Fall Protection - Ladders - Evaluate Loading for Ladder Rung

[franklineng]

Our company is considering purchasing a flexible cable fall protection system. The manufacture requires the climbing structure be capable of supporting the loads imposed by the system. For calculation purposes the required bracket load may be assumed to be distributed evenly between the number of rung attachments. The load required for each rung for a single user system is 1,125 lbs. per rung (3,375 lbs./3).
The load required for each rung for a four user system is 2,100 lbs. per rung (6,300 lbs./3).

From my machinery's handbook, would the rung be evaluated as simple supported at both ends with a load at center or fixed at both ends with a load at center? Our completely different calculation?

One of the rungs appear to be 3/4" diameter rebar, 13.5" in length.

Stress at Center (supported), -(W*l)/(4*Z)

W=2,100 lbs.
l=13.5 inches
Z= (Pi*d^3)/32 assuming 3/4" diameter = 0.0413 in^3

Stress at Center = -(2,100*13.5)/(4*0.0413)= 171.6 KSI?

Doesn't appear possible to use the system?

Other opinion?

 

[civilperson]

If the ends are rigidly fixed, maximum moment is PL/8 and stress is 85ksi. The system needs bigger diameter rungs.

 

[JLNJ]

I would look at the load and see if I couldn't reasonably reduce it. Multiple attachments might justify using less impact than a single user's attachment..

What allowable are you camparing your stresses against? You may be able to take a stress increase for the nature of the temporary load.

It might be optimistic to rigidly fix the ends unless they tie into something quite stiff.

 

[franklineng]

The flexible cable system has a pretension load requirement of 750 lbs. The load requirements are outlined by the manufacturer of the cable fall prevention equipment based on their own tests.

 

[DMcGrath]

Isn't the OSHA anchorage requirement for fall protection 5,000 lbs? (assuming you are in the US).

 

[gwynn]

I believe you can have loads adjusted with certified testing in some jurisdictions.

I have no idea what the set-up of the system you are looking at is, but is there a reason it needs to be attached to the centre of the rungs? You can lower the bending stresses considerably by placing the brackets off centre.

 

[franklineng]

I've been reviewing tread ladder rung specifications and the companies are giving a safe allowable concentrated load calculated based on a simple beam calculation.

Wouldn't this over estimate the safe allowable load?

 

[EngineerTex]

It seems to me that if the ends of the rungs can transfer moment into the rails that the simple beam calculation would underestimate the safe allowable load.

-T


Engineering is not the science behind building things. It is the science behind not building things.

 

[franklineng]

Compared the two formulae's again. If the bending stress were equal for both the simple supported and the fixed end, the simple supported beam would have half the load as the fixed end supported beam. But the deflection would be greater for the simple supported beam.

 

[MiketheEngineer]

OSHA says 5,000 lbs or twice the anticipated load. Our testing using a 6' shock absorbing lanyard shows forces in the 1,000 lb. range. So twice that is 2,000 lbs. - for each man.

Remember - during a fall - one guy will often pull another guy with him or knock him off also.

That said - you or the mfg should test the thing. We have found that our equipment will hold the loads but can get pretty bent up and must be discarded.

 

[geoffdale]

Also, check the welds. Normally just tacked for the most part unless spec'd otherwise.