The influence of gap- and chord-widths for multi-box girders: Superposition of flat plate flutter derivatives and section model tests

A two dimensional model based on the superposition of Theodorsen’s flat plate theory generalized to approximate the aeroelastic effects on an arbitrary multi-box bridge girder is developed. The model is compared with wind tunnel test results of a single box girder and two triple-box girders having different gap-widths between the center box and the external boxes. Reasonable agreement between the flutter derivatives obtained from single degree of freedom harmonic forced motion tests and superposition of flat plate flutter derivatives is obtained. For vertical motion, the aerodynamic damping is correlated with the sum of chord-widths of the interconnected boxes. The aerodynamic damping during harmonic torsional motion is correlated with the distance from the elastic axis of the section to the mid-chords of the external boxes. The torsional aerodynamic damping agrees with the superposition of flat plate derivatives while the torsional aerodynamic stiffness disagrees. This means that the flat plate model overestimates the critical flutter wind speed for the present sections, but it can be used to approximate the flutter derivatives: H1, H2, H3 and A2. Empirical results are used to tune the superposed flat plate model, which is then used to approximate the critical flutter wind speed for a 2200 m single span suspension bridge featuring a light-weight triple-box girder. The wind tunnel tests indicate that the light-weight triple-box girder is aerodynamically stable up to at least 70 m/s even though the torsional frequency is lower than the vertical.