Assessment of the dynamic response of a floating pontoon bridge with a fiber reinforced polymer superstructure

Abstract

Floating bridges are found at locations where deep water must be crossed for a long distance, such as in Norwegian fjords. The superstructure of the existing floating bridges is usually constructed from steel. In a marine environment these bridges are exposed to dynamic wave loads. As such, the current floating bridges suffer severe damage due to corrosion and fatigue. Using fiber reinforced polymer instead of steel could alleviate these phenomena and reduce maintenance costs. However, the lower stiffness could cause undesired vibrations in the floating bridge. Therefore a parametric model is developed to investigate the influence of design parameters on the dynamic response of a floating pontoon bridge.

A literature study is performed to find suitable concepts and techniques to develop the floating bridge model. A floating pontoon bridge can be schematized with rigid bodies, Euler-Bernoulli beam elements and linear springs and dashpots. The fluid-structure interaction is taken into account by including the added mass, hydrodynamic damping, hydrostatic stiffness and wave force transfer functions of the pontoons. A frequency domain approach is used to compute the dynamic response.

A parametric model of a floating pontoon bridge to predict the dynamic response is developed in Python. The hydrodynamic properties of the pontoons are computed by Diffrac and are used as input for the Python model. The Bergsøysund bridge is used as a reference case, because measurement data of this bridge’s dynamic response is available and can be used for validation.
The geometrical and structural properties of the Bergsøsund bridge are used to model its dynamic response. Wave conditions with a peak frequency at 2 rad/s are used. The results show that the response of the pontoons is governing compared to the response of the superstructure and that the dominant degree of freedom is sway. The fourth sway mode is the main contributor to this dynamic response. A comparison with measurement data shows that the produced response spectra are in good agreement with the measurements in terms of the peak locations.

In the parametric study the influence of single design parameters on the dynamic response of a floating bridge is investigated independently. The results show that the length of the superstructure has the biggest influence on the dynamic response of the floating bridge. In general, a reduction in stiffness in the superstructure leads to a lower overall frequency response function and thus to a lower dynamic response. Increasing the stiffness or reducing the mass of the bridge shifts the eigenfrequency of the fourth sway mode to a higher frequency and vice versa. Finally, when the damping is increased, the peaks in the frequency response function decrease and thereby the dynamic response in resonance reduces.

In conclusion, the dynamic response of a floating end-supported pontoon bridge is mainly influenced by the stiffness of the superstructure. A fiber reinforced polymer superstructure should be designed sufficiently stiff, especially in lateral direction, to keep the overall frequency response function low enough.