Page 118 - Marine Structural Design

94 Part I Structural Design Principles

should be good enough. The total loading must be balanced so that the reaction forces at the
boundaries approach zero.
When modeling, the model length of the ship structure should be sufficient to minimize
boundary condition effects over the analyzed area. ABS (2002) requires 3 cargo holds to be
covered for models of tankers, bulk carriers, or container ships; LR "Direct calculation -
Guidance Notes" (1996) requires that 2 cargo holds be covered for the model of a bulk carrier.
All continuous longitudinal elements should be restrained to remain plane under the effects of
the hull girder bending and must be rotationally fixed about the vertical axis if the calculated
deformations or forces are not available at the free ends of the model. Conditions of symmetry
should be applied at each end of the finite element model. Rotation about the two axes in the
plane of symmetry is to be constrained where there is symmetry imposed at the centerline or at
the ends of the model. The model should be supported vertically by distributed springs with
shipsides and longitudinal bulkheads at the intersections of the transverse bulkheads.
5.3.3 Type of Elements

The types of elements are chosen to provide a satisfactory representation of the deflections and
stress distributions within the structure. The conventional frame analysis may be carried out
with a beam model. It has significant advantages for its modeling simplicity and
computational efficiency. However, thanks to the availability of powerhl computers,
computational efficiency is no longer a concern. More refined and accurate element types can
be used.
In a research conducted by the ISSC, Zillottto et al. (1991), nine different finite element
models were applied to different combinations of beams, trusses, rods, membranes, planes,
and shell elements. A considerable scatter was observed in the results. The conclusion was that
a detailed analysis of the deformations and stress levels in all the elements of the transverse
frames should be performed using a refined finite element model for all the different types of
structures and ships.
In "Direct Calculation-Guidance Notes", LR (1996) suggests that all areas of the plating
should be modeled by shell elements, secondary stiffeners by line elements (bars or rods),
double bottom girders and floors by three or more plate elements over the depth of these
members, and side shells by plate or bar elements.
In general, if the structure is not subjected to lateral bending, membrane and rod elements may
be applied. Otherwise, plate and beam elements, which have both bending and membrane
resistance, should be employed. The selection of element types depends on many aspects, such
as the type of structure, the load application approach, the type of analysis performed, the
results generated, and the accuracy expected. There is no substitute for engineering judgement.
5.3.4 Post-Processing
The design is a complicated and iterative process in which building and solving a FE model is
simply the first step. A more important step is that designers use their knowledge and
judgment to analyze the results and, if necessary, redesign or reinforce the structure.
First, the engineer must ensure that the results calculated by the FE program are reasonable,
and that the model and the load application are correct. This can be achieved by plotting stress
contour, the deformation, the reactions & applied load equilibrium, force & moment diagrams,
etc. The next step is to check the strength of the structure against relevant design criteria. Load
combinations and stress combinations are not always straightforward. Assumptions are usually