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CONTENTS
Volume 16, Number 4, April 2013
 

Abstract
Recent major post-hurricane damage assessments in the United States have reported that the most common damages result from the loss of building roof coverings and subsequent wind driven rain intrusion. In an effort to look further into this problem, this paper presents a full-scale (Wall of Wind--WoW--) investigation of external and underneath wind pressures on roof tiles installed on a low-rise building model with various gable roofs. The optimal dimensions for the low-rise building that was tested with the WOW are 2.74 m (9 ft) long, 2.13 m (7 ft) wide, and 2.13 m (7 ft) high. The building is tested with interchangeable gable roofs at three different slopes (2:12; 5:12 and 7:12). The field tiles of these gable roofs are considered with three different tile profiles namely high (HP), medium (MP), and low profiles (LP) in accordance with Florida practice. For the ridge, two different types namely rounded and three-sided tiles were considered. The effect of weather block on the \"underneath\" pressure that develops between the tiles and the roof deck was also examined. These tests revealed the following: high pressure coefficients for the ridge tile compared to the field tiles, including those located at the corners; considerably higher pressure on the gable end ridge tiles compared to ridge tiles at the middle of the ridge line; and marginally higher pressure on barrel type tiles compared to the three-sided ridge tiles. The weather blocking of clay tiles, while useful in preventing water intrusion, it doesn\'t have significant effect on the wind loads of the field tiles. The case with weather blocking produces positive mean underneath pressure on the field tiles on the windward side thus reducing the net pressures on the windward surface of the roof. On the leeward side, reductions in net pressure to a non-significant level were observed due to the opposite direction of the internal and external pressures. The effect of the weather blocking on the external pressure on the ridge tile was negligible.

Key Words
full-scale; ridge tiles; field tiles; tile profile; wind pressure; turbulence; mitigation; low-rise building; underneath pressure

Address
Amanuel Tecle and Serge Fuez : 1Laboratory for Wind Engineering Research (LWER), International Hurricane Research Center (IHRC)/Department of Civil and Environmental Engineering (CEE), Florida International University (FIU), Miami, Florida 33174, USA
Girma T. Bitsuamlak : WindEEE Research Institute, Western University, London, On, Canada
Nakin Suskawang and Arindam Gan Chowdury :LWER, IHRC/CEE, FIU, Miami, Florida 33174, USA

Abstract
An active mass damper system for flutter control of bridges is presented. Flutter stability of bridge structures is improved with the help of eccentric rotational actuators (ERA). By using a bridge girder model that moves in two degrees of freedom and is subjected to wind, the equations of motion of the controlled structure equipped with ERA are established. In order to take structural nonlinearities into consideration, flutter analysis is carried out by numerical simulation scheme based on a 4th-order Runge-Kutta algorithm. An example demonstrates the performance and efficiency of the proposed device. In comparison with known active mass dampers for flutter control, the movable eccentric mass damper and the rotational mass damper, the power demand is significantly reduced. This is of advantage for an implementation of the proposed device in real bridge girders. A preliminary design of a realization of ERA in a bridge girder is presented.

Key Words
vibration control; active mass damper; eccentric rotational actuator; bridge; flutter

Address
R. Korlin and U. Starossek : Structural Analysis and Steel Structures Institute, Hamburg University of Technology, Denickestr. 17, 21073 Hamburg, Germany

Abstract
This paper presents an experimental investigation concerning the peak amplitudes of oscillation of a square prism due to Vortex-Induced-Vibrations (VIV) as a function of the mass damping parameter m*S (the so called Griffin--plot); m*and S being, respectively, the non-dimensional mass and the mechanical (structural) damping ratio. With this purpose in mind, an electromagnetic actuator has been employed to provide controlled damping. During the experiments the mass--damping parameter was in the range 0.15 < m*S < 2.4. Experiments show that there is a value of m*S below which VIV appears combined with galloping and the prism oscillation increases monotonically with the incoming flow velocity. For m*S >0.3 the present experiments show a well-defined VIV phenomenon and, consequently, a Griffin-plot can be defined.

