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5The present paper investigates the possibilities for avoiding classical flutter and 6 static divergence for very long span suspension bridges with a novel, flexible and 7 lightweight triple box girder. Previous studies have shown that the critical classical 8 flutter wind speed tends to decline with the main span width. Other studies have 9 shown that flutter can be avoided if the torsional frequency is lower than the vertical. 10The road to get there in practice, however, is complicated. The possibility for tuning 11 the torsional natural frequency without affecting the vertical frequency is utilized in 12 the present paper. The effect on aerodynamic stability is analyzed in detail for low 13 torsional-to-vertical frequency ratios typical for very long span bridges with lightweight 14 and flexible girders. The present study includes non-linear finite element analysis and 15 static, free vibration and forced motion wind tunnel tests. Aerodynamic stability have 16 1 PhD Student, Corresponding author, University of Southern Denmark, Department of Civil and Architectural Engineering, Campusvej 55, DK-5230, Odense M. Current affiliation: Consultant, Svend Ole Hansen ApS, Sankt Jørgens Allé 5C, DK-1615 been obtained in a section model test with a lightweight setup corresponding to a bridge 17 girder mass of only 6.38 t/m in full scale. Flutter was not observed for any torsional-18 to-vertical frequency ratios in the range from 0.97 ≤ γ ω ≤ 1.55. Stability was observed 19 up to wind speeds of approximately 88 m/s in full scale for a 2125.2 m span. The aero-20 dynamic stability obtained in the configurations of the present section at γ ω ≈ 1.20 21 shows that this might be an economic feasible solution for future long span suspension 22 bridges because aerodynamic stability is achieved even though the torsional stiffness 23 and the mass of the deck is low. 24 25 ers; Lightweight bridges 26 35 is the key to minimize the total costs(Richardson 1981; Brown 1999). A decrease 36 in mass could cause the deck to be more easily excited by wind compared to a 37 heavier deck. While the mass of the deck is optimized, caution should be paid 38 to severe aeroelastic instability phenomena such as flutter and static divergence.39 Aerodynamic instability is not allowed to occur below a desired critical wind 40 speed, typically 1.5 times the characteristic 10 minute mean velocity at bridge-41 deck height (Dyrbye and Hansen 1997), which corresponds to a sufficient high 42 2Andersen, August 21, 2018 annual probability of failure. 43Enhanced torsional stiffness of the girder is a known measure to avoid insta- 44 bilities, but tend to increase the mass. Furthermore, the torsional girder stiffness 45 is inverse proportional to the span length. If bridge decks can be designed aero-46 dynamically stable with low torsional stiffness, the cross wind dimension, i.e. the 47 depth, and thus the mass and the costs can be reduced considerably. The concept 48 of obtaining aerodynamic stability against flutter by designing the bridge deck to 49 h...

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