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CONTENTS
Volume 14, Number 2, March 2011
 

Abstract
Wind turbine structures are long slender columns with a rotor and blade assembly placed on the top. These slender structures vibrate due to dynamic environmental forces and its own dynamics. Analysis of the dynamic behavior of wind turbines is fundamental to the stability, performance, operation and safety of these systems. In this paper a simplied approach is outlined for free vibration analysis of these long, slender structures taking the soil-structure interaction into account. The analytical method is based on an Euler-Bernoulli beam-column with elastic end supports. The elastic end-supports are considered to model the flexible nature of the interaction of these systems with soil. A closed-form approximate expression has been derived for the first natural frequency of the system. This new expression is a function of geometric and elastic properties of wind turbine tower and properties of the foundation including soil. The proposed simple expression has been independently validated using an exact numerical method, laboratory based experimental measurement and field measurement of a real wind turbine structure. The results obtained in the paper shows that the proposed expression can be used for a quick assessment of the fundamental frequency of a wind turbine taking the soil-structure interaction into account.

Key Words
mono-pile; natural frequency; beam theory; wind-turbine; soil stiffness.

Address
S. Adhikari : College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK
S. Bhattacharya :Department of Civil Engineering, University of Bristol, 2.37, Queen\'s Building,
University Walk, Clifton, Bristol, BS8 1TR, UK

Abstract
Excessive wind-induced motion in tall buildings can cause discomfort, affect health, and disrupt the daily activities of the occupants of a building. Dynamic vibration absorbers such as the tuned mass dampers (TMDs) can be used to reduce the wind-induced motion below a specified tolerable serviceability limit state (SLS) criterion. This study investigates whether the same probability of not exceeding specified wind-induced motion levels can be achieved by torsionally sensitive structures without/with linear/nonlinear TMDs subjected to partially correlated wind forces, if they are designed to just meet the same SLS criterion. For the analyses, different structures and the uncertainty in the response, wind load and perception of motion is considered. Numerical results indicate that for structures that are designed or retrofitted without or with optimum linear TMDs and satisfying the same SLS criterion, their probability of exceeding the considered criterion is very consistent, if the inherent correlation between the wind forces is considered in design. However, this consistency deteriorates if nonlinear TMDs are employed. Furthermore, if the correlation is ignored in the design, in many cases a slightly unconservative design, as compared to the designed by considering correlation, is achieved.

Key Words
tuned mass dampers; torsional response; probabilistic analysis; limit state function, serviceability limit state.

Address
A. Pozos-Estrada, H.P. Hong and J.K. Galsworthy :Department of Civil and Environmental Engineering, University of Western Ontario, Canada N6A 5B9

Abstract
For a detailed investigation of the dynamic behaviour of slender bridges under wind action especially the motion-induced fluid forces should be available not only for harmonic motions but also for more general ones. If linear transfer behaviour is assumed, the force-displacement relation for almost arbitrary motions can be handled in the frequency domain using aerodynamic transfer functions. In aerospace engineering as well as in bridge engineering, these functions are usually approximated by special kinds of complex-valued rational functions which depend on complex frequencies. The quality of this approximation is evaluated for several bridge cross sections in this article. It is shown that rational functions are for some sections scarcely suitable to realistically represent the transfer behaviour of motioninduced aerodynamic forces for arbitrarily complex frequencies.

Key Words
bridges; rational function approximation; state-space model; flutter; divergence.

Address
Arno Kirch and Udo Peil : Institute of Steel Structures, Technische Universitat Carolo-Wilhelmina zu Braunschweig,
Beethovenstrabe, 51, 38106 Braunschweig, Germany

Abstract
The alternative solution for flutter and buffeting stability of a long suspension bridge will be a passive control using flaps. This method not only enables a lightweight economic stiffening girder without an additional stiffness for aerodynamic stability but also avoid the problems from the malfunctions of control systems and energy supply system of an active control by winglets and flaps. A mechanically control using flaps for increasing flutter speed and decreasing buffeting response of a suspension bridge is experimentally studied through a two dimensional bridge deck model. The result shows that the flutter speed is increased and the buffeting response is decreased through the mechanical drive of the flaps.

Key Words
flutter; buffeting; suspension bridge; passive control; flaps.

Address
Duc - Huynh Phan : Department of Civil Engineering, The University of Education and Technology, VietNam
Hiroshi Kobayshi : Ritsumeikan University, Japan

Abstract
This paper presents the results of measurements relating to the aerodynamic forces on flat square plates which were allowed to rotate at different speeds about their horizontal axis, by modifying the velocity of the incoming flow. A 1 m square test-sheet and a 0.3 m square test-sheet were fitted with a number of pressure sensors in order to obtain information relating to the instantaneous pressure distribution acting on the test-sheet; a compact gyroscope to record the angular velocity during the rotational motion was also implemented. Previous work on autorotation has illustrated that the angular velocity varies with respect to the torque induced by the wind, the thickness and aspect ratio of the testsheet,any frictional effects present at the bearings, and the vorticity generated through the interaction between the plate and the wind flow. The current paper sets out a method based on the solution of the equation of motion of a rotating plate which enables the determination of angular velocities on autorotating elements to be predicted. This approach is then used in conjunction with the experimental data in order to evaluate the damping introduced by the frictional effects at the bearings during steady autorotation.

Key Words
autorotation; aerodynamic forces; windborne debris.

Address
P. Martinez-Vazquez, M. Sterling, C.J. Baker, A.D. Quinn : School of Civil Engineering, University of Birmingham, UK
P.J. Richards :Department of Mechanical Engineering, University of Auckland, New Zealand


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