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CONTENTS
Volume 20, Number 5, May 2015
 

Abstract
This study presents a dynamic response analysis of operational and parked wind turbines in order to gain better understanding of the roles of wind loads on turbine blades and tower in the generation of turbine response. The results show that the wind load on the tower has a negligible effect on the blade responses of both operational and parked turbines. Its effect on the tower response is also negligible for operational turbine, but is significant for parked turbine. The tower extreme responses due to the wind loads on blades and tower of parked turbine can be estimated separately and then combined for the estimation of total tower extreme response. In current wind turbine design practice, the tower extreme response due to the wind loads on blades is often represented as a static response under an equivalent static load in terms of a concentrated force and a moment at the tower top. This study presents an improved equivalent static load model with additional distributed inertial force on tower, and introduces the square-root-of-sum-square combination rule, which is shown to provide a better prediction of tower extreme response.

Key Words
extreme response; wind loading; equivalent static loads; combination rule; wind turbine

Address
Kuangmin Gong and Xinzhong Chen: National Wind Institute, Department of Civil and Environmental Engineering, Texas Tech University, Lubbock, TX 79409, USA

Abstract
Several international codes have been developed for evaluating wind loads on structures; however, the wind structure interaction could not be accurately captured by these codes due to the gusty nature of wind and the dynamic behavior of structures. Therefore, the alternative wind tunnel testing was introduced. In this study, an introduction to the available approaches for wind load calculations for tall buildings was presented. Then, a comparative study between different codes: the Egyptian code, ECP 201-08, ASCE 7-05, BS 6399-2, and wind tunnel test results was conducted. An investigation has been carried out on two case studies tall buildings located within the Arabian Gulf region. Numerical models using (ETABS) software were produced to obtain the relation between codes analytical values and wind tunnel experimental test results for wind loads in the along and across wind directions. Results for the main structural responses including stories forces, shears, overturning moments, lateral displacements, and drifts were presented graphically in order to give clear comparison between the studied methods. The conclusions and recommendations for future works obtained from this research are finally presented to help improving Egyptian code provisions and show limitations for different cases.

Key Words
tall buildings; wind tunnel test; Egyptian code; ASCE 7-05; BS 6399-2; drift control

Address
Abdulmonem A. Badri, Manar M. Hussein and Walid A. Attia: Department of structural engineering, faculty of engineering, Cairo university, Egypt

Abstract
The wind tunnel test of large-scale sectional model and computational fluid dynamics (CFD) are employed for the purpose of studying the aerodynamic appendices and mechanism on suppression for the vortex-induced vibration (VIV). This paper takes the HongKong-Zhuhai-Macao Bridge as an example to conduct the wind tunnel test of large-scale sectional model. The results of wind tunnel test show that it is the crash barrier that induces the vertical VIV. CFD numerical simulation results show that the distance between the curb and crash barrier is not long enough to accelerate the flow velocity between them, resulting in an approximate stagnation region forming behind those two, where the continuous vortex-shedding occurs, giving rise to the vertical VIV in the end. According to the above, 3 types of wind fairing (trapezoidal, airfoil and smaller airfoil) are proposed to accelerate the flow velocity between the crash barrier and curb in order to avoid the continuous vortex-shedding. Both of the CFD numerical simulation and the velocity field measurement show that the flow velocity of all the measuring points in case of the section with airfoil wind fairing, can be increased greatly compared to the results of original section, and the energy is reduced considerably at the natural frequency, indicating that the wind fairing do accelerate the flow velocity behind the crash barrier. Wind tunnel tests in case of the sections with three different countermeasures mentioned above are conducted and the results compared with the original section show that all the three different countermeasures can be used to control VIV to varying degrees.

Key Words
bridge deck with long projecting slab; vortex-induced vibration; wind fairing; CFD; large-scale sectional model test; mechanism

Address
Zhiyong Zhou, Ting Yang, Quanshun Dingand Yaojun Ge: State Key Laboratory for Disaster Reduction in Civil Engineering, Transport Industry Key Laboratory for Wind Resistance Technique in Bridge Engineering, Tongji University, Shanghai 200092, China

