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CONTENTS
Volume 10, Number 5, September 2007
 

Abstract
Field measurements of the wind-induced response of two residential reinforced concrete buildings, among the tallest in the world, have been performed during two typhoons. Natural periods and damping values have been determined and compared with other field measurements and empirical predictors. Suitable and common empirical predictors of natural period and structural damping have been obtained that describe the trend of tall, reinforced concrete buildings whose structural vibrations have been measured in the collection of studies in Hong Kong compiled by the authors. This data is especially important as the amount of information known about the dynamic parameters of buildings of these heights is limited. Effects of the variation of the natural period and damping values on the alongwind response of a tall building for serviceability-level wind conditions have been profiled using the gust response factor approach. When using this approach on these two buildings, the often overestimated natural periods and structural damping values suggested by empirical predictors tended to offset each other. Gust response factors calculated using the natural periods and structural damping values measured in the field were smaller than if calculated using design-stage values.

Key Words
residential; tall buildings; typhoons; full-scale measurement; dynamic characteristics.

Address
CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, Hong Kong S.A.R., P.R.China

Abstract
A two-dimensional flutter analysis method (2d-3DOF method) was developed to simultaneously investigate the relationship between oscillation parameters and aerodynamic derivatives of three degrees of freedom, and to clarify the coupling effects of different degrees of freedom in flutter instability. With this method, the flutter mechanism of two typical bridge deck sections, box girder section and two-isolated-girder section, were numerically investigated, and both differences and common ground in these two typical flutter phenomena are summarized. Then the flutter stabilization effect and its mechanism for long-span bridges with box girders by using central-slotting were studied by experimental investigation of aerodynamic stability and theoretical analysis of stabilizing mechanism. Possible explanation of new findings in the evaluation trend of critical wind speed through central vent width is finally presented.

Key Words
aerodynamic instability; flutter analysis; long span bridge; coupling effect; flutter mechanism; central slotting; vent width

Address
State Key Lab for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China

Abstract
A non-translating, long duration thunderstorm downburst has been simulated experimentally and numerically by modelling a spatially stationary steady flow impinging air jet. Velocity profiles were shown to compare well with an upper-bound of velocity measurements reported for full-scale microbursts. Velocity speed-up over a range of topographic features in simulated downburst flow was also tested with comparisons made to previous work in a similar flow, and also boundary layer wind tunnel experiments. It was found that the amplification measured above the crest of topographic features in simulated downburst flow was up to 35% less than that observed in boundary layer flow for all shapes tested. From the computational standpoint we conclude that the Shear Stress Transport (SST) model performs the best from amongst a range of eddy-viscosity and second moment closures tested for modelling the impinging jet flow.

Key Words
downburst; speed-up; topography; impinging jet.

Address
M.S. Mason; School of Civil Engineering, University of Sydney, Sydney, NSW 2006, Australia
G.S. Wood; Cermak Peterka Petersen, Pty. Ltd., St. Peters, NSW 2044, Australia
D.F. Fletcher; School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSW 2006, Australia

Abstract
A series of wind tunnel sectional model dynamic tests of a twin-deck bridge were conducted at the CLP Power Wind/Wave Tunnel Facility (WWTF) of The Hong Kong University of Science and Technology (HKUST) to investigate the effects of gap-width on the self-excited vibrations and the dynamic and aerodynamic characteristics of the bridge. Five 2.9 m long models with different gap-widths were fabricated and suspended in the wind tunnel to simulate a two-degrees-of-freedom (2DOF) bridge dynamic system, free to vibrate in both vertical and torsional directions. The mass, vertical frequency, and the torsional-to-vertical frequency ratio of the 2DOF systems were fixed to emphasize the effects of gap-width. A free-vibration test methodology was employed and the Eigensystem Realization Algorithm (ERA) was utilized to extract the eight flutter derivatives and the modal parameters from the coupled free-decay responses. The results of the zero gap-width configuration were in reasonable agreement with the theoretical values for an ideal thin flat plate in smooth flow and the published results of models with similar cross-sections, thus validating the experimental and analytical techniques utilized in this study. The methodology was further verified by the comparison between the measured and predicted free-decay responses. A comparison of results for different gap-widths revealed that variations of the gap-width mainly affect the torsional damping property, and that the configurations with greater gap-widths show a higher torsional damping ratio and hence stronger aerodynamic stability of the bridge.

Key Words
wind-induced vibration; flutter derivative; system identification; twin-deck bridge; wind tunnel dynamic test.

Address
X.R. Qin; CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Institute of Mechanical Design and Its Theory, School of Mechanical Engineering, Tongji University, 1239 Si-Ping Road, Shanghai, 200092, China
K.C.S. Kwok, C.H. Fok and P.A. Hitchcock; CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Y.L. Xu; Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

Abstract
Downbursts are transient phenomena that produce wind profiles that are distinctly different from synoptic boundary layers. Wind field data from Computational Fluid Dynamics (CFD) simulations of isolated downburst-like impinging jets, are used to investigate structural loads of tall buildings due to these high intensity winds. The base shear forces and base moments of tall buildings of heights between 120 and 250 m produced by downburst winds of various scales are compared with the forces from the equivalent boundary layer gust winds, with matched 10-metre wind velocity. The wind profiles are mainly functions of the size of the downburst and the radial distance from the centre of the storm. Wind forces due to various downburst profiles are investigated by placing the building at different locations relative to the storm center as well as varying the size of the downburst. Overall it is found that downbursts larger than approx. 2,000 m in diameter might produce governing design wind loads above those from corresponding boundary layer winds for tall buildings.

Key Words
downbursts; boundary layers; structural loads; tall buildings.

Address
Alan G Davenport Wind Engineering Group, The University of Western Ontario, London, Ontario, Canada


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