Anchor point for fall arrest device 3

[haggis]

I wonder if anyone can help with my problem.

We are installing a fall arrest device for maintenance personnel within the plant.

The specs on the device are as follows:

Drop forged carabiner - tensile strength 5000lbs.
Drop forged harness snap hook - tensile strength 5000lbs.
Capacity - One person - 310lbs.

My question is this. Why would the literature say that the supporting structure to which the device is attached, be capable of supporting a load of 5000lbs.

The wearer of the harness attached to the device would free fall approx. 3 ft. when the fall is arrested.

Does this fall under the same premise that given the rated capacity of a hoisting device, the support structure must be capable of sustaining that load. I can see this in the case of a hoist, but a fall arrest device?

The device is going to be hung mid span from a clevis welded to a member spanning 9'-0", clamped both ends to building joists. Now in choosing a member, and keeping the bending stress at around 14.5 ksi, I come up with say a C8 x 11.5 channel.

Just seems awfully big for what it has to do. Any help greatly appreciated.

Thanks in advance.

 

[Sparweb]

Sorry for coming in so late, but just for the sake of helping out anyone who stumbles on this thread in the future...

Haggis, Jetmaker,

You both prefer to know the origin of the "numbers" you're supposed to follow. The way to pick the OSHA nubers apart is to equate the kinetic energy, generated by falling a given distance, with the potential energy acquired by the rope/lanyard as it catches the falling worker.

Many workplace falls can be assigned a fall factor of "1". That means that the worker's belay (anchor) point is just as high as his center of mass. Were he to fall the same distance, but have a belay point way below his feet, the fall factor increases, to the extreme "2" where he has to fall the entire length of the lanyard just to pass the belay point, and then fall the distance again before he's caught.

When you get into the math, it's the fall factor, not the absolute distance, that counts. The modulus of elasticity of the wire rope is the spring constant as the worker is being caught. A fall that is 3 feet, 6 feet, or 20 feet down will always apply the same load to the worker, assuming the fall factor and the type of rope are the same.
This is what makes possible OSHA's standard 5000 pounds strength. Unless you postulate a higher fall factor (I don't think that can happen) or rope stiffer than steel, the shock load doesn't get higher.

I'd also like to add that synthetic ropes usually have a curved stress-strain graph, meaning "E" drops as the load increases. The impact on the worker as the rope catches him is gentler. Steel would be the stiffest stuff, hence the use of inertia reels. When fall factors get higher than 25%, the fall can injure the worker, just by catching him, unless it can stretch generously. "Dynamic ropes" are another solution.

For more info, the NFPA standard 1983 gives a good explanation of the principles involved.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout