On Stability of Submerged Floating Foundation

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Recently many demands of bridges still exist, but the distance between towers becomes much longer and the depth of the sea bed to obtain firm foundation becomes larger. In order to build bridges in such bad-conditioned sites, one may need new materials which is much lighter and at the same time much stronger. Or one can use floating foundations to support at least some part of dead loads. Such floating foundations are usually moored by cables anchored at the sea bed. Cable structures have essential disadvantage that the tension within cables must be high enough to support structures and that the total performance show highly non-linear behavior.In considering floating foundation, oneof the worst conditions to affect its supporting function is wave of sea surface. Such oscillating forces on flexible structures by cables require fatigue resistance. In order to avoid this difficulty, the floating foundation here is sub-merged in the sea, so that the largest force except the load from upper structures is the tidal force which may be considered quasi-static. Furthermore to introduce tension forces within cables large enough to support loads, a big floating body is attached at the top of the cable structuresand its buoyancy acts just like a pre-tensioning mechanism as well as for supporting loadings. Numerical simulation of such system yields following results. Such system shows quite linear resisting behavior in the range of the actual loading conditions and thus the designing method may become very easy because the total system can be estimated as alinear-spring systme. As far as the sinkage and rotating angles are restricted, the behavior shows linear and stable to make its design easy enough with simple model.