14. Modal Analysis of a Floating Bridge Without Side-Mooring

The Norwegian Public Roads Administration is currently planning a ferry-free Coastal Highway Route E39. Floating bridges represent feasible options in this project with already two long span floating bridges in function, i.e. the Bergsøysund and Nordhordaland Bridges. In connection with this project, one of the main objectives is to quantify the accuracy of the numerical methods used to predict dynamic behaviour of floating bridges. An extensive monitoring system is installed to measure structural response as well as environmental actions from wind and waves on an existing floating bridge: the Bergsøysund Bridge. These measurements are used to estimate the modal system properties of the structure. The system identification is performed using a parametric time-domain Stochastic Subspace Identification method as well as the Frequency Domain Decomposition method. Challenges of system identification for highly damped structural systems, such as a floating bridge, are especially emphasized. The results are also compared with numerical predictions from a two part combined linear frequency-domain model set-up. The first part consists of a hydrodynamic model, including wave excitation as well as fluid-structure interaction, and relies on linearized potential theory. The results from this are thereafter introduced into a finite element model, for a complete structural dynamic analysis.