Key Words
Vortex-Induced Vibrations; Griffin plot; square prism

Address
A. Barrero-Gil and P. Fernandez-Arroyo : Aerospace Thermal and Fluids Mechanics Department, School of Aeronautics, Universidad Politecnica de Madrid, Plaza Cardenal Cisneros 3, 28040, Madrid, Spain

Abstract
The Gringorten estimator has been extensively used in extreme value analysis of wind speedrecords to obtain unbiased estimates of design wind speeds. This paper reviews the derivation of the Gringorten estimator for the mean plotting position of extremes drawn from parents of the exponential type and demonstrates how it eliminates most of the bias caused by the classical Weibull estimator. It is shown that the coefficients in the Gringorten estimator are the asymptotic values for infinite sample sizes, whereas the estimator is most often used for small sample sizes. The principles used by Gringorten are used to derive a new Consistent Linear Unbiased Estimator (CLUE) for the mean plotting positions for the Fisher Tippett Type 1, Exponential and Weibull distributions and for the associated standard deviations. Analytical and Bootstrap methods are used to calibrate the bias error in each of the estimators and to show that the CLUE are accurate to better than 1%.

Key Words
plotting position; linear unbiased estimators; extreme values; peak-over-threshold; weibull distribution; exponential distribution; Fisher Tippett Type 1 distribution; method of independent storms

Address
Nicholas John Cook and Raymond Ian Harris : RWDI, Unit 4, Lawrence Way, Dunstable, Bedfordshire, LU6 1BD, UK

Abstract
This is the first of two companion papers that analyse ten years of on-site monitoring data for the Confederation Bridge to determine the validity of the original wind speeds and wind loads predicted in 1994 when the bridge was being designed. The check of the original design values is warranted because the design wind speed at the middle of Northumberland Strait was derived from data collected at shore-based weather stations, and the design wind loads were based on tests of section and full-aeroelastic models in the wind tunnel. This first paper uses wind, tilt, and acceleration monitoring data to determine the static and dynamic responses of the bridge, which are then used in the second paper to derive the static and dynamic wind loads. It is shown that the design ten-minute mean wind speed with a 100-year return period is 1.5% less than the 1994 design value, and that the bridge has been subjected to this design event once on November 7, 2001. The dynamic characteristics of the instrumented spans of the bridge including frequencies, mode shapes and damping are in good agreement with published values reported by others. The on-site monitoring data show bridge response to be that of turbulent buffeting which is consistent with the response predicted at the design stage.

Key Words
full-scale; long-span; bridge damping; design wind speed; bridge acceleration; spectral analysis; mode shape; frequency; structural health monitoring

Address
Bilal Bakht : Ministry of Transportation and Infrastructure, KAMLOOPS BC, Canada
J. Peter C. King : Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London ON, Canada
F.M. Bartlett : Department of Civil and Environmental Engineering, University of Western Ontario, London ON,
Canada

Abstract
This paper uses ten years of on-site monitoring data for the Confederation Bridge to derive wind loads and investigate whether the bridge has experienced its design wind force effects since its completion in 1997. The load effects derived using loads from the on-site monitoring data are compared to the load effects derived using loads from the 1994 and 2009 wind tunnel aerodynamic model tests. The research shows, for the first time, that the aerodynamic model-based methodology originally developed in 1994 is a very accurate method for deriving wind loads for structural design. The research also confirms that the bridge has not experienced its specified (i.e., unfactored) wind force effects since it was opened to traffic in 1997, even during the most severe event that has occurred during this period.

Key Words
full-scale, long-span, RMS accelerations, load effects, aerodynamic damping, wind tunnel testing, full-aeroelastic model, mode shape, frequency

Address
Bilal Bakht : Ministry of Transportation and Infrastructure, KAMLOOPS BC, Canada
J. Peter C. King: Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London ON, Canada
F. M. Bartlett : Department of Civil and Environmental Engineering, University of Western Ontario, London ON, Canada


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