Modeling of Structures Subjected to Wind Waves

The difficulties inherent in the direct determination of loads on off-shore structures which are exposed simultaneously to wind and waves make it desirable to test a model of the structure in the laboratory. It is shown herein that scaling of the structure and the loads due to waves is possible by using waves which are generated by blowing air over the surface of a laboratory channel, and by choosing a model material with an appropriate modulus of elasticity. Fully developed wind generated waves are found to be determined in magnitude, frequency, and spectral shape by Froude number scaling. The structural response obeys Cauchy number similarity, which is interpreted as implying equality of the ratios of peak frequency of the spectrum to natural frequency of the structure. The wave modeling aspects of the method are illustrated with an example. Some consideration is also given to dynamic wind load modeling and to the question whether simultaneous modeling of both the wave and the wind forces is possible in a laboratory. In principle, this appears feasible, but for sufficiently large scale models, fetch lengths may be required which are larger than typical laboratory channels.