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CONTENTS
Volume 14, Number 3, May 2011
 

Abstract
Based on the ANSYS, an approach of full-mode aerodynamic flutter analysis for long-span suspension bridges has been presented in this paper, in which the nonlinearities of structure, aerostatic and aerodynamic force due to the deformation under the static wind loading are fully considered. Aerostatic analysis is conducted to predict the equilibrium position of a bridge structure in the beginning, and then flutter analysis of such a deformed bridge structure is performed. A corresponding computer program is developed and used to predict the critical flutter wind velocity and the corresponding flutter frequency of a long-span suspension bridge with double main span. A time-domain analysis of the bridge is also carried out to verify the frequency-domain computational results and the effectiveness of the approach proposed in this paper. Then, the nonlinear effects on aerodynamic behaviors due to aerostatic action are discussed in detail. Finally, the results are compared with those of traditional suspension bridges with single main span. The results show that the aerostatic action has an important influence on the flutter stability of long-span suspension bridges. As for a suspension bridge with double main spans, the flutter mode is the first anti-symmetrical torsional vibration mode, which is also the first torsional vibration mode in natural mode list. Furthermore, a double main-span suspension bridge is better in structural dynamic and aerodynamic performances than a corresponding single main-span structure with the same bridging capacity.

Key Words
suspension bridge; double main spans; full-mode; flutter; aerostatic action.

Address
W.M. Zhang and Y.J. Ge : State Key Laboratory for Disaster Reduction in Civil Engineering, Department of Bridge Engineering,Tongji University, Shanghai 200092, P.R.China
M.L. Levitan : Civil and Environmental Engineering Department, Louisiana State University, Baton Rouge, LA 70803, United States

Abstract
The cable system is generally considered to be a structural solution to increase the spanning capacity of suspension bridges. In this work, based on the Runyang Bridge over the Yangtze River, three case suspension bridges with different 3D cable systems are designed, structural dynamic characteristics, the aerostatic and aerodynamic stability are investigated numerically by 3D nonlinear aerostatic and aerodynamic analysis, and the cable system favorable to improve the wind-induced instability of long-span suspension bridges is also proposed. The results show that as compared to the example bridge with parallel cable system, the suspension bridge with inward-inclined cable system has greater lateral bending and tensional frequencies, and also better aerodynamic stability; as for the suspension bridge with outward-inclined cable system, it has less lateral bending and tensional frequencies, and but better aerostatic stability; however the suspension bridge is more prone to aerodynamic instability, and therefore considering the whole wind-induced instability, the parallel and inward-inclined cable systems are both favorable for long-span suspension bridges.

Key Words
long-span suspension bridge; 3D cable system; dynamic characteristics; aerostatic stability; aerodynamic stability.

Address
Xin-Jun Zhang : College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310032, P.R. China

Abstract
The estimated response of large-scale engineering structures to severe wind loads is prone to modelling uncertainties that can only ultimately be assessed by full-scale testing. To this end ambient vibration data from full-scale monitoring of the historic Clifton Suspension Bridge has been analysed using a combination of a frequency domain system identification method and a more elaborate stochastic identification technique. There is evidence of incipient coupling action between the first vertical and torsional modes in strong winds, providing unique full-scale data and making this an interesting case study. Flutter derivative estimation, which has rarely previously been attempted on full-scale data, was performed to provide deeper insight into the bridge aerodynamic behaviour, identifying trends towards flutter at higher wind speeds. It is shown that, as for other early suspension bridges with bluff crosssections, single-degree-of-freedom flutter could potentially occur at wind speeds somewhat below requirements for modern designs. The analysis also demonstrates the viability of system identification techniques for extracting valuable results from full-scale data.

Key Words
full-scale; system identification; ambient vibration data; flutter derivatives.

Address
Nikolaos Nikitas and John H.G. Macdonald : Department of Civil Engineering University of Bristol, University Walk, Bristol, BS8 1TR, UK
Jasna B. Jakobsen : Department of Mechanical and Structural Engineering and Material Science University of Stavanger, N-4036, Stavanger, Norway

Abstract
The wind-induced mean, background and resonant responses of Beijing National Stadium are investigated in this paper. Based on the concepts of potential and kinetic energies, the mode participation factors for the background and the resonant components are presented and the dominant modes are identified. The coupling effect between different modes of the resonant response and the coupling effect between the background and resonant responses are analyzed. The coupling effects between the background and resonant components and between different modes are found all negligible. The mean response is approximately analogous to the peak responses induced by the fluctuating wind. The background responses are significant in the fluctuating responses and it is much larger than the resonant responses at the measurement locations.

Key Words
background response; resonant response; mode participation coefficient; coupling effect; mode participation factor; stadium.

Address
YANG Q.S.and TIAN Y.J. : School of Civil Engineering, Beijing Jiaotong University, Beijing 100044

Abstract
This paper presents the field measurement results of wind effects on a super-tall building (CITIC Plaza, 391 m high) located in Guangzhou. The field data such as wind speed, wind direction and acceleration responses were simultaneously and continuously recorded from the tall building by a wind and vibration monitoring system during two typhoons. The typhoon-generated wind characteristics including turbulence intensity, gust factor, peak factor, turbulence integral length scale and power spectral density of fluctuating wind speed were presented and discussed. The dynamic characteristics of the tall building were determined based on the field measurements and compared with those calculated from a 3D finite element model of the building. The measured natural frequencies of the two fundamental sway modes of the building were found to be larger than those calculated. The damping ratios of the building were evaluated by the random decrement technique, which demonstrated amplitude-dependent characteristics. The field measured acceleration responses were compared with wind tunnel test results, which were found to be consistent with the model test data. Finally, the serviceability performance of the super-tall building was assessed based on the field measurement results.

Key Words
wind effect; reinforced concrete structure; tall building; typhoon; full-scale measurement; dynamic response; wind tunnel test.

Address
Lunhai Zhi and Z.N. Li:Key Laboratory of Building Safety and Energy Efficiency of Ministry of Education, College of Civil Engineering, Hunan University, Changsha, Hunan, P.R. China
Q.S. Li and J.R. Wu : Department of Building and Construction, City University of Hong Kong,Tat Chee Avenue, Kowloon, Hong Kong
J.R. Wu : School of Civil Engineering, Guangzhou University, Guangzhou, Guangdong, P.R. China


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