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CONTENTS
Volume 10, Number 1, January 2007
 

Abstract
The suppression of aerodynamic response of long-span suspension bridges during erection and after completion by using single TMD and multi TMD is presented in this paper. An advanced finite-element-based aerodynamic model that can be used to analyze both flutter instability and buffeting response in the time domain is also proposed. The frequency-dependent flutter derivatives are transferred into a time-dependent rational function, through which the coupling effects of three-dimensional aerodynamic motions under gusty winds can be accurately considered. The modal damping of a structure-TMD system is analyzed by the state-space approach. The numerical examples are performed on the Akashi Kaikyo Bridge with a main span of 1990 m. The bridge is idealized by a three-dimensional finite-element model consisting of 681 nodes. The results show that when the wind velocity is low, about 20 m/s, the multi TMD type 1 (the vertical and horizontal TMD with 1% mass ratio in each direction together with the torsional TMD with ratio of 1% mass moment of inertia) can significantly reduce the buffeting response in vertical, horizontal and torsional directions by 8.6-13%. When the wind velocity increases to 40 m/s, the control efficiency of a multi TMD in reducing the torsional buffeting response increases greatly to 28%. However, its control efficiency in the vertical and horizontal directions reduces. The results also indicate that the critical wind velocity for flutter instability during erection is significantly lower than that of the completed bridge. By pylon-to-midspan configuration, the minimum critical wind velocity of 57.70 m/s occurs at stage of 85% deck completion.

Key Words
aerodynamic response; suspension bridges; Akashi Kaikyo Bridge; tuned mass dampers.

Address
Virote Boonyapinyo; Dept. of Civil Engineering, Thammasat University, Rangsit Campus, Pathumthani 12120, Thailand
Adul Aksorn; Faculty of Architecture, Khon Kean University, Khon Kean 40002, Thailand
Panitan Lukkunaprasit; Dept. of Civil Engineering, Chulalongkorn University, Bangkok 10330, Thailand

Abstract
Wind tunnel experiments on small scale groups of tanks are reported in the paper, with the aim of evaluating the pressure patterns due to group effects. A real tank configuration is studied in detail because one tank buckled during a hurricane category 3. Three configurations are studied in a wind tunnel, two with several tanks and different wind directions, and a third one with just one blocking tank. The pressures were measured in the cylindrical part and in the roof of the tank, in order to obtain pressure coefficients. Next, computational buckling analyses were carried out for the three configurations to evaluate the buckling pressure of the target structure. Finally, imperfection-sensitivity was investigated for one of the configurations, and moderate sensitivity was found, with reductions in the maximum load of the order of 25%. The results help to explain the buckling of the tank for the levels of wind experienced during the hurricane.

Key Words
buckling; shells; tanks; wind tunnel; wind pressures.

Address
Genock Portela; General Engineering Dept., University of Puerto Rico, Mayag?z, Puerto Rico 00681-9044, USA
Luis A. Godoy; Civil Infrastructure Research Center, Dept. of Civil Engineering and Surveying, Univ. of Puerto Rico, Mayag?z, Puerto Rico 00681-9041, USA

Abstract
Transmission tower is a vital component in electrical system. In order to accurately compute the dynamic response and reliability of transmission tower under the excitation of wind loading, a new method termed as probability density evolution method (PDEM) is introduced in the paper. The PDEM had been proved to be of high accuracy and efficiency in most kinds of stochastic structural analysis. Consequently, it is very hopeful for the above needs to apply the PDEM in dynamic response of wind-excited transmission towers. Meanwhile, this paper explores the wind stochastic field from stochastic Fourier spectrum. Based on this new viewpoint, the basic random parameters of the wind stochastic field, the roughness length z0 and the mean wind velocity at 10 m heigh U10, as well as their probability density functions, are investigated. A latticed steel transmission tower subject to wind loading is studied in detail. It is shown that not only the statistic quantities of the dynamic response, but also the instantaneous PDF of the response and the time varying reliability can be worked out by the proposed method. The results demonstrate that the PDEM is feasible and efficient in the dynamic response and reliability analysis of wind-excited transmission towers.

Key Words
transmission towers; wind; stochastic Fourier spectrum; probability density evolution; dynamic response; reliability.

Address
Lin-lin Zhang and Jie Li; Dept. of Building Engineering, School of Civil Engineering, Tongji University, Siping Road 1239, Shanghai, 200092, China

Abstract
This paper presents a novel finite element (FE) model for analyzing coupled flutter of long-span bridges using the commercial FE package ANSYS. This model utilizes a specific user-defined element Matrix27 in ANSYS to model the aeroelastic forces acting on the bridge, wherein the stiffness and damping matrices are expressed in terms of the reduced wind velocity and flutter derivatives. Making use of this FE model, damped complex eigenvalue analysis is carried out to determine the complex eigenvalues, of which the real part is the logarithm decay rate and the imaginary part is the damped vibration frequency. The condition for onset of flutter instability becomes that, at a certain wind velocity, the structural system incorporating fictitious Matrix27 elements has a complex eigenvalue with zero or near-zero real part, with the imaginary part of this eigenvalue being the flutter frequency. Case studies are provided to validate the developed procedure as well as to demonstrate the flutter analysis of cable-supported bridges using ANSYS. The proposed method enables the bridge designers and engineering practitioners to analyze flutter instability by using the commercial FE package ANSYS.

Key Words
long-span bridge; coupled flutter; instability; complex eigenvalue analysis; finite element (FE) model; ANSYS.

Address
X. G. Hua; Dept. of Civil and Structural Eng., The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Z. Q. Chen; College of Civil Eng., Hunan University, Changsha, Hunan 410083, P. R. China
Y. Q. Ni and J. M. Ko; Dept. of Civil and Structural Eng., The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Abstract
In the last decades there have been frequent reports of oscillations of slender tension members under simultaneous action of rain and wind - characterized by large amplitudes and low frequencies. The members, e.g. cables of cable-stayed bridges, slightly inclined hangers of arch bridges or cables of guyed-masts, show a circular cross section and low damping. These rain-wind induced vibrations negatively affect the serviceability and the lifespan of the structures. The present article gives a short literature review, describes a mathematical approach for the simulation of rain-wind induced vibrations, sums up some examples to verify the calculated results and discusses measures to suppress the vibrations.

Key Words
cable vibration; rain; wind; numerical simulation; aeroelastic instability.

Address
U. Peil and O. Dreyer; Institute for Steel Structures, Technical University, Braunschweig, Germany


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