Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 506

geometry or determined through analysis. A single element represents the
member behavior.
In some software, the type of failure must be defined and anticipated for
each element of the structural component and loading mode before starting
the analysis, and the element type and its nonlinear characteristics must be
defined accordingly. Element types include:

Elastic members ●

Buckling members ●

Beam members

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Frame members

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Joints.

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Material nonlinearity and initial imperfections are taken into consideration
in this method and the material data will be obtained from direct test results.
The yield of the section may be determined from the full section under the
yield stress.

Shell Finite Element Models
Any member or joint-can component may also be modeled by shell elements, or
it can be modeled with a combination of shell elements and beam elements.
Steel usually yields before it fractures because the uni-axial yield stress is
significantly less than the stress required for a cleavage fracture. However, in
three-dimensional stress fields, the individual stress components can be much
greater than the uni-axial yield stress. Without yield occurring in the vicinity
of a crack tip, a simple elastic calculation of the stress distribution combined
with the von Mises equation predicts that the stresses may reach 2.5 times
the uni-axial yield stress before yield occurs.
The fracture mechanism is usually complex, as it is much more dependent
on the largest stress and is less affected by the tri-axial stress pattern. Therefore,
in the tri-axial stress field, the applied stress may exceed the yield stress, so that
fracture becomes the failure mechanism. The main factor in the fracture is the
plane strain condition due to effect of the Poisson ratio.
Brittle fracture is more likely to occur in cold conditions and at high rates of
loading (e.g., under impact). The toughness grade of material specified for any
application should be carefully selected to account for these factors as well as
the likely stress levels, crack growth through fatigue, inspection procedures and
consequences of failure.

Modeling the Element
The sliding action of piles within legs should be modeled with the approximate
constraint conditions, which allow unrestrained differential axial displacement
and rotations but couple the lateral displacements of piles and legs.