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COMPRESS provides options to account for
the action of vortex shedding. The designer
must initiate the vortex shedding analysis
when desired. To do so: Edit the
Wind Code and click the button labeled
"Vortex Shedding / Damping Coefficients";
the Vortex Shedding Calculations dialog will be
displayed. "Check" the conditions for
which the vortex shedding analysis should be performed
(eg: Operating, Empty, and Vacuum).
When the wind vortex shedding option is applied
COMPRESS first determines the critical wind speed of
the vessel. If this is greater than the design wind
speed then no further analysis is performed because the
necessary forcing condition for vortex shedding would
not be encountered. If the critical wind speed is less
than the design wind speed, COMPRESS uses the method described
in the National Building Code (NBC) of Canada - 1995 to
perform an analysis for the additional loads imposed on
the vessel by the vortex shedding condition.
The vortex shedding analysis defined in the NBC applies a static
load to the top 1/3 height of the vessel in a direction perpendicular
to the wind. The static load is determined based on the vessel
geometry, dynamic characteristics (natural frequency)
and the critical wind speed.
When the critical wind speed is less than the design wind
speed COMPRESS performs additional calculations to
determine the bending moments in the vessel due to the
combined effect of the additional lateral force due to
vortex shedding and the basic wind load corresponding to
the critical wind speed.
Additional summary tables are provided to report the wind shear,
bending moment and deflection due to this load condition.
In "Design Mode", COMPRESS will increase shell
thicknesses if necessary to maintain the calculated stresses
at or below the allowable stress.
Note that the vortex shedding calculations reported are
based on forces resulting from a wind speed corresponding
to the critical wind speed. For example, if the
design wind speed is 90 miles per hour (mph) but the
critical wind speed is only 20 mph, then the results
(wind shear, bending moment, deflection) are calculated based
on the 20 mph critical wind speed in combination with an
additional vortex shedding force applied normal to the wind
flow. Thus there is no "mixing" of design
wind speed of 90 mph with vortex shedding forces applied at 20 mph.
If the deflection due to wind (with or without vortex shedding
action) exceeds the maximum specified in the Wind Code dialog
and design mode is used, COMPRESS will automatically increase
the shell and skirt thicknesses so as to increase the moment of
inertia to a value so that the deflection is within the
specified limits.
See these related technical papers on the Codeware website:
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