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CONTENTS
Volume 4, Number 1, February 2001
 

Abstract
The classical two-degree-of-freedom (2-d-o-f)

Key Words
suspension bridges; wind effects; sectional model; nonlinear dynamics; vortex shedding.

Address
Vincenzo Sepe and Giuliano Augusti, Dipartimento di Ingegneria Strutturale e Geotecnica, Universita di Roma \"La Sapienza\" , via Eudossiana 18, 00184 Roma, Italy

Abstract
This paper concerns computational response predictions when a tuned mass damper is intendedrnto be used for the suppression of vortex shedding induced vibrations of e.g., a bridge deck. A generalrnfrequency domain theory is presented and its application is exemplified on a suspension bridge (wherernvortex shedding vibrations have been observed and where such an installation is a possible solution).rnRelevant load data are taken from previous wind tunnel tests. In particular, the displacement responsernstatistics of the tuned mass damper as well as the bridge deck are obtained from time domain simulations,rnshowing that after the installation of a TMD peak factors between three and four should be expected.

Key Words
tuned mass damper; vortex shedding; dynamic response.

Address
Einar Str

Abstract
Mean concentrations of ammonia gas released as a tracer from an isolated low-rise buildingrnhave been measured and predicted. Predictions were calculated using computational fluid dynamics (CFD)rnand two dispersion models: a diffusion model and a Lagrangian particle tracking technique. Explicit accountrnwas taken of the natural variation of wind direction by a technique based on the weighted summation ofrnindividual steady state wind direction results according to the probability density function of the windrndirection. The results indicated that at distances >3 building heights downstream the weighted predictionsrnfrom either model are satisfactory but that in the near wake the diffusion model is less successful. Weightedrnsolutions give significantly improved predictions over unweighted results. Lack of plume spread isrnidentified as the main cause of inaccuracies in predictions and this is linked to inadequate resolution ofrnflow features and mixing in the CFD model. Further work on non-steady state simulation of wake flowsrnfor dispersion studies is recommended.

Key Words
computational fluid dynamics; dispersion; tracer gas; modelling; building wake.

Address
A.D. Quinn, M. Wilson, A.M. Reynolds, S.B. Couling and R.P. Hoxey, Silsoe Research Institute, Wrest Park, Silsoe, Bedfordshire. MK45 4HS, U.K.

Abstract
This paper is focused on the torsional effects that are induced on bridge piers by unbalanced windrnbuffeting on the deck during double cantilever erection stages. The case of decks with variable cross section isrnconsidered in particular as this characteristic is typical of most frame bridges that are built by the cantileverrnmethod. The procedure outlined in the paper is basically an application of the method that Dyrbye and Hansenrn(1996) have illustrated for decks with constant cross section. This format was chosen because it is suitable forrndesign purposes and may easily be implemented in structural codes. As a complement, the correspondence withrnthe format that is adopted in the Canadian code (NBCC 1990) for the gust factor is established, which might bernuseful to bridge designers used to the North-American approach to the gust effects on structures. Onlyrnalongwind turbulence and horizontal movements of the deck are considered. The combination of torsional andrnbending effects is also discussed and it is illustrated with an example of application.

Key Words
bridges; alongwind effects; torsion of piers; combined bending and torsion; cantilever erection stages

Address
Pedro A. Mendes and Fernando A. Branco, ICIST, Department of Civil Engineering, IST, Av. Rovisco Pais, 1096 Lisbon, Portugal

Abstract
This paper presents results of a study on wind loads and wind induced dynamic response of bridgernjointed twin-towered buildings. Utilizing the high-frequency force balance technique, the drag and momentrncoefficients measured in wind tunnel tests, and the maximum acceleration rms values on the top floor of towers,rnare analyzed to examine the influence of building\'s plan shapes and of intervals between towers. The alongwind,rnacrosswind and torsional modal force spectra are investigated for generic bridge jointed twin-towered buildingrnmodels which cover twin squares, twin rhombuses, twin triangles, twin triangles with sharp corners cut off, twinrnrectangles and individual rectangle with the same outline aspect ratio as the twin rectangles. The analysis of thernstatistical correlation among three components of the aerodynamic force corroborated that the correlationrnbetween acrosswind and torsional forces is significant for bridge jointed twin-towered buildings.

Key Words
twin-towered buildings; wind loads; wind-induced response; high-frequency force balance technique.

Address
Z.-H. Ni, C.-K. He, Z.-N. Xie, B.-Q. Shi and D.-J. Chen, Department of Civil Engineering, Shantou University, Shantou, Guangdong, 515063, P.R. China

Abstract
Analysis of pressures measured on the roof of the full-scale Texas Tech building and a 1/50rnscale model of a typical house showed that the pressure fluctuations on cladding fastener and cladding-trussrnconnection tributary areas have similar characteristics. The probability density functions of pressurernfluctuations on these areas are negatively skewed from Gaussian, with pressure peak factors less than -5.5. Thernfluctuating pressure energy is mostly contained at full-scale frequencies of up to about 0.6 Hz. Pressurerncoefficients, Cp and local pressure factors, Kl given in the Australian wind load standard AS1170.2 arerngenerally satisfactory, except for some small cladding fastener tributary areas near the edges.

Key Words
wind load; cladding; cladding fastener; batten-truss connection; external pressure; internal pressure; net pressure; pressure coefficient; probability density function; Gaussian distribution; pressure spectrum.

Address
J. D. Ginger, Cyclone Structural Testing Station (CSTS), School of Engineering, James Cook University, Townsville, QLD 4811, Australia


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