ssc-367 - Ship Structure Committee

Screeninq Process
For a preliminarydesign ofa structure it is common practice to use
a blanket SCF of 5.0 or 6.0 for all joints, depending upon dynamic
effects. If the structure is susceptible to dynamic amplification
the higher blanket SCF should be used. Once the fatigue sensitive
joints are identified during this screening process, the SCFS for
these joints should be determined.
In the determinationofSCFs a parametricstudyofvariablesd/Dand
t/T should be considered. The joint fatigue life is a function of
nominal brace stress and SCF. To increase joint fatigue life, the
nominal brace stress or the SCF should be reduced. An increase in
brace diameter can dramaticallyreduce nominal brace stress without
a significant increase in SCF. This is particularlytrue for brace
members intersecting large diameter legs. However, where members
are more similar in size, an increase in brace diameter also
requires an increase in chord diameter.
By increasing the brace diameter rather than increasing the brace
thickness, a more effective section can be used and prohibitively
low diameter to thickness ratios can be avoided. Increasing the
brace diametermay be the easiestwayto increasejoint fatiguelife
during preliminary design. The chord diameter may also have to be
increased to offset the SCF increasesif the brace area and section
modulus are increased.
Comprehensive Desiqn
Once the member diameters are finalized a comprehensive fatigue
analysis and design may be carried out. The parameter most easily
modified during this stage is the member thickness. An increase in
brace thickness increases brace axial and bending section
properties, which will reduce brace nominal stress. However, as
stated above, the chord thickness should be increased accordingly.
Otherwise the brace nominal stress reduction will be offset by the
joint SCF increase, resulting in marginal difference in fatigue
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