Rope Rescue: Vector Forces & Rigging
In rescue operations, understanding vector forces and their effect on rigging components is crucial to ensure the safety and success of the operation. When an internal angle greater than 0° exists between rigging components or anchor points, vector forces become apparent and relevant. This article aims to summarize the information regarding vector forces, emphasizing the importance of considering these forces when undertaking rigging tasks.
When a load of 100kg is suspended equally from two slings with no internal angle, each sling shares half of the load's weight. In this scenario, each sling and anchor point bear 50kg or 50% of the load's weight.
The Ideal Angle
As the internal angle between rigging slings increases, additional vector forces are applied to each sling and anchor point. The 'ideal angle' for rigging ropes is approximately 45°, which distributes 54% of the load's weight to each anchor device. Although this is over half the original weight, it still provides an advantage by sharing the load between the two anchor points.
The 'OK' Angle
An internal angle of 90° between ropes and rigging components is often referred to as the 'OK' angle. At this angle, each anchor component carries 71% of the load's weight. By staying at or below this angle, excessive forces on anchor components can be avoided. Estimating a right angle during rigging tasks simplifies the process and ensures the anchor components are not overloaded.
The Critical Angle
The critical angle is defined as an internal angle of 120°. At this angle, equilibrium is achieved, resulting in the load's weight being exerted on each anchor point and rigging equipment. For example, if the load weighs 100kg, each anchor point and rigging item will experience 100kg or 100% of the load's weight.
Mathematical formulas can be used to calculate vector forces. It is important to convert the load's mass (in kilograms) to weight (in Newtons) to accurately determine the resultant force. When rigging components equally share the load's weight, as in a 'Y' hang configuration, the following equation can be used:
F = W * sin(α)
Where: F represents the resultant force exerted on each anchorage. W is the weight of the load. α is the internal angle between the two slings.
Vector Force Chart
The accompanying chart illustrates the resultant forces applied to each anchor point and rigging component in a Y-hang rigging configuration. The chart also provides the force ratio in percentages, facilitating force calculations based on the specific weight of the load. Notably, when the critical angle of 120° is exceeded, the forces increase dramatically. In extreme cases, such as achieving an angle of 175°, each anchor component may experience significantly higher forces, which could have adverse effects on tensioned lines, cross hauls, and tyroleans.
Understanding vector forces is paramount when it comes to rescue work and rigging tasks. By recognizing the impact of internal angles between rigging components, the load's weight can be distributed effectively and within safe working load capacities. By considering these factors, rescuers can mitigate risks, prevent catastrophic failures, and ensure the success of their operations.