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The Insidious Threat: Shock Loads in Rigging Operations 

In the world of rigging, understanding shock loads is paramount for safety. Let's delve into the dangers of these invisible forces.

Static vs. Dynamic Loads: Setting the Stage

Most forces encountered during lifts are a combination of two types:

Static Loads: Stationary (non-moving) forces acting on a load.

Dynamic Loads: Forces acting on a load that is in motion.

Even seemingly slow lifting involves some dynamic forces due to factors like swinging and slight accelerations.

Shock Loads: The Hidden Danger

Our focus here is on a specific type of dynamic force: the shock load (also known as a shock force). This is a rapid, impulsive force that occurs much faster than typical dynamic forces.

Causes of Shock Loads

Several factors can trigger shock loads:

Sudden Jerking of Slack: An operator quickly taking up slack in a sling with a jerk can introduce a shock load.

Rapid Acceleration/Deceleration: Rapidly speeding up or slowing down the load can create shock loads.

Uncontrolled Movement: Failure of equipment like fairleads or sheave guides can allow a slack line to roll out, potentially causing a shock load when tension is applied.

The Devastating Impact of Shock Loads

The magnitude of a shock load can be several times greater than the weight of the load itself. This is why the safe working load (SWL) of rigging equipment is significantly lower than its breaking strength. SWL considers a "safety factor" to account for potential shock loads.

Calculating Shock Loads: A Challenge

Calculating the exact force of a shock load is complex. Factors like load weight, travel distance, and the elasticity of the rigging material all play a role.

While precise calculation can be intricate, many resources acknowledge the difficulty of estimating shock loads.

A Real-World Example

Gelrum's "Stage Rigging Handbook" provides a telling example: A 75-foot, ¼-inch diameter galvanized cable sling subjected to a shock load from a suddenly dropped 500-pound load (6-inch drop) experiences a shock force of 2,296 pounds – over four times the weight of the load!

Conclusion:

Shock loads are a serious threat in rigging operations. By understanding their causes and potential consequences, riggers can take steps to mitigate these risks and ensure safe lifting practices.