Abstract
This study investigates the use of time-frequency coherence analysis for detecting and evaluating coherent \"structures\" of surface pressures and wind turbulence components, simultaneously on the time-frequency plane. The continuous wavelet transform-based coherence is employed in this time-frequency examination since it enables multi-resolution analysis of non-stationary signals. The wavelet coherence quantity is used to identify highly coherent \"events\" and the \"coherent structure\" of both wind turbulence components and surface pressures on rectangular prisms, which are measured experimentally. The study also examines, by proposing a \"modified\" complex Morlet wavelet function, the influence of the time-frequency resolution and wavelet parameters (i.e., central frequency and bandwidth) on the wavelet coherence of the surface pressures. It is found that the time-frequency resolution may significantly affect the accuracy of the time-frequency coherence; the selection of the central frequency in the modified complex Morlet wavelet is the key parameter for the time-frequency resolution analysis. Furthermore, the concepts of time-averaged wavelet coherence and wavelet coherence ridge are used to better investigate the time-frequency coherence, the coherently dominant events and the time-varying coherence distribution. Experimental data derived from physical measurements of turbulent flow and surface pressures on rectangular prisms with slenderness ratios B/D=1:1 and B/D=5:1, are analyzed.

Key Words
bluff body; time-frequency analysis; turbulence; pressure distribution; flow separation/attachment/reattachment

Address
Thai-Hoa Le:Department of Civil and Environmental Engineering, Northeastern University, 360 Huntington Ave., Boston, MA 02115, USA;
Department of Engineering Mechanics and Automation, Vietnam National University, Hanoi, 144 Xuanthuy Rd., Hanoi, Vietnam
Luca Caracoglia: Department of Civil and Environmental Engineering, Northeastern University, 360 Huntington Ave., Boston, MA 02115, USA

Abstract
Lighting poles and antenna masts are typically high, slender and light structures. Moreover, they are often characterized by distributed eccentricities that make very complex their shape. Experience teaches that this structural type frequently suffers severe damage and even collapses due to wind actions. To understand and interpret the aerodynamic and aeroelastic behavior of lighting poles and antenna masts, this paper presents the results of static and aeroelastic wind tunnel tests carried out on a complex prismatic element representing a segment of the shaft of such structures. Static tests are aimed at determining the aerodynamic coefficients and the Strouhal number of the test element cross-section; the former are used to evaluate the critical conditions for galloping occurrence based on quasi-steady theory; the latter provides the critical conditions for vortex-induced vibrations. Aeroelastic tests are aimed at reproducing the real behavior of the test element and at verifying the validity and reliability of quasi-steady theory. The galloping hysteresis phenomenon is identified through aeroelastic experiments conducted on increasing and decreasing the mean wind velocity.

Key Words
complex structure; galloping; hysteresis; quasi-steady theory; vortex-induced vibrations; wind-induced instability; wind tunnel test

Address
Cung Huy Nguyen, Andrea Freda, Giovanni Solari and Federica Tubino: Department of Civil, Chemical and Environmental Engineering (DICCA), University of Genoa, Via Montallegro 1, 16145 Genoa, Italy

Abstract
The joint distribution of wind speed and wind direction at a bridge site is vital to the estimation of the basic wind speed, and hence to the wind-induced vibration analysis of long-span bridges. Instead of the conventional way relying on the weather stations, this study proposed an alternate approach to obtain the original records of wind speed and the corresponding directions based on field measurement supported by the Structural Health Monitoring System (SHMS). Specifically, SHMS of Sutong Cable-stayed Bridge (SCB) is utilized to study the basic wind speed with directional information. Four anemometers are installed in the SHMS of SCB: upstream and downstream of the main deck center, top of the north and south tower respectively. Using the recorded wind data from SHMS, the joint distribution of wind speed and direction is investigated based on statistical methods, and then the basic wind speeds in 10-year and 100-year recurrence intervals at these four key positions are calculated. Analytical results verify the reliability of the recorded wind data from SHMS, and indicate that the joint probability model for the extreme wind speed at SCB site fits well with the Weibull model. It is shown that the calculated basic wind speed is reduced by considering the influence of wind direction. Compared to the design basic wind speed in the Specification of China, basic wind speed considering the influence of direction or not is much smaller, indicating a high safety coefficient in the design of SCB. The results obtained in this study can provide not only references for further wind-resistance research of SCB, but also improve the understanding of the safety coefficient for wind-resistance design of other engineering structures in the similar area.

Key Words
joint distribution; wind speed; wind direction; Sutong Cable-stayed Bridge (SCB); field measurement; Structural Health Monitoring System (SHMS); basic wind speed

Address
Hao Wang, Tianyou Tao, Jianxiao Mao and Aiqun Li: Key Laboratory of C&PC Structures of Ministry of Education, Southeast University, No. 2 Sipailou, Nanjing 210096, China
Teng Wu: Department of Civil, Structural and Environmental Engineering, University at Buffalo, State University of New York, Buffalo, NY 14126, USA


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