Experimental study of floating bridge global response when subjected to waves and current

Abstract

The present paper investigates the wave- and current-induced responses of the Bjørnafjord phase 5 K12 floating bridge concept based on small-scale model tests. Due to the extensive length of the bridge concept combined with relatively small wave heights governing the design, the experimental model represents a truncated section of the original bridge concept. This truncated section includes a stay-cable tower and ten floating pontoons supporting a horizontally curved bridge girder at varying altitude. The bridge girder truncation points coincide with the tower column and the location just above the first moored pontoon. The girder boundary conditions are simplified as fixed for all degrees of freedom (DOF) on either side of the model while allowing for free rotation around the longitudinal and vertical axes at the location above the otherwise moored pontoon. The instrumentation of the experimental model includes three DOF translational motions captured at 13 locations along the bridge and six DOF motion of each pontoon. Force transducers are used to capture axial forces in all ten tower stay-cables and at the two ends of the bridge girder, while strain gauges capture shear forces and bending- and torsional moments measured at 12 locations along the bridge girder.

The dynamic properties of the model are investigated by subjecting the model to both regular waves and broad-banded, long-crested waves, propagating at two different wave directions corresponding to waves travelling in and out of the fjord. These tests are performed with and without collinear current to investigate the effect of current on the bridge responses. Subsequently, the effect of current is investigated for two important long-crested wave conditions, i.e. the 100-year wind wave condition and the 10000-year swell wave condition. For both wave conditions the model tests are performed without current and with 1-year and 10-year current velocities, respectively. Finally, the short-term response from the long-crested wind and swell wave conditions without current are compared to those of short-crested waves without current.

Results show that both current and the directional distribution of the waves have a significant influence on the structural responses. It is recommended that both effects are properly accounted for in any future design.