There exist eight underwater observation towers in Japan. Seven of them have been built by vertical circular cylindrical shell structure, which are fixed at the sea bed like cantilever. And the other one is constructed of floating structure. Among the seven fixed-type structures, one stands under the frozen sea. The underwater observation towers are surrounded by sea water, and the inside is perfectly a close-spaced. Sightseers and several interested people visit to observe the undersea scene. Therefore, top priority must be given to structure safety. There is a vast existence of various creatures within the outstanding underwater scene which needed to be maintained to proliferate life. In order to satisfy with convenient access and outstanding scene, most structures have been built at the breaker zone. This paper presents the following contents: 1) Assuming that the underwater observation tower is constructed by vertical circular cylindrical shell, natural frequency analysis will be shown using present simple equation of added mass. We developed this equation uniquely based on fluid-structure interaction problem through water tank experiment and systematic numerical calculation. 2) Sawaragi, et al had presented wave breaking force acting on the vertical cylinder through the experiment. With reference to the force given by total force, we have newly extended its force to the wave pressure distribution so as to correspond to FEM analysis. We will show this developed method with regard to wave pressure distribution on the surface of cylindrical shell. 3) Using this simple equation of added mass and nodal forces obtained from wave breaking force, a procedure of calculation will be shown with demonstration. The evaluation and the validity in terms of added mass, load arrangement and responses will be discussed through the comparison of experimental result.

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