ssc-367 - Ship Structure Committee

a
fn = ~ (EI/mi L4)%
where:
the
moment of inertia of the beam
3.52 for a beam with fix-free ends (cantilever)
9.87 for a beam with pin-pin ends
15.4 for a beam with fix-pin ends
22.4 for a beam with fix-fix ends
1ength
mode of vibration
mass per unit length
The amount of member fixity assumed in the analysis has a large
effect on vortex shedding results, because of its impact on member
stiffness, natural period, amplitude of displacement, and member
stress. Hence, careful consideration should be given to member end
conditions. Members framing into relatively stiff members can
usually be assumed to be fixed. Other members, such as caissons and
risers, may act as pinned members if supports are detailed to allow
member rotation.
For members with non-uniform spans, complex support arrangements or
non-uniform mass distribution, the natural frequency should be
determined from either a dynamic analysis or from Tables provided in
References D.5 and D.6. Reid (Reference D.7) provides a discussion
and a model to predict the response of variable geometry cylinders
subjected to a varying flow velocities.
The natural frequency of a member is a function of the member’s
stiffness and mass. For the purposes of vortex shedding analysis and
design, the member’s stiffness properties are computed from the
D-6