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CONTENTS
Volume 17, Number 2, August 2013
 

Abstract
Pressure field and velocity profiles in a thunderstorm downburst are significantly different from that of an atmospheric boundary layer wind. A model of the pressure field in a downburst is presented in accordance with the experimental and numerical results. Large eddy simulation method is employed to investigate transient pressure field on impingement ground of a downburst. In addition, velocity profiles of the downburst are studied, and good agreement is achieved between the present results and the data obtained from empirical models.

Key Words
thunderstorm downburst; pressure field; large eddy simulation

Address
Z. Tang and L.Y. Lu : Key Laboratory for RC & PC Structures of Ministry of Education, Southeast University, Nanjing 210096, China

Abstract
The majority of experiments to characterize the turbulence in the surface layer have been performed in flat, open expanses. In order to characterize the turbulence in built-up terrain, two mobile towers were deployed during Hurricane Ike (2008) in close proximity, but downwind of different terrain conditions: suburban and open. Due to the significant non-stationarity of the data primarily caused by changes in wind direction, empirical mode decomposition was employed to de-trend the signal. Analysis of the data showed that the along-wind mean turbulence intensity of the suburban terrain was 37% higher than that of the open terrain. For the mean vertical turbulence intensity, the increase for the suburban terrain was as high as 74%, which may have important implications in structural engineering. The gust factor of the suburban terrain was also 16% higher than that of the open terrain. Compared to non-hurricane spectral models, the obtained spectra showed significantly higher energy in low frequencies especially for the open terrain.

Key Words
hurricane; suburban; turbulence intensities; gust factors; integral scales; power spectra

Address
S. Jung : Department of Civil and Environmental Engineering, Florida A&M University –Florida State University College of Engineering, Tallahassee, FL 32310, USA
F.J. Masters : Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL 32611, USA

Abstract
An accurate identification of the aerodynamic characteristics of vehicles and the bridge is the premise for the coupled vibration analysis of a wind-vehicle-bridge system. At present, the interaction of aerodynamic forces between the road vehicles and bridge is ignored in most previous studies. In the present study, an experimental setup was developed to measure the aerodynamic characteristics of vehicles and the bridge for different cases in a wind tunnel considering the aerodynamic interference. The influence of the wind turbulence, the wind speed, the vehicle interference, and the vehicle position on the aerodynamic coefficients of vehicles, and the influence of vehicles on the static coefficients of the bridge were investigated, based on the experimental results. The variations in the aerodynamic characteristics of vehicles and the bridge were studied and the measured results were validated according to the results of surface pressure measurements on the vehicle and the bridge. The measured results were further validated by comparing the measured results with values derived numerically. The measured results showed that the wind turbulence, the vehicle interference, and the vehicle position significantly affected the aerodynamic coefficients of vehicles. However, the influence of the wind speed on the aerodynamic coefficients of the studied vehicle is small. The static coefficients of the bridge were also significantly influenced by the presence of vehicles.

Key Words
wind-vehicle-bridge system; cross winds; aerodynamic characteristics; aerodynamic interference; wind- tunnel tests; numerical simulation

Address
Yan Han, Jiexuan Hu, C.S. Caiand Chunguang Li : School of Civil Engineering and Architecture, Changsha University of Science & Technology,Changsha, Hunan, China, 410004
C.S. Cai: Department of Civil and Environmental Engineering, Louisiana State University,Baton Rouge, USA, LA 70803
Zhengqing Chen : Wind Engineering Research Center, College of Civil Engineering, Hunan University, Changsha, Hunan, China, 410082

Abstract
The goal of this study was to investigate the vulnerability of roof tile systems and metal shutters to roof tile debris. Three phases addressed the performance of tile roof systems and metal shutters impacted by roof tile debris. The first phase experimentally evaluated the tile fragment size and quantity generated by a tile striking a tile roof system. The second phase experimentally quantified the puncture vulnerability of common metal panel shutter systems as a function of tile fragment impact speed. The third phase provided context for interpretation of the experimental results through the use of a tile trajectory model. The results provide supporting evidence that while metal panel window shutters provide significant protection against a prevalent form of windborne debris, these systems are vulnerable to tile fragment puncture in design level tropical cyclones. These findings correlate with field observations made after Hurricane Charley (2004).

Key Words
windborne debris; missile impact; roof tiles; window shutters; puncture

Address
Sylvia T. Laboy-Rodriguez, Daniel Smith, Kurtis R. Gurley and Forrest J. Masters : Department of Civil and Coastal Engineering, University of Florida, USA

Abstract
An analysis framework for vehicle-bridge dynamic interaction system under turbulent wind is proposed based on the relevant theory of wind engineering and dynamics. Considering the fluctuating properties of wind field, the stochastic wind velocity time history is simulated by the Auto-Regressive method in terms of power spectral density function of wind field. The bridge is represented by three-dimensional finite element model and the vehicle by a multi-rigid-body system connected by springs and dashpots. The detailed calculation formulas of unsteady aerodynamic forces on bridge and vehicle are derived. In addition, the form selection of wind barriers, which are applied as the windbreak measures of newly-built railways in northwest China, is studied based on the suggested evaluation index, and the suitable values about height and porosity rate of wind barriers are studied. By taking a multi-span simply-supported box-girder bridge as a case study, the dynamic response of the bridge and the running safety indices of the train traveling on the bridge with and without wind barriers are calculated. The limit values of train speed with respect to different wind velocities are proposed according to the allowance values in the design code.

Key Words
wind-train-bridge coupled system; turbulent wind field; wind barrier; unsteady aerodynamic force; running safety of train

Address
T. Zhang, H. Xia and W.W. Guo : School of Civil Engineering, Beijing Jiaotong University, Beijing, 100044, China

Abstract
Non-stationary extreme winds such as thunderstorm downbursts are responsible for many structural damages. This research presents a time domain approach for estimating along-wind load effects on tall buildings using multiple wind speed time history samples, which are simulated from evolutionary power spectra density (EPSD) functions of non-stationary wind fluctuations using the method developed by the authors\' earlier research. The influence of transient wind loads on various responses including time-varying mean, root-mean-square value and peak factor is also studied. Furthermore, a simplified model is proposed to describe the non-stationary wind fluctuation as a uniformly modulated process with a modulation function following the time-varying mean. Finally, the probabilistic extreme response and peak factor are quantified based on the up-crossing theory of non-stationary process. As compared to the time domain response analysis using limited samples of wind record, usually one sample, the analysis using multiple samples presented in this study will provide more statistical information of responses. The time domain simulation also facilitates consideration of nonlinearities of structural and wind load characteristics over previous frequency domain analysis.

Key Words
non-stationary winds; evolutionary power spectra density function; time history samples; tall building response; extreme value distribution

Address
Guoqing Huang, Haili Liao and Mingshui Li : 1Research Center for Wind Engineering, School of Civil Engineering,
Southwest Jiaotong University, Chengdu, Sichuan 610031, China
Xinzhong Chen: Wind Science and Engineering Research Center, Department of Civil and Environmental Engineering,
Texas Tech University, Lubbock, Texas 79409, USA


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