Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 51)

Wind and Seismic Effects on Vessel Stability 47
22.0 Wind and Seismic Effects on Vessel Stability
22.1 Overview
Other than forces resulting from the impact of a vehicle, wind
and seismic forces are the most important external forces
which might affect a weigh vessel. The threat from vehicular
traffic can be guarded against using properly designed guard
rails. The effects of wind and seismic forces, where they are a
factor, must be accounted for in the design of a weigh vessel.
At a minimum, consideration of these forces might affect the
capacity of load cells selected. In more extreme cases they
may dictate the use of additional restraints on a vessel. In
general, weigh modules have a lift-off capacity of 150% of
capacity, and a side-load capacity of 100% of capacity.
Figure 22-1.
In general, these forces act horizontally at the center of gravity
(CG) of the weigh vessel. Figure 22-1illustrates a four-legged
vertical cylindrical vessel and the forces acting on it in the
absence of wind or seismic forces. W is the vessel’s weight
(an empty and full vessel should be considered separately, as
either one may be the limiting case), and it acts through the
vessel’s center of gravity. Assuming that the four legs are
arranged symmetrically, then each leg will exert a force of
1/4W on each mount.
Figure 22-2 illustrates the same vessel with the addition of a
horizontal force F (the result of wind or seismic activity.) The
vessel exerts a horizontal force of 1/4 F on each load cell
mount. Also, there is an additional force of F
0T
acting on the
left-hand side load cell mounts, which means that each is now
carrying a load of 1/4W + F
0T
. On the right-hand side load cell
mounts, a force of F
0T
is also induced as a result of F,
however, this force is in the opposite direction to the existing
1/4W and the total force here is reduced to 1/4W - F
0T
.
Hence, you will see that load is being transferred from the
mounts on one side of the vessel to those on the other. The
load cell capacity selected must be capable of withstanding
this additional force for the extremes of wind or seismic forces
expected. If F was increased to where F
0T
equaled W/4 then
there would be zero load on the right hand mounts and the
load would have doubled to W/2 on the left-hand mounts.
Further increase in F will cause the vessel to lift up on the
right-hand mounts and may, in the extreme case, cause the
vessel to tip.
Figure 22-2.
The relationship between F
0T
and F may be stated as follows
for the vessel shown in Figure 22-2:
F
0T
= .7Fh/D
where h = height to the center of gravity and D = vessel
diameter.
It is desirable to reduce F
0T
; this can be done as might be
expected by reducing F or h or by increasing D. Dimension h
can be reduced by reducing the vessel height (not always
practical) or by placing the mounts at the vessel’s center of
gravity as illustrated earlier. In this case h = 0 and hence F
0T
=
0.
It is interesting to compare the stability of a vessel supported
on 3 and 4 load cell mounts. Figure 22-3 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.