Fall Arrest - Energy Absorption

Energy Absorption

To arrest a fall in a controlled manner, it is essential that there is sufficient energy absorption capacity in the system. Without this designed energy absorption, the fall can only be arrested by applying large forces to the worker and to the anchorage, which can result in either or both being severely affected.

An analogy for this energy absorption is to consider the difference in dropping an egg onto a stone floor or dropping it into soft mud. Even for the same fall distance and weight of egg (the input energy), there will be more damage with the stone floor as the arrest distance is smaller and so forces must be higher to dissipate the energy. For the soft mud, the arrest distance is longer and so arrest forces are lower but the egg is still stopped and is hopefully undamaged.

Because fall arrest designs require high-rate-energy capacity design methods, fundamental fall arrest design is tedious and esoteric. Thus, most fall arrest parts and systems are designed to the force standards contained in Federal OSHA 29CFR1910.66 appendix c, a force-type design standard which accounts for required energy considerations. The standard mitigates PPE interchangeability problems, allows wide use by designers not versed in high rate energy methods, and it limits the force into the worker to a survivable level.

Actual loads on the user and anchor-anchorage vary widely with user weight, height of fall, geometry, and type of line/rope. Excessive energy into the support and user is avoided by the use of energy absorbing PPE designed for the 1800 lbs maximum of the referenced Federal OSHA standard. (Designers should be cautioned that the force values of the standard are based on high rate energy system design and thus its force values are not necessarily inter-related.)

The most common fall arrest system is the vertical lifeline: a stranded rope that is connected to an anchor above, and to which the user's PPE is attached either directly or through a "shock absorbing" (energy absorbing) lanyard. Once all of the components of the particular lifeline system meet the requirements of the standard, the anchor connection is then referred to as an anchorage, and the system as well as the rope is then called a "lifeline".

Anchors used for lifeline anchorages are designed for 5000 lbs force per connecting user, and the standard permits an anchor to deform in order to absorb energy (adhesive anchors have higher design requirements because of aging loss).

The rope can be lifeline rope, which stretches to lengthen the fall distance as it absorbes energy; or static rope, which does not stretch and thus limits the fall distance, but requires the fall energy be absorbed in other devices. It is essential that the PPE be rated for Fall Arrest and PPE used with static line include an energy absorber. While the energy absorbing lanyards hold in excess of 5000 pounds when fully absorbed, most limit the load during the fall to under 1400 lbs.

Another common system is an HLL (Horizontal Life Line). These are linear anchoring devices, which allow workers to move along the whole length of the anchor, usually without needing to disconnect and fixing points of the anchorage.

It is normally essential to include energy (or shock) absorbers within HLL in addition to those within the workers' PPE. Without such absorbers, the horizontal life line cannot deform significantly when arresting the fall. Because of the geometry of pulling across the horizontal line, this in turn results in large resolved forces being generated within the anchor system, sufficient to cause failure of the anchorage. This can occur even with energy absorbers being included in the PPE of the worker.

The load and horizontal line geometry in horizontal lifelines usually creates falls in excess of the 6 ft limit of the standard, limiting HLL design to standard-defined "qualified persons". (The recognition of these basic weaknesses have resulted in most temporary "wrapped structure" HLL anchors, which were anchors made from a wire rope wrapped around a structure and its ends fastened together by wire rope clips, being replaced by fixed-point anchors or HLL systems designed by defined "qualified" persons.)