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CONTENTS
Volume 8, Number 1, January 2005
 

Abstract
A simplified analysis able to point out the most relevant geometrical and aerodynamic parameters that can influence the flutter of long span modern bridges is the aim of the paper. With this goal, by using a continuous model of the suspension bridge and by a quasi stationary approach, a simple formula of the combined vertical/torsional flutter wind speed is given. A good agreement is obtained comparing the predictions from the proposed formula with the flutter speeds of three modern suspension or cable stayed bridges: the Great Belt East Bridge, the Akashi and Normandie bridges. The paper ends with some comments and comparisons with the well known Selberg formula.

Key Words
flutter of bridges, aerodynamic instability, long span bridges.

Address
Department of Civil Engineering, University of Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy

Abstract
This paper presents an analysis of wind velocity and pressure data obtained in a rural environment with a view to identifying the vortex structures present within the flow and examining the relationship between pressure and dynamic pressure. The data is analysed using both conventional analysis and conditional sampling. A method examining the eigenvalues of a matrix formed by the addition of thernsquare of the strain tensor and the square of the vorticity tensor is also investigated. This method illustrates that there are a number of vortex structures present in the flow. The work presented in this paper suggests that the extreme events occur as a result of the superposition of two independent mechanisms.

Key Words
vortex; vorticity; flow structures; conditional sampling; pressure fluctuations.

Address
M. Sterling and C. J. Baker; School of Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UKrnA. D. Quinn and R. P. Hoxey; Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4HS, UK

Abstract
Wind loading is very important in structural design of large-span single-layer reticulated shell structures. In this paper, a geometrically nonlinear wind-induced vibration analysis strategy for large-span single-layer reticulated shell structures based on the nonlinear finite element method is introduced. According to this strategy, a computation program has been developed. With the information of the wind pressure distribution measured simultaneously in the wind tunnel, nonlinear dynamic analysis, includingrndynamic instability analysis, for the wind-induced vibration of a single-layer reticulated shell is conducted as an example to investigate the efficiency of the strategy. Finally, suggestions are given for dynamic wind-resistant analysis of single-layer reticulated shells.

Key Words
large-span single-layer reticulated shells; wind-induced vibration; nonlinear dynamic analysis; dynamic stability

Address
Yuan-Qi Li; Department of Building Engineering, Tongji University, Shanghai 200092, ChinarnYukio Tamura; Wind Engineering Research Center, Tokyo Polytechnic University, Atsugi 243-0297, Japan

Abstract
A tornado changes its wind speed and direction rapidly; therefore, it is difficult to study the effects of a tornado on buildings in a wind tunnel. The status of the tornado-structure interaction and various models of the tornado wind field found in literature are surveyed. Three dimensional computer modeling work using the turbulence model based on large eddy simulation is presented. The effect of arntornado on a cubic building is considered for this study. The Navier-Stokes (NS) equations are approximated by finite difference method, and solved by an semi-implicit procedure. The force coefficients are plotted in time to study the effect of the Rankine combined vortex model. The tornado is made to translate at a 0o and 45o angle, and the grid resolution is refined. Some flow visualizations are also reported to understand the flow behavior around the cube.

Key Words
CFD; tornado; forces; building; cube; wind engineering.

Address
Department of Civil Engineering, University of Arkansas, Fayetteville, AR 72701, USA

Abstract
This paper presents results from an investigation of the structural behaviour of a very slender 90 m high steel chimney erected at Vaxjo in southern Sweden in 1995. The chimney is equipped with a mechanical friction-type damper at the top. Due to a mistake during erection and installation of the chimney the transport fixings of the damper were not released properly and the chimney developed extensive oscillations in the very first period of service. This caused a great number of fatigue cracks to occur within a few months of service. After the functioning of the damper had been restored and the fatigue cracks were repaired an extensive program was initiated in 1996 to monitor the structural behaviour of the chimney under wind loading. In the investigation data were collected for more than six years of continuous measurements and regular observations of the chimney. The data obtained have some general relevance with respect to wind data, behaviour of a slender structure under wind loading, and the effect of a mechanical damper. Also some theoretical studies were performed as part of the investigation of the chimney.

Key Words
full scale measurements; cross wind oscillations; vortex shedding; chimney; mechanical damper and dynamic wind loading

Address
Par Tranvik; Alstom Power Sweden AB, Box 1233, SE-351 12 Vaxjo, SwedenrnGoran Alpsten; Stalbyggnadskontroll AB, Bergshamra Alle 139, SE-170 77 Solna, Sweden


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