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CONTENTS
Volume 19, Number 3, September 2014
 

Abstract
Vortex-induced oscillation is a type of aeroelastic phenomenon, to which extended structures such as long-span bridges are most susceptible. The vortex-induced vibration (VIV) behaviors of a concerned bridge were investigated conventionally in virtue of wind tunnel tests on string-mounted sectional models. This necessitates the building of a linkage between the response of the sectional model and that of the prototype structure. Although many released literatures have related to this issue and provided suggestions, there is a lack of consistency among them. In this study, some theoretical models describing the vortex-induced structural motion, including the linear empirical model, the nonlinear empirical model and the modified (or generalized) nonlinear empirical model, are firstly reviewed. Then, the concept of equivalent mass density is introduced based on the principle that an equal input of energy should result in identical structural amplitudes. Based on these, the theoretical linkages between the amplitude of a section model and that corresponding to the prototype bridge are discussed with different analytical models. Theoretical derivation indicates that such connections are dependent mainly on two factors, one is the presupposed shape of deformation, and the other is the theoretical VIV model employed. The theoretical analysis in this study shows that, in comparison to the nonlinear empirical models, the linear one can result in obvious larger estimations of the full bridges\' responses, especially in cases of cable-stayed bridges.

Key Words
bridge; vortex shedding; aeroelastic; oscillation; sectional model; full scale

Address
Zhitian Zhang: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China; Wind Engineering Research Center, Hunan University, Changsha, China
Yaojun Ge: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, ShangHai, China
Zhengqing Chen: Wind Engineering Research Center, Hunan University, Changsha, China

Abstract
Through comparing the mean wind profiles observed overland during the passages of four typhoons, and the gradient wind speeds calculated based on the sea level pressure data provided by a numerical model, the present paper discusses, (a) whether the gradient balance is a valid assumption to estimate the wind speed in the height range of 1250 m ~ 1750 m, which is defined as the upper-level mean wind speed, in a tropical cyclone over land, and (b) if the super-gradient feature is systematically observed below the height of 1500 m in the tropical cyclone wind field over land. It has been found that, (i) the gradient balance is a valid assumption to estimate the mean upper-level wind speed in tropical cyclones in the radial range from the radius to the maximum wind (RMW) to three times the RMW, (ii) the super-gradient flow dominates the wind field in the tropical cyclone boundary layer inside the RMW and is frequently observed in the radial range from the RMW to twice the RMW, (iii) the gradient wind speed calculated based on the post-landfall sea level pressure data underestimates the overall wind strength at an island site inside the RMW, and (iv) the unsynchronized decay of the pressure and wind fields in the tropical cyclone might be the reason for the underestimation.

Key Words
boundary layer height; field measurements; gradient balance; super-gradient feature; typhoon mean wind profile

Address
K.T. Tse and C.Q. Lin:Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
S.W. Li: CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
P.W. Chan: Hong Kong Observatory, 134A Nathan Road,Kowloon, Hong Kong

Abstract
The impact of artefacts in archived wind observations on the design wind speed obtained by extreme value analysis is demonstrated using case studies. A signpost protocol for detecting candidate artefacts is described and its performance assessed by comparing results against previously validated data. The protocol targets artefacts by exploiting the serial correlation between observations. Additional \"sieve\" algorithms are proposed to identify types of correctable artefact from their \"signature\" in the data. In extreme value analysis, artefacts displace valid observations only when they are larger, hence always increase the design wind speed. Care must be taken not identify large valid values as artefacts, since their removal will tend to underestimate the design wind speed.

Key Words
wind observations; validation; bias; artefacts; extreme values; design wind speed

Address
Nicholas J. Cook: Consultant, RWDI, Unit 4, Lawrence Way, Dunstable, Bedfordshire, LU6 1BD, UK

