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CONTENTS
Volume 7, Number 3, May 2004
 

Abstract
The evaluation of the accumulation of permanent set for inelastic structures due to wind action is important in establishing a criterion to select a reduced design wind load and in incorporating the beneficial ductile behaviour in wind engineering. A parametric study of the accumulation of the permanent set as well as the ductility demand for bilinear single-degree-of-freedom (SDOF) systems is presented in the present study. The dynamic analysis of the inelastic SDOF system is carried out using the method of Newmark for artificially generated time history of wind speed. Simulation results indicate that the mean of the normalized damage rate is highly dependent on the natural frequency of vibration. This mean value is relatively insensitive to the damping ratio if the damping ratio is larger than 5%. Thernscatter associated with the accumulation of the permanent set is very significant. The consideration of the postyield stiffness can significantly reduce the accumulation of the permanent set if the ratio of the yield strength to the expected peak response is small. The results also show that the ductility demand due to the wind action over a period of one hour for flexible structures can be much less than that for rigid structures or structures with larger damping ratio if the SDOF systems are designed with a reduced peak response caused by the fluctuating wind.

Key Words
inelastic behaviour; ductility; wind loads; permanent set.

Address
H.P. Hong; Department of Civil and Environmental Engineering, University of Western Ontario, London, ON, N5A 6B9 Canada

Abstract
The study of how buildings affect wind flow is an important part of the research being conducted on urban climate and urban air quality. NJU-UCFM, a standard k- e turbulence closure model, is presented and is used to simulate how the following affect wind flow characteristics: (1) an isolated building, (2) urban canyons, (3) an irregular shaped building cluster, and (4) a real urban neighborhood. The numerical results are compared with previous researchers

Key Words
urban micro-climate; building; numerical simulation; urban canopy.

Address
Ning Zhang and Weimei Jiang ; Department of Atmospheric Science, Nanjing University, Nanjing, 210093 ChinarnFei Hu; Key Laboratory of Atmospheric Physics and Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029 China

Abstract
Flutter derivatives provide the basis of predicting the critical wind speed in flutter and buffeting analysis of long-span cable-supported bridges. In this paper, one popular stochastic system identification technique, covariance-driven Stochastic Subspace Identification(SSI in short), is firstly presented for estimation of the flutter derivatives of bridge decks from their random responses in turbulent flow. Secondly, wind tunnel tests of a streamlined thin plate model and a P type blunt bridge section model are conducted in turbulent flow and the flutter derivatives are determined by SSI. The flutter derivatives of the thin plate model identified by SSI are very comparable to those identified by the unifying least-square method and Theodorson

Key Words
flutter derivative; modal parameter identification; stochastic subspace identification (SSI); wind-induced vibration; bridge structure.

Address
Xian-rong Qin and Ming Gu; State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, 1239 Si-ping Road, Shanghai, 200092, P. R. China

Abstract
Wind tunnel experiments are often performed for the identification of aeroelastic parameters known as flutter derivatives that are necessary for the prediction of flutter instability for flexible structures. Experimental determination of all the eighteen flutter derivatives for a section model facilitates complete understanding of the physical mechanism of flutter. However, work in the field of identifying allrnthe eighteen flutter derivatives using section models with all three degree-of-freedom (DOF) has been limited. In the current paper, all eighteen flutter derivatives for a streamlined bridge deck and an airfoil section model were identified by using a new system identification technique, namely, Iterative Least Squares (ILS) approach. Flutter derivatives of the current bridge and the Tsurumi bridge are compared. Flutter derivatives related to the lateral DOF have been emphasized. Pseudo-steady theory for predicting some of the flutter derivatives is verified by comparing with experimental data. The three-DOF suspension system and the electromagnetic system for providing the initial conditions for free-vibration of the section model are also discussed.

Key Words
system identification; elastic suspension system; flutter derivatives; wind tunnel testing; pseudo-steady theory.

Address
Arindam Gan Chowdhury and Partha P. Sarkar; Department of Aerospace Engineering, Iowa State University, 2271 Howe Hall, Ames, Iowa, 50011-2271, USA

Abstract
This paper describes a method for developing a multi-degree-of freedom aero-elasto-plastic model of a base-isolated mid-rise building. The horizontal stiffness of isolators is modeled by several tension springs and the vertical support is performed by air pressure from a compressor. A lead damper and a steel damper are modeled by a U-shaped lead line and an aluminum line. With this model, thernfrequency ratio of torsional vibration to sway vibration, and plastic displacements of isolation materials can be changed easily when needed. The results of isolation material tests and free vibration tests show that this model provides the object performance. The peak displacement factors are about 4.5 regardless of wind speed in wind tunnel tests, but their gust response factor decreases with increment of wind speed.

Key Words
base-isolated building; wind tunnel test model; multi-degree-of-freedom model; elasto-plastic model.

Address
Takeshi Ohkuma; Department of Architecture and Building Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Yokohama, Kanagawa 221-8686, JapanrnHachinori Yasui and Hisao Marukawa; Urban Environment Research Center, Izumi Sohken Engineering Co., Ltd., 51, Minami-sode, Sodegaura, Chiba 299-0268, Japan


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