5003 Lectures - Faculty of Engineering and Applied Science

E5003 - Ship Structures I 213
© C.G. Daley
When we a lateral load (a bending load) to a beam
or ship frame, the web carries the load and tends
to bend. The flange is attached at the edges of the
web and as the web bends, its edge shortens (or
lengthens) and tends to pull the flange with it. To
pull on the flange, a shear stress of applied to the
edge of the flange. As shown in the sketch, the
flange is acted upon along its edge. Its as if the
flange is pinched along its edge, causing the flange
to compress more near the web and less away from
the web.
Unfortunately there are no general analytical
solutions for shear lag and effective width. Certain
approximate solutions have been postulated (see
PNA, VI, pp 247-250)
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