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Wind and Structures Volume 12, Number 5, August 2009 , pages 413424 DOI: https://doi.org/10.12989/was.2009.12.5.413 


Effects of frequency ratio on bridge aerodynamics determined by freedecay sectional model tests 

X.R. Qin, K.C.S. Kwok, C.H. Fok and P.A. Hitchcock


Abstract  
A series of wind tunnel freedecay sectional model dynamic tests were conducted to examine the effects of torsionaltovertical natural frequency ratio of 2DOF bridge dynamic systems on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around
2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency of the system held constant for all the tests. Three 2.9 m long twindeck bridge sectional models, with a zero, 16% (intermediate gap) and 35% (large gap) gaptowidth ratio, respectively, were tested to determine whether the effects of frequency ratio are dependent on bridge deck crosssection shapes. The results of wind tunnel tests suggest that for the model with a zero gapwidth, a model to approximate a thin flat plate, the flutter derivatives, and consequently the aerodynamic forces, are relatively independent of the torsionaltovertical frequency ratio for a relatively large range of reduced wind velocities, while for the models with an
intermediate gapwidth (around 16%) and a large gapwidth (around 35%), some of the flutter derivatives, and therefore the aerodynamic forces, are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the two models with nonzero gap (16% and 35% gapwidth). The test results clearly show that the effects of the frequency ratio on the flutter derivatives and the aerodynamic forces were dependent on the aerodynamic crosssection shape of
the bridge deck.  
Key Words  
windinduced vibration; flutter derivative; system identification; frequency ratio; wind tunnel
test.  
Address  
X.R. Qin, K.C.S. Kwok, C.H. Fok and P.A. Hitchcock ; CLP Power Wind/Wave Tunnel Facility, HKUST, H.K. S.A.R. P.R. China  