Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 26)

SYSTEM DESIGN
1-24
Wind and Seismic Effects
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 ac-
counted 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.
In general, these forces act horizontally at the center of gravity
(CG) of the weigh vessel. Figure 1-26 illustrates 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 1-27 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
Wind and Seismic Effects on Vessel Stability
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 addi-
tional force for the extremes of wind or seismic forces expected. If
F was increased to where F
0T
equalled 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.
The relationship between F
0T
and F may be stated as follows for the
vessel shown in Figure 1-27:
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.
Figure 1-27
Figure 1-26