Stochastic dynamic analysis of floating bridges exposed to inhomogeneous and irregular waves

Abstract

As future floating bridges gets longer, the likelihood of significant inhomogeneous wave conditions across them increase. Consequently, a proper assessment of the effects of inhomogeneous wave conditions is needed. In this paper, we present an approach to model inhomogeneous sea states by representing the stochastic process using generalized harmonic decomposition, retaining the coherency inherent in Airy wave theory. The methodology is applied to a concept model of a floating bridge to cross the Bjørnafjord in Norway, which is more than five kilometres wide. The paper emphasizes on frequency-domain simulation, but the required steps to generate time-domain representations are given. To suggest what might be critical considerations in design, the response effects due to some selected conceptual inhomogeneities are highlighted. For varying wave heights alone, it is found that it is likely conservative to approximate the inhomogeneous sea state as homogeneous by assuming the harshest conditions across the full width of the strait. By modulating the mean wave angle of a 100-year swell sea state, a spherical wave front matching the curvature of the bridge girder is found to produce significantly more severe response than the homogeneous reference swell; an increase of 14% of the maximum axial force response is found. Furthermore, a proper treatment of the coherency for swell sea states is found to be crucial, in contrast to typical wind sea conditions.