Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 117

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FIGURE 3.22 Local axis technique.

Member Effective Lengths
The effective length of a member under axial compression should reflect the
relative joint stiffnesses at the end of the member. The appropriate effective
length factor K should be selected from the recommended values in the design
codes. Consideration should be given to the constraining effect provided by
intermediate members along the length of the member; the effective length of
a member buckling about its y-y axis is often different from the effective length
about the z-z axis.
The compression flange (or critical flange) of a member may buckle under
bending lateral torsional buckling. The effective length of a member under
bending should reflect the degree of torsional restraint offered by the end con-
nections of the member and by intermediate members along the length of the
member. The bending effective length of a member should be calculated
using the appropriate factors given in the design codes.

Joint Eccentricities
Eccentric joints in jacket structures should be modeled using member-end
offsets. For topside type structures, joint eccentricities should be modeled
using discrete elements, thereby allowing easy extraction of joint forces
from the output.
When required, the deck plate should be modeled as a structural element
using it as a membrane plate. Note that the plate elements need not be offset.
Alternatively, pinned-end axial brace members may be used in lieu of plate
elements.
A problem the structural engineer always faces is how to model the pile inside
the leg. Most software provides a wishbone member that should be modeled at all
horizontal bracing levels of the jacket to account for pile-to-jacket leg interaction,
as shown in Figure 3.23 . If the pile-to-jacket leg annulus is to be grouted, then a
rigid connection between the pile and leg should be modeled.
Generally, appurtenances do not contribute to the structural stiffness of the
primary structure. Appurtenances may be modeled to facilitate automatic load
generation by the program and are sometimes referred to as nonstructural mem-
bers. Appurtenances may be modeled by assigning a small modulus of elasticity
or small stiffness properties to these members. It is important to ensure any
member-end releases, etc., accurately reflect the actual support conditions for
the appurtenance and that no spurious forces enter the structure due to poor