Theatre Design & Technology - Spring 2013 - 48

understanding

Shock
hen most people think of the cause of a line (chain,
wire, or fiber rope) failing, they think of the load exceeding the tensile breaking strength of the line. While
this situation can certainly cause the line to fail, there is another possible
cause-the dynamic load (or shock load) exerted on the line is greater
than the line's energy-absorbing capacity.
A shock load is the force that results when an object suddenly accelerates or decelerates, but we most often associate it with the abrupt
stopping of a falling object. Remember the old adage: "It is not the fall that
kills you, it is the sudden stop at the end." A shock load force is exerted
on both the object and whatever stops the object's fall. While this force is
momentary, it can be tremendous.
Three factors determine the magnitude of the shock load: the weight
of the object, the speed that the object is traveling before it starts to decelerate, and the rate of deceleration (the stopping distance).
With a free-falling object, the stopping distance (or time it takes for
the object to stop falling) is critically important in determining shock
loads, and the amount of stretch/elasticity in the material stopping the
fall determines this distance. The more a material stretches, the more
energy it absorbs. Bungee cord (a.k.a. shock cord) has a great deal
of elasticity and is an excellent shock absorber. Therefore, falls on
bungee cord typically result in relatively low shock loads. On the other
hand, steel cable, and especially chain, has very little elasticity, and falls

46

theatre design

& technology

involving these materials usually result in extremely high shock loads.
The tensile breaking strength of a line is not related to its energyabsorbing capacity. In fact, many lines with very high tensile strengths that
allow them to hold extremely heavy loads have very low energy-absorbing
capacities and can break easily when subjected to dynamic shock load.
A common situation where we might want to calculate a shock load
relates to fall protection. How much shock load 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:

(

)

Fall Distance +1
Force = Weight × Free
Stopping Distance

s p r i n g 2013

Note: The "1" 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 shock load), then
these two zeros cancel each other out, and Force = Weight × 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.



Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 2013

Contents
Theatre Design & Technology - Spring 2013 - 1
Theatre Design & Technology - Spring 2013 - 2
Theatre Design & Technology - Spring 2013 - 3
Theatre Design & Technology - Spring 2013 - Contents
Theatre Design & Technology - Spring 2013 - 5
Theatre Design & Technology - Spring 2013 - 6
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