Page 154 - Marine Structural Design

130 PART I Structural Design Principles

plate stiffening upon the ductility of the structure should not be overlooked. Furthermore, if
detailed stiffening is added, the analyst should consider the fabrication and inspection
consequences of stiffening. For examples, the questions may be given: “Can the welder get
suffkient access to the area?”, “Will the weld type be limited (e.g. only single sided welding
possible)?”, “Will the weld detail cause a local stress concentration?”, “What are the
possibilities for inspection of the weld post fabrication and in-service, if required?” etc.
Plated sections of beams, i.e. web and flange sections or the walls of box sections will be
defined as standard sections in the finite element program and will be checked against the
appropriate code without the need for additional hand-checks. However, for joints in
particular, forces will often need to be taken from the finite element analyses and used in hand
or spreadsheet calculations to establish if sufficient strength exists.
The finite element program will generally center both the panel and the stiffeners on the nodal
points for stiffened panels. Therefore, a horizontal deck panel’s plate will appear to run
through the center of the stiffeners rather than being supported on the stiffener ends, see
Figure 7.1. There may also be a small inconsistency with the elevation since the nodes may be
based on Top Of Steel (TOS) or on the Bottom Of Steel (BOS) coordinates rather than on the
centerline of the plate as would be modeled. In both cases, offsets can be modeled to give the
correct visual appearance; however, this is generally unnecessary in terms of the calculation of
stresses in the model.
NORSOK N-004 gives a useful reference table for buckling checks of plate panels under
different loading conditions. The recommended reference for the check is in NORSOK, NS
3472 or Eurocode 3. The most useful are the limiting values in the following section that state
where buckling checks are not necessary. These tables are reproduced in Table 7.1.
7.2.3 Shell Structures
Unstiffened and ring-stiffened cylindrical shells subjected to axial forces, bending moments,
and hydrostatic pressures may be designed as tubular members, or in a more refined analysis
as a shell structure.
A tubular section in air, with a diameter to thickness ratio in excess of 60, is likely to fail by
local buckling at an axial stress less than the material yield strength. The capacity of members
failing due to local buckling is more sensitive to geometric imperfections than members that
can sustain yielding over the thickness, which allows some redistribution of local stress due to
yielding. The failure of such members is normally associated with a descending post-critical
behavior compared to that of a brittle structure. Structures with this behavior are denoted as
shells.
Thin-walled shell structures might not be adequately covered by the formulations for tubular
members and joints, which are included in finite element programs that handle truss and beam
models. Therefore, in general, shells should not simply be defined as thin-walled tubulars and
treated in the same manner. Rather, a more complex finite element mesh should be developed
and analyzed, particularly where the shell includes ring and/or longitudinal stiffening, see
Figure 7.2.