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CONTENTS
Volume 13, Number 1, January 2010
 

Abstract
The recommended factored design wind load effects for overhead lattice transmission line towers by codes and standards are evaluated based on the applicable wind load factor, gust response factor and design wind speed. The current factors and design wind speed were developed considering linear elastic responses and selected notional target safety levels. However, information on the nonlinear inelastic responses of such towers under extreme dynamic wind loading, and on the structural capacity curves of the towers in relation to the design capacities, is lacking. The knowledge and assessment of the capacity curve, and its relation to the design strength, is important to evaluate the integrity and reliability of these towers. Such an assessment was performed in the present study, using a nonlinear static pushover (NSP) analysis and incremental dynamic analysis (IDA), both of which are commonly used in earthquake engineering. For the IDA, temporal and spatially varying wind speeds are simulated based on power spectral density and coherence functions. Numerical results show that the structural capacity curves of the tower determined from the NSP analysis depend on the load pattern, and that the curves determined from the nonlinear static pushover analysis are similar to those obtained from IDA.

Key Words
Transmission tower; wind load; nonlinear static pushover analysis; incremental dynamic analysis; capacity curve.

Address
S.S. Banik, H.P. Hong and Gregory A. Kopp; Department of Civil and Environmental Engineering, University of Western Ontario, Canada N6A 5B9

Abstract
A new full scale testing apparatus generically named the Wall of Wind (WoW) has been built by the researchers at the International Hurricane Research Center (IHRC) at Florida International University (FIU). WoW is capable of testing single story building models subjected up to category 3 hurricane wind speeds. Depending on the relative model and WoW wind field sizes, testing may entail blockage issues. In addition, the proximity of the test building to the wind simulator may also affect the aerodynamic data. This study focuses on the Computational Fluid Dynamics (CFD) assessment of the effects on the quality of the aerodynamic data of (i) blockage due to model buildings of various sizes and (ii) wind simulator proximity for various distances between the wind simulator and the test building. The test buildings were assumed to have simple parallelepiped shapes. The computer simulations were performed under both finite WoW wind-field conditions and in an extended Atmospheric Boundary Layer (ABL) wind flow. Mean pressure coefficients for the roof and the windward and leeward walls served as measures of the blockage and wind simulator proximity effects. The study uses the commercial software FLUENT with Reynolds Averaged Navier Stokes equations and a Renormalization Group (RNG) k-? turbulence model. The results indicated that for larger size test specimens (i.e. for cases where the height of test specimen is larger than one third of the wind field height) blockage correction may become necessary. The test specimen should also be placed at a distance greater than twice the height of the test specimen from the fans to reduce proximity effect.

Key Words
full scale testing; blockage; wind simulator proximity; CFD; pressure coefficient; turbulence.

Address
Girma T. Bitsuamlak, Agerneh Dagnew and Arindam Gan Chowdhury; Laboratory for Wind Engineering Research, International Hurricane Research Center, Department of
Civil and Environmental Engineering, Florida International University, Miami, Florida 33174, USA.

Abstract
In this paper South Africa is divided into strong wind climate zones, which indicate the main sources of annual maximum wind gusts. By the analysis of wind gust data of 94 weather stations, which had continuous climate time series of 10 years or longer, six sources, or strong-wind producing mechanisms, could be identified and zoned accordingly. The two primary causes of strong wind gusts are thunderstorm activity and extratropical low pressure systems, which are associated with the passage of cold fronts over the southern African subcontinent. Over the eastern and central interior of South Africa annual maximum wind gusts are usually caused by thunderstorm gust fronts during summer, while in the western and southern interior extratropical cyclones play the most dominant role. Along the coast and adjacent interior annual extreme gusts are usually caused by extratropical cyclones. Four secondary sources of strong winds are the ridging of the quasi-stationary Atlantic and Indian Ocean high pressure systems over the subcontinent, surface troughs to the west in the interior with strong ridging from the east, convergence from the interior towards isolated low pressure systems or deep coastal low pressure systems, and deep surface troughs on the West Coast.

Key Words
strong winds; wind climate; climate zones; South Africa.

Address
A.C. Kruger; Climate Service Division, South African Weather Service, Private Bag X097, Pretoria 0001, South Africa
A.M. Goliger; Division of Built Environment, CSIR, P. O. Box 395, Pretoria 0001, South Africa
J.V. Retief; Department of Civil Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
S. Sekele; Climate Service Division, South African Weather Service, Private Bag X097, Pretoria 0001, South Africa

Abstract
The 486m-long roof of Shenzhen Citizens Centre is one of the world\'s longest spatial lattice roof structures. A comprehensive numerical study of wind effects on the long-span structure is presented in this paper. The discretizing and synthesizing of random flow generation technique(DSRFG) recently proposed by two of the authors (Huang and Li 2008) was adopted to produce a spatially correlated turbulent inflow field for the simulation study. The distributions and characteristics of wind loads on the roof were numerically evaluated by Computational Fluid Dynamics (CFD) methods, in which Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes Equations (RANS) Model were employed. The main objective of this study is to explore a useful approach for estimations of wind effects on complex curved roof by CFD techniques. In parallel with the numerical investigation, simultaneous pressure measurements on the entire roof were made in a boundary layer wind tunnel to determine mean, fluctuating and peak pressure coefficient distributions, and spectra, spatial correlation coefficients and probability characteristics of pressure fluctuations. Numerical results were then compared with these experimentally determined data for validating the numerical methods. The comparative study demonstrated that the LES integrated with the DSRFG technique could provide satisfactory prediction of wind effects on the longspan roof with complex shape, especially on separation zones along leading eaves where the worst negative wind-induced pressures commonly occur. The recommended LES and inflow turbulence generation technique as well as associated numerical treatments are useful for structural engineers to assess wind effects on a long-span roof at its design stage.

Key Words
long-span roof; computational fluid dynamics (CFD); large eddy simulation; wind effect; wind tunnel test.

Address
Chao Li; Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China, Department of Building and Construction, City University of Hong Kong, Hong Kong
Q.S. Li; Department of Building and Construction, City University of Hong Kong, Hong Kong
S.H. Huang; School of Engineering Science, University of Science and Technology of China, Hefei, 230026, China
J.Y. Fu; Department of Civil Engineering, Guangzhou University, Guangzhou 510632, China
Y.Q. Xiao; Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China

Abstract
Wind tunnel measurements on T-shaped free-standing walls and inclined free-standing walls have been carried out. Mean net pressure coefficients have been derived and compared with previous research. It was observed that the high loads at the free ends are differently distributed than those derived from the pressure coefficients for free-standing walls in EN 1991-1-4. In addition net pressure coefficients based on extreme value analysis have been obtained. The lack of correlation of the wind induced pressures at windward and leeward side result in lower values for the net pressure coefficients when based on extreme value analysis. The results of this wind tunnel study have been included in Dutch guidelines for noise barriers.

Key Words
wind load; wind tunnel test; free-standing wall; mean values; extreme value analysis.

Address
Chris Geurts and Carine van Bentum; TNO Built Environment and Geosciences, PO Box 49, 2600 AA Delft, the Netherlands


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