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.

Key Words
wind loads; wind tunnel tests; buildings; building design; structural dynamics; dynamic analysis; random

Address
Xinzhong Chen:1National Wind Institute, Texas Tech University, USA
Dae-Kun Kwon and Ahsan Kareem:NatHaz Modeling Lab., University of Notre Dame, USA

Abstract
The high frequency base balance (HFBB) technique is a convenient and relatively fast wind tunnel testing technique for predicting wind-induced forces for tall building design. While modern tall building design has seen a number architecturally remarkable buildings constructed recently, the characteristics of those buildings are significantly different to those that were common when the HFBB technique was originally developed. In particular, the prediction of generalized forces for buildings with 3-dimensional mode shapes has a number of inherent uncertainties and challenges that need to be overcome to accurately predict building loads and responses. As an alternative to the more conventional application of general mode shape correction factors, an analysis methodology, referred to as the linear-mode-shape (LMS) method, has been recently developed to allow better estimates of the generalized forces by establishing a new set of centers at which the translational mode shapes are linear. The LMS method was initially evaluated and compared with the methods using mode shape correction factors for a rectangular building, which was wind tunnel tested in isolation in an open terrain for five incident wind angles at 22.5o increments from 0o to 90o. The results demonstrated that the LMS method provides more accurate predictions of the wind-induced loads and building responses than the application of mode shape correction factors. The LMS method was subsequently applied to a tall building project in Hong Kong. The building considered in the current study is located in a heavily developed business district and surrounded by tall buildings and mixed terrain. The HFBB results validated the versatility of the LMS method for the structural design of an actual tall building subjected to the varied wind characteristics caused by the surroundings. In comparison, the application of mode shape correction factors in the HFBB analysis did not directly take into account the influence of the site specific characteristics on the actual wind loads, hence their estimates of the building responses have a higher variability.

Key Words
HFBB; linear-mode-shape method; real tall building application; various site wind conditions; effects of surroundings

Address
K.T. Tse and X.J. Yu: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology,
Hong Kong
P.A. Hitchcock: CLP Power Wind/Wave Tunnel Facility, Hong Kong University of Science and Technology, Hong Kong

Abstract
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.

Key Words
correlation; coherence; base wind loading; wind tunnel testing, dual high-frequency force balance; tall twin buildings; building coupling

Address
Juntack Lim: Construction Technology Center, Samsung Corporation, Korea
Bogusz Bienkiewicz: Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80523, USA

Abstract
A summary of the main results from an international comparative study for the high-frequency base balance is given. Two buildings were specified – a \"basic\" and an \"advanced\" building. The latter had more complex dynamic response with coupled modes of vibration. The predicted base moments generally showed good agreement amongst the participating groups, but less good agreement was found for the roof accelerations which are dominated by the resonant response, and subject to measurement errors for the generalized force spectra, to varying mode shape correction techniques, and different methods used for combining acceleration components.

Key Words
base-balance; building; tall; vibration; wind-loading

Address
John D. Holmes: JDH Consulting, Mentone, Victoria 3194, Australia
Tim K.T. Tse: Department of Civil and Environmental Engineering, HKUST, Clearwater Bay, Hong Kong


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