Rigging
Math
(Made
Simple)
A
Primer by
Delbert
L.
Hall, Ph.D.
ETCP
Certified
Rigger
ETCP
Recognized Trainer
Lesson
11:
Calculating Shockloads
What
is a Shockload?
A shockload is the
result of an object coming to a
sudden stop, and this force is exerted on both the object and whatever
stops
the objectÕs fall. While this force is momentary, it
can be
tremendous. Three factors determine the magnitude of the shockload: the weight of the
object, the speed that the
object is traveling when it starts to decelerate, and how fast it
decelerates. In the problems presented in this lesson, we
will assume
that the object is freefalling, but instead of using speed (which we
could do),
we will be using the Free Fall Distance. And instead of using
deceleration
time, we
will be using Stopping Distance. Using these knowns will also
make our calculations much simplier.
The
Stopping Distance is critially important in determining shockloads, and the amount of
stretch in the material stopping the fall determines distance. The more a material
stretches, the more
shock force it absorbs. Bungee
cord (a.k.a. shock cord) stretches a lot and is an excellent shock
absorber. Therefore,
falls on bungee cord typically result in relatively low shockloads.
On the other hand, steel cable and
chain have very little stretch, and falls involving these materials
usually
result in extremely high shockloads.
A
common situation where we might want to
calculate shockloads
relates to fall
protection. How much shockload
results when a
technician falls a specified distance, is caught by his harness and
lanyard,
and stops over a specified distance as the lanyard stretches or the
shock
absorber on the lanyard expands?
The
equation for solving this problem is: Force
= Weight 
Note: The
Ò1Ó represents the weight of the falling
object. Without it there would be no
load on the line AFTER the initial shock
occurs, so do not forget to include it. If
both
the free fall distance and the stopping distance are zero (there is no fall and therefore no shockload) then these two zeros
cancel each other out and Force = Weight X 1 or just the weight of the object. If
the stopping distance is zero and the free fall distance is greater than zero,
then force would be infinite, which is not possible. So, the
stopping distance MUST be greater than zero.
Example:
A 200-pound man wearing a safety harness and lanyard falls six
feet. As
he stops, the harness and lanyard stretch 6 inches. What is
the force on
him and the rigging that supports him?
Force
= Weight x ((Free Fall Distance/ Stopping Distance) +1)
or
Force
= 200 x ((6/.5) +
1) or
Force
= 200 x (12 + 1) or
Force
= 200 x 13 or
Force
= 2600
pounds
If
this number seems large to you, it is; but it is also
correct. Shockloads
can be huge; that is why you want to avoid them
at all costs.
