Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 27)

1-25
SYSTEM DESIGN Wind and Seismic Effects
It is interesting to compare the stability of a vessel supported on 3
and 4 load cell mounts. Figure 1-28 shows a top view of a vertical
cylindrical vessel supported at 3 and 4 points (broken and solid
lines respectively). The vessel will tend to tip about a straight line
drawn between adjacent support points; the greater the distance
from the center of gravity to this line the more stable the vessel will
be. A vessel supported at 3 points will be approximately 29% less
stable than if it were supported at 4 points.
Because of the many variables in vessel design and site conditions,
it is impossible to deal comprehensively with the calculation of
wind and seismic forces in this text. However, the following
subsections deal with these forces in general terms and point out
the information necessary for a complete analysis. Refer to the
Uniform Building Code (UBC) for further details.
While the effects of both wind and seismic forces should be
considered, it is acceptable to consider these forces in isolation.
To perform a complete wind force analysis, the following
information is necessary:
■ Vessel: The vessel’s dead and live weights, number of supports,
and overall dimensions such as height, length of legs, diameter,
etc.
■ Minimum basic wind speed: This may be taken from Figure
1-30, which is a map of the USA superimposed with wind speed
contours. This map is based on a 50-year mean recurrence interval
which has traditionally been accepted as a reasonable risk. If local
records indicate higher 50-year wind speeds, then the higher
values should be used. This map does not consider the effects of
tornadoes.
Figure 1-28: Top view of cylindrical vessel
Wind Forces
Consideration must be given to the effects of wind loading when a
weigh vessel is installed outdoors. This is particularly important
for tall slender vessels, vessels installed in exposed locations (for
example, facing a large body of water), or those installed in a high
wind-speed location. In analyzing the effects of wind loading, it
must be assumed that the wind may blow at a vessel in any
horizontal direction.
Figure 1-29 illustrates the effect of wind blowing at a vertical
cylindrical vessel. Note that not only is there a force exerted against
the windward side of the vessel, but there is also a suction force on
the leeward side. These forces are additive, and tend to tip the vessel
in the direction of the wind. At right angles to the wind direction
are suction forces pulling on each side due to the increased speed
of the wind at these points. Since these are equal and opposite in
direction, they have no net effect on the stability of the vessel.
WIND DIRECTION
Figure 1-29
.25D .35D
CG
.5D
D = Diameter
CG = Center of Gravity
= 3 point support
= 4 point support