Stiffness Evaluation of Submerged Floating Foundation Moored by Four Cables

Abstract

As an application of cable structures to bridge structures, a submerged floating foundation moored by four cables is studied. The applied forces considered are the buoyancy, vertical load and horizontal force partly as a model of the tidal current. In order to examine the resistance against the overturn moment action, the governing equations with geometrical nonlinearity are solved by a direct integration method.As far as the sufficiently large tensile forces are introduced into all four cables, the system shows stable and almost linear responses, which may lead to a simplified design code generation based on a simple linear theory.Key Words: Cable Structure, Submerged Floating Foundation INTRODUCTIONCooperative constructions in the developing countries and development of natural resources near the polar circles are parts of the most important tasks recently assigned to civil engineers in our country. Such cases often require the construction under very severe conditions.For example in bridge construction over the deep sea, it is so difficult to secure rigid foundations that the span length of the bridges becomes long, and the cost becomes high. At the same time, such huge structures are likely to destroy the environment in the aesthetic sense. Considering such circumstances, introduction of submerged but stiff floating foundations not only removes such difficulties but also extends the freedom of designing.Since the floating bodies are generally moored by cables, they are sensitive to the dynamic effects of wave and tidal current and show nonlinear mechanical behavior. Consequently the recent researches are mainly related to the nonlinear dynamic analysis of the floating and moored bodies including the effect of slack cables. However we here restrict our attention within studies of submerged floating bodies as a model of the floating foundation, which undergoes relatively small influence of surface waves. Therefore in order to carry out the feasibility study of such structures, the system is modeled by a rigid submerged body moored by a number of cables anchored on the sea bed. Only the static and elastic analyses are carried out to examine their spatial stiff ness.The major subject of studying such floating structures subjected to the buoyancy has been the socalled mooring probleme. g. 1), 2), and most systems have not been permanent structures nor foundations but floating bodies under construction. The complication caused by the nonlinear behavior of cables or chains which anchor the floating body in the water or the hydraulically dynamic effect like the tidal current and wave action may be the factors against development of such structural systems. In the analyses the large displacement analysis3) of the cable must be also taken into account, and the dynamic contact4) problem of cables at anchor may become important.The buoyancy acting along the cable has been sometimes approximated by usage of the underwater weight, but it has been reported5), 6), 7) that such an approximation leads to significant discrepan...