Modeling and Calculating a Floating Body with RFEM

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To correctly model and calculate floating bodies (special rafts, pontoons, floating jetties, dredgers, floating houses, inflatable islands, floating cranes, houseboats, et cetera), a two-stage calculation is necessary.

In the first stage, the submersion depth of the floating body is determined. If the submersion depth is determined correctly, the lateral water pressure can be applied up to the waterline in the second stage. The buoyancy of the floating body can be considered via an elastic surface foundation. Submersion depth and buoyancy are linearly correlated. If a cube with an edge length of 1 m is completely submerged into the water, it creates a buoyancy of 10 kN. This means that an elastic foundation coefficient of cz = 10 kN/m³ has to be applied for bottom surfaces.

Figure 01 - Elastic Foundation

This makes it possible to determine the correct submersion depth, already.

Figure 02 - Submersion Depth of Floating Body

However, a prerequisite is that the floating body barely twists and that the side walls are perpendicular.

For each load combination, the submersion depth is determined first. An additional load case that contains the respective lateral loads is then added to each load combination. In the example, a load combination CO 1 with characteristic loads was created. A deformation uZ = 308 mm is obtained for the nodes 5 and 6, and uZ = 325 mm for nodes 7 and 8. To facilitate entering the loads, the help nodes 11 to 14 were respectively created at the locations.

Figure 03 - Help Node for the Waterline

The water loads have been applied via free polygon loads and for reasons of clarity, only one surface has been selected for each load. The load direction is local z. This means that the load is perpendicular to the surface. The load distribution is linear. Then, three load values have to be specified. The selected points were always two points at the water surface where the load is zero, as well as the most deeply submerged point as the third point, applied with 3.35 kN/m² or 3.08 kN/m².

Figure 04 - Water Pressure

Due to the fairly large deformations, the large deformation analysis has been set for the calculation of the load combination.

Figure 05 - Calculation According to the Large Deformation Analysis

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RFEM Main Program
RFEM 5.xx

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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