Physics of a Fall

The Relationship of the Fall to the Impact

Examining fall distances and impact forces is a worthwhile exercise for any “at-height” operation. In the occupational safety realm the implication of workplace falls above 4 feet becomes quite clear when viewing the figures below. This is in part why States such as Washington have moved away from the federally mandated 6 foot trigger height for fall protection to a more conservative 4 feet.

When looking at impact forces through the lens of rope access or rescue we can substitute the concept of a falling worker with that of a dynamic event resulting from a failure within our rope system. With more than 1 person in the system the impact forces will naturally compound to both the individual(s) and to the anchor.

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Its helpful to remember:

  • With a 6 foot lanyard tied off even with your D-ring you will free-fall 6 feet (the maximum allowable fall distance per OSHA) before your Personal Fall Arrest System (PFAS) engages.

  • For every 1 foot below your D-ring that your lanyard is tied off you will free-fall 2 additional feet before PFAS engages. OR, for every 1 foot above your D-ring that you tie off you will free fall 2 feet less.

Calculating Your Ground Clearance

Knowing your fall clearance distance is essential when working and rigging off of the ground without the normal protections of guardrails or parapets. The formula below is helpful in recognizing just how much height you’ll need to protect workers utilizing horizontal lifelines or working near unprotected edges.

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*OSHA 29 CFR limits deceleration distance (shock absorbing lanyard extension) to 3.5 feet.

* Safety Factor of 2 feet is typically used to add a buffer for error in calculating total fall clearance.

Kyle DunganComment