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CONTENTS
Volume 13, Number 5, September 2010
 

Abstract
This article provides a time-resolved characterisation of the wind field in a recentlycommissioned, downdraft outflow simulator at The University of Western Ontario. A large slot jet approach to physical simulation was used. The simulator performance was assessed against field observations from a 2002 downdraft outflow near Lubbock, Texas. Outflow wind speed records were decomposed according to classical time series analysis. Length scales, characterising the coarse and fine flow structure, were determined from the time-varying mean and residual components, respectively. The simulated downdraft outflow was approximately 1200 times smaller in spatial extent than the 2002 Lubbock event.

Key Words
downdraft outflow; downburst; microburst; localised high-intensity wind; wind tunnel; slot jet; mixing layer; transient signal; time series analysis.

Address
Department of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario, Canada, N6A 5B9

Abstract
The focus of this article is on the assessment of vertical wind vector components and their aerodynamic impact on lattice framework, specifically two distinct sections of a guyed transmission tower. Thunderstorm winds, notably very localized events such as convective downdrafts (including downbursts) and tornadoes, result in a different load on a tower structural system in terms of magnitude and spatial distribution when compared to horizontal synoptic winds. Findings of previous model-scale experiments are outlined and their results considered for the development of a testing rig that allows for rotation about multiple body axes through a series of wind tunnel tests. Experimental results for the wind loads on two unique experimental models are presented and the difference in behaviour discussed. For a model cross arm with a solidity ratio of approximately 30%, the drag load was increased by 14% when at a pitch angle of 20o. Although the effects of rotation about the vertical body axis, or the traditional angle of attack, are recognized by design codes as being significant, provisions for vertical winds are absent from each set of wind loading specifications examined. The inclusion of a factor to relate winds with a vertical component to the horizontal speed is evaluated as a vertical wind factor applicable to load calculations. Member complexity and asymmetric geometry often complicate the use of lattice wind loading provisions, which is a challenge that extends to future studies and codification. Nevertheless, the present work is intended to establish a basis for such studies.

Key Words
lattice tower; wind tunnel testing; thunderstorm winds; non-horizontal wind; downburst simusimulation; transmission tower.

Address
T.G. Mara: The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, London, ON, Canada N6A 5B9
J.K. Galsworthy: Rowan Williams Davies & Irwin Inc., 650 Woodlawn Road West, Guelph, ON, Canada, N1K 1B8
E. Savory: Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, Canada N6A 5B9

Abstract
Wind tunnel model tests were conducted for a residential apartment block located within the complex terrain of The Hong Kong University of Science and Technology (HKUST). The test building is typical of medium-rise residential buildings in Hong Kong. The model study was conducted using modelling techniques and assumptions that are commonly used to predict design wind loads and pressures for buildings sited in regions of significant topography. Results for the building model with and without the surrounding topography were compared to investigate the effects of far-field and near-field topography on wind characteristics at the test building site and wind-induced external pressure coefficients at key locations on the building facade. The study also compared the wind tunnel test results to topographic multipliers and external pressure coefficients determined from nine international design standards. Differences between the external pressure coefficients stipulated in the various standards will be exacerbated when they are combined with the respective topographic multipliers.

Key Words
wind-induced pressure; medium-rise building; topography; wind tunnel; international design standards and codes of practice.

Address
P.A. Hitchcock: CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, Hong Kong SAR
K.C.S. Kwok: School of Engineering, University of Western Sydney, Australia
K.S. Wong and K.M. Shum: CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology, Hong Kong SAR

Abstract
The majority of weather-related failures of transmission line structures that have occurred in the past have been attributed to high intensity localized wind events, in the form of tornadoes and downbursts. A numerical scheme is developed in the current study to assess the performance of transmission lines under tornado wind load events. The tornado wind field is based on a model scale Computational Fluid Dynamic (CFD) analysis that was conducted and validated in a previous study. Using field measurements and code specifications, the CFD model data is used to estimate the wind fields for F4 and F2 full scale tornadoes. The wind forces associated with these tornado fields are evaluated and later incorporated into a nonlinear finite element three-dimensional model for the transmission line system, which includes a simulation for the towers and the conductors. A comparison is carried between the forces in the members resulting from the tornadoes, and those obtained using the conventional design wind loads. The study reveals the importance of considering tornadoes when designing transmission line structures.

Key Words
tornado; finite element; transmission line; transmission tower; wind load.

Address
A. Hamada and A.A. El Damatty: Department of Civil and Environmental Engineering, Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9
H. Hangan: Alan G. Davenport Wind Engineering Group, The Boundary Layer Wind Tunnel Laboratory, Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada
A.Y. Shehata: Atomic Energy of Canada Limited, Mississauga, Ontario, Canada

Abstract
In comparison with the common two-tower suspension bridge, due to the lack of effective longitudinal restraint of the center tower, the three-tower suspension bridge becomes a structural system with greater flexibility, and more susceptible to the wind action. By taking a three-tower suspension bridge-the Taizhou Bridge over the Yangtze River with two main spans of 1080 m as example, effects of structural parameters including the cable sag to span ratio, the side to main span ratio, the deck bearing system, longitudinal structural form of the center tower and the cable system on the aerodynamic stability of the bridge are investigated numerically by 3D nonlinear aerodynamic stability analysis, the favorable structural system of three-tower suspension bridge with good wind stability is discussed. The results show that good aerodynamic stability can be obtained for three-tower suspension bridge as the cable sag to span ratio is assumed ranging from 1/10 to 1/11, the central buckle are provided between main cables and the deck at midpoint of main spans, the longitudinal bending stiffness of the center tower is strengthened, and the spatial cable system or double cable system is employed.

Key Words
three-tower suspension bridge; aerodynamic stability; structural parameters.

Address
College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310032, P.R. China


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