Long floating bridges supported by pontoons with span-widths between 100 m and 200 m are discrete hydro-elastic structures with many critical eigenmodes. The response of the bridge girder is dominated by vertical eigenmodes and coupled horizontal modes (lateral) and rotational modes (about the longitudinal axis of the bridge girder). This paper explores the design principles used to reduce the response with regards to these eigenmodes. It is shown for a floating bridge with 200 m span-width that by inserting a bottom flange the vertical eigenmodes can be lifted out of wind-driven wave regime. It is also shown that selecting a pontoon length that leads to cancelation of horizontal excitation forces is beneficial, and that the geometrical shaping of the pontoon can be efficient to decrease the bridge response.

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