Page 119 - Marine Structural Design

Chapter 5 Ship Hull scantling Design by Analysis 95

made to certain degrees both in creating the model and in solving the model. The designers
must bear this in mind and be familiar with the FE program being used, in order to account for
the assumptions adopted, to evaluate the calculated results, and, if necessary, to modify the
results.

Yielding Check
The yield check ensures that the stress level on each structural member is below the allowable
stress. The allowable stress is defined as the yield limit of the material divided by a safety
factor. Stresses calculated fiom different models are combined to derive the equivalent von
Mises stress and evaluated against the yield criterion. Component stresses, such as axial stress,
bending stress, normal stress in x-direction, normal stress in y-direction, shear stress, etc. as
well as combined stresses, are to be evaluated. The combination of global and local stresses
should account for actual stress distributions and phases. If the phase information is limited or
uncertain, the maximum design value for each component may be combined as the worst
scenario. Possible load offset due to the simplified assumptions made in the FE analysis
should be accounted for in the stress combinations.
Buckling Check
Structural members subjected to compressive loads may normally buckle before reaching the
yield limit. Various buckling modes should therefore be evaluated. Four different modes of
buckling are usually recognized:
Mode 1: simple buckling of the plate panel between stiffeners and girders.
Mode 2: flexural buckling of the individual stiffener along with its effective width of
plating in a manner analogous to a simple column.
Mode 3: lateral-torsion or tripping mode. The stiffener is relatively weak in torsion, and
failure may be initiated by twisting the stiffener in such a way that the joint between
stiffener and plate does not move laterally.
Mode 4: overall grillage buckling.
See Part I1 of this book for more information. To ensure that the local bending stress resulting
from loads, acting directly on stiffeners, are included in the buckling code check, the lateral
pressure should be explicitly included in the capacity check, combined with membrane
stresses calculated from the FE analysis. Relevant combinations of buckling load checks
should include evaluation of the capacity with relevant lateral pressure applied to either side of
the plate. Compressive stresses calculated from global and local models are to be
superimposed. Each structural member is to be designed to withstand the maximum combined
buckling loads, of which the critical load cases and wave phases may be different to those
pertaining to the yield check.

5.4 Fatigue Damage Evaluation
General
The fatigue strength of welded joints (structural details) in highly dynamically stressed areas
needs to be assessed to ensure structural integrity and to optimize the inspection effort. The
analysis of fatigue strength should be based on the combined effects of loading, material
properties, and flaw characteristics. At the global scantling design level, the fatigue strength
check for hull-girder members can be conducted for screening purposes. At the final design