Estimation of Aerodynamic Forces and Moments on Bridge Decks Based on Two Dimensional Velocity Fields

In order to predict the structural responses under random wind loads to avoid catastrophe failures, structural dynamic analysis is necessary, which requires a reasonable description of wind loads on structures. Based on the equations of fluid mechanics, equations for aerodynamic forces for structures in motion and coefficients of drag, lift and pitching moment for static structures are derived based on two dimensional velocity fields. Since the pressure terms in the equations are eliminated under 2-D velocity assumptions, the equations can be used to obtain the instantaneous aerodynamic forces based on the velocity information along the segments enclosing the bridge deck. In the present study, the divergence theorem is used to convert the control volume integral to the control surface integral. Similarly, the equations for pitching moments are proposed in a similar approach based on the conservation of the momentum moment. This makes it possible to predict the aerodynamic forces and moment for bridge desks of large span bridges, which are more vulnerable to wind induced vibrations. Therefore, the direct relationship between the loads from wind on structures and the velocity field can be founded. Accordingly, the equations for the coefficient of wind forces can be obtained for static bridge decks. In order to validate the equations at a higher Reynolds number (Re), velocity in the wind field around static bridge decks are obtained from both numerical simulations and wind tunnel Particle Image Velocimetry (PIV) experiments at Re being about 10 4 . Two types of sections including a square cylinder and a twin box girder are used as the prototypes of the bridge decks in the present study. PIV experiments are set up in a wind tunnel and the fluid domain in the numerical simulation is set with the same size of the wind tunnel. Based on the sensitivity analyses, several cases with different sizes of control volumes and space resolutions are used to obtain the wind force coefficient for both of the square cylinder section and the twinbox girder section. Good agreements are found for all the cases for the drag coefficient and acceptable agreements are found for certain cases for the lift coefficient. However, discrepancies are found for some cases for the coefficient of lift and pitch moment. Discussions are made upon the agreements and discrepancies associated with the predicted wind force coefficients. With a 1 Graduate Student 2 Professor 3 Edwin B. and Norma S. McNeil Distinguished Professor fine resolution both in time and space domain, a possible new method to predict wind loads on bridge decks is suggested.