Over the past several years, we have seen many a "jury rigged" fall protection "system" on the roof tops of buildings. By this we mean a chaotic assembly of hand-tied ropes and / or cables, attached to roof vents, plumbing, HVAC equipment stands, conduits or other 'structures', thus spreading the 'load' of the suspended worker(s) and their powered platforms or boatswain chairs, or the load of a worker falling from an unprotected roof edge or walking / working surface.
FIGURE:1
Effect of a rigged horizontal line being used to suspend a 1,250 lb. working load, when the sag is only 1.2" and the line is 20' in length; the load in the line is 41,663 lbs. !!! Sag must be sufficiently increased so the load in the line is less than 5,000 lbs.
Problem #1
Ropes are suspect materials that degrade over time due to ultra-violet rays from the sun, due to heat exposure, due to chemicals in the air, due to mold and water damage, and due to friction and rubbing from use and being dragged on asfault and tar rooftops. Thus with age, ropes do not have a particular strength rating that can be guaranteed, unless they are perfectly maintained, cleaned and cared for.
Problem #2
Tieing knots in ropes weakens them by as much as 70%, and the knot itself could come loose. There is no way to eliminate the human element as a variable with knots.
Problem #3
The arrangement of the ropes and cables may in fact spread out the load, but how do you know that the peak load applied during a fall incident isn't being more applied to one or another of the structures, knots or ropes in the 'system' of rigging. This isn't a system and since the arrangement of structures could be multi-variant distances, it is very difficult to calculate the lengths of rope and the angles being created by them.
Problem #4
None of the structures being attached to have ever been pull-tested by a qualified person, have never been reviewed by a licensed professional engineer for structural integrity to support the maximum dynamic load that could be applied, and could by damaged by the ropes and cables being wrapped around them. Such damage could be latent water leaks, dents, strains, bending; such damage could affect the safe and proper operation of the electrical, plumbing, HVAC or other systems on your building. Latent water damage could create a mold problem and a toxic air problem in your building, which could costs many tens of thousands of dollars to fix.
Problem #5
When a load is applied to a horizontal line, in about the center of that line, that load is not divided in half, as most people think. The only way the load is exactly divided in half, is if the horizontal line sags so much, that it is effectively two (2) vertical lines. In other words, the sag would have to be very large as compared to the distance between the two (2) points that horizontal line is attached to.
The reality is, if the sag is nearly zero (0), then the forces within the horizontal line are greatly multiplied. A way to visualize this is to imagine a 1 lb. weight being suspended between two people with a 20' length of rope. Let's assume the rope has no weight.
If the people pull on the rope just enough to lift the 1 lb. off the ground, the sag will be about 5', and the tension in the rope experience by each person will be about 1.25 lbs..
Now, if the two people pull on the rope as hard as they can, so as to lift that 1 lb. weight higher off the ground (not to bounce it, but in a slow and steady motion), that rope becomes flatter and flatter, the sag becomes smaller and smaller, and the force they experience becomes greater and greater.
They find it more difficult to lift that 1 lb. weight higher and higher off the ground. If they pull so the sag is only 1', the tension force in the rope will be 3.33 lbs.. If they continue to pull so that the sag is only 0.1', then the tension will be 33.33 lbs.!
OSHA demands fall protection anchors have safety factors built in, thus OSHA requires a 5,000 lb. force of maximum dynamic load before fracture or pull-out with anchors.
If sag is very small dividing 5,000 by 33.33 gives 150 lbs. -- that means a 150 lbs. person would be able to fracture or bend a certified anchor when suspended from the middle of a 20' horizontal line that has only 0.1' of sag. If the sag is reasonably large, say 1' on a 20' line, then dividing 5,000 by 3.33 we get 1,500 lbs.
Properly engineered and installed horizontal lifelines have the proper sag built-in, or they have tension sensing devices and deploy extra length of line in order to create the extra sag needed to relieve the system of excessive force that might damage the building or the anchorages or the cable itself.
Engineered horizontal lifelines are designed with OSHA / ANSI / IWCA compliant anchor points and intermediate anchors that are connected to a building and tested to withstand the ultimate dynamic loading that may be applied.
Any anchor that deforms, fractures, bends or otherwise doesn't return to it's original position and condition is defective and non-conforming - unknown loads or excessive loading can and will damage even certified anchors, so care and caution must be taken to avoid inadvertent misuse.
Finally, suspending from a manually rigged horizontal lifeline is EXTREMELY hazardous and should never be undertaken. Lives would be in danger.
Rarely will any reputable fall protection company design a horizontal cable for the purposes of performing suspended work, particularly because of the complexities of such a design and because of the risks involved.
Suspended work must ONLY be done using certified safety anchors, davits and proper transportable equipment and/or permanently installed equipment, with a written safety plan approved by the site owner / developer / fiduciary.
Good luck and be safe!
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