Abstract
Starting from an overview on the research on thunderstorms in the last forty years, this paper provides a general discussion on some emerging issues and new frameworks for wind loading on structures in mixed climates. Omitting for sake of simplicity tropical cyclones and tornadoes, three main aspects are pointed out. The first concerns the separation and classification of different intense wind events into extra-tropical depressions, thunderstorms and gust fronts, with the aim of improving the interpretation of the phenomena of engineering interest, the probabilistic analysis of the maximum wind velocity, the determination of the wind-induced response and the safety format for structures. The second deals with the use of the response spectrum technique, not only as a potentially efficient tool for calculating the structural response to thunderstorms, but also as a mean for revisiting the whole wind-excited response in a more general and comprehensive framework. The third involves the statistical analysis of extreme wind velocities in mixed climates, pointing out some shortcomings of the approaches currently used for evaluating wind loading on structures and depicting a new scenario for a more rational scheme aiming to pursue structural safety. The paper is set in the spirit of mostly simplified analyses and mainly qualitative remarks, in order to capture the conceptual aspects of the problems dealt with and put on the table ideas open to discussion and further developments.

Key Words
depression; gust front; mixed climate; response spectrum; thunderstorm; wind monitoring

Address
Giovanni Solari:Department of Civil, Chemical and Environmental Engineering (DICCA), University of Genoa,
Via Montallegro, 1, 16145 Genoa, Italy


Abstract
This study has developed an Enhanced Remote-Sensing (ERS) scale to improve the accuracy and efficiency of using remote-sensing images of residential building to predict their damage conditions. The new scale, by incorporating multiple damage states observable on remote-sensing imagery, substantially reduces measurement errors and increases the amount of information retained. A ground damage survey was conducted six days after the Joplin EF 5 tornado in 2011. A total of 1,400 one- and two-family residences (FR12) were selected and their damage states were evaluated based on Degree of Damage (DOD) in the Enhanced Fujita (EF) scale. A subsequent remote-sensing survey was performed to rate damages with the ERS scale using high-resolution aerial imagery. Results from Ordinary Least Square regression indicate that ERS-derived damage states could reliably predict the ground level damage with 94% of variance in DOD explained by ERS. The superior performance is mainly because ERS extracts more information. The regression model developed can be used for future rapid assessment of tornado damages. In addition, this study provides strong empirical evidence for the effectiveness of the ERS scale and remote-sensing technology for assessment of damages from tornadoes and other wind events.

Key Words
tornadoes; damage assessment; remote-sensing; enhanced remote-sensing scale; Enhanced Fujita scale; degree of damage

Address
Jianjun Luo:National Wind Institute, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA;
AIR Worldwide, 131 Dartmouth Street, Boston, MA 02116, USA
Daan Liang: National Wind Institute, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA
Cagdas Kafali and Ruilong Li: AIR Worldwide, 131 Dartmouth Street, Boston, MA 02116, USA
Tanya M. Brown: Insurance Institute for Business and Home Safety, 5335 Richburg Road, Richburg, SC 29729, USA

Abstract
This paper discusses the appropriate duration for basic gust wind speeds in wind loading codes and standards, and in wind engineering generally. Although various proposed definitions are discussed, the \'moving average\' gust duration has been widely accepted internationally. The commonly-specified gust duration of 3-seconds, however, is shown to have a significant effect on the high-frequency end of the spectrum of turbulence, and may not be ideally suited for wind engineering purposes. The effective gust durations measured by commonly-used anemometer types are discussed; these are typically considerably shorter than the \'standard\' duration of 3 seconds. Using stationary random process theory, the paper gives expected peak factors, gu, as a function of the non-dimensional parameter (T/), where T is the sample, or reference, time, and  is the gust duration, and a non-dimensional mean wind speed, U ̅.T /Lu, whereU is a mean wind speed, and Lu is the integral length scale of turbulence. The commonly-used Durst relationship, relating gusts of various durations, is shown to correspond to a particular value of turbulence intensity Iu, of 16.5%, and is therefore applicable to particular terrain and height situations, and hence should not be applied universally. The effective frontal areas associated with peak gusts of various durations are discussed; this indicates that a gust of 3 seconds has an equivalent frontal area equal to that of a tall building. Finally a generalized gust response factor format, accounting for fluctuating and resonant along-wind loading of structures, applicable to any code is presented.

Key Words
anemometer; codes; gust duration; gust response factor; peak factor; wind load

Address
John D. Holmes: JDH Consulting, PO Box 269, Mentone, Victoria, 3194, Australia
Andrew C. Allsop : Arup AT+R, 13 Fitzroy Street, London, W1T 4BQ, England, U.K.
John D. Ginger: School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland, 4811, Australia



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