Shear lag and diminished effective width are most important
in cases of ;
• wide flanges (large
b)
• short frames (small
L/b)
• proximity to free
ends
• proximity to
concentrated loads.
Finite element
programs, when shell or brick elements are used to model the frames, will
naturally show the
shear lag effects. There have been experiments on hull girder models that have
shown not only a variation in deck stresses, but actual stress reversals. This means
that even when the average deck stress is compressive, there may be a part of
the deck (at center) where the stresses are tensile, with the deck edges in
exaggerated compression. (See PNA [Principles of Naval Architecture Vol. I,
SNAME], p250).
Classification society
rules have various approaches to account for effective width. We will
consider deck plate
buckling in the next ship structures course. In that case we will consider
another type of effective width of plating, but one that describes a buckled
plate’s reserve
capacity.
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