ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
be easily designed and extruded. For this reason, there are a variety of different extrusions used
in marine construction, but few to any standard cross-section.
1.3 Structural Design
The basic principles for structural design with aluminum are similar to those for design
with steel. Consideration needs to be made for the reduced elastic modulus of aluminum, which
means reduced buckling strength and stiffness. In aluminum the strength of welds and the
adjacent heat-affected-zone are significantly less than in the base metal, and the designer needs
to be aware of this. Design codes generally address the issue of weaker welded properties by
designing all of the structure for this reduced strength, although there are instances when the
properties of the base metal are used.
Naval authorities and classification societies document methods for design of aluminum
hull structures. These methods have historical backing, but new materials, emerging
technologies, different hull forms, and high-speed operation mean that application of these
methods will require interpretation. The designer must be aware of the basic principles, and not
try to apply blindly a method to a use for which it was not intended.
For the smaller, high-speed craft that are being fabricated with aluminum, the design
loads are different from those of larger vessels in that the loads are reduced if the craft is to
operate in a more benign environment. Furthermore, even though equations are given for
determining the design loads, for most of these craft model testing or hydrodynamic analysis is
necessary and required for determining the design loads actually used. Likewise, even though
equations are given by the classification societies to compute the structural response to the loads,
in most situations detailed finite element analysis is required to determine the scantlings.
1.4 Structural Details
Many of the same structural problems are posed in aluminum structure as in steel
structure for details such as intersections of structural members or avoidance of discontinuities.
In many cases the solution to the problem, the structural detail selected, will be the same in
aluminum as in steel. However, considerations of fatigue, which is a far greater concern in
aluminum, will dictate the use of details that have lower stress concentrations.
Opportunities in aluminum, particularly for the ease with which unique structural shapes
can be extruded, leads to structural details that are unique to aluminum structure. The result is
generally lighter structure at a reduced total cost. However, such details often have
discontinuities for which detailed stress analysis, including fatigue analysis should be performed,
and fatigue testing of such details is needed.
1.5 Welding and Fabrication
Aluminum welding is generally performed with gas-metal arc welding (MIG), similar to
the use of the process with steel. The only other process used with aluminum is gas-tungsten arc
welding (TIG), which is used for thinner material. Although the processes in aluminum are
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