Ladder Safety Systems

[JSgam]

Hey all. Attached is a ladder safety system used commonly on longer length ladders. Most all manufacturer's I've spoken with rely on not exceeding the ultimate strength of the rungs, and bending is expected under these loads, reaching up to 3,600lbs (split amongst 2-3 rungs). Every manufacturer will state the supporting structure needs to be able to support the loads, so I'm assuming any ladder using these just needs to make sure the ultimate stress of the rungs is not exceeded. For steel ladders, the rungs are usually plug slot welded at the ends. Also, these systems are typically installed onto existing ladders. For new ladders, I'd just used thick enough rungs, but on existing ladders they can have 3/4" round bar rungs, and assuming it's A36 steel there isn't a ton of capacity in those.

My questions are:
1) For analysis, is it as simple as comparing the ultimate stress of the rung material to an applied stress of M/Z? or is an FEA program more useful here when exceeding the nominal moment FyZ (from AISC 15th edition section F11)?
2) For the welds at the ends, is it acceptable to use the applicable allowable strengths from Section J of AISC, or is FEA more appropriate when deformation is expected?
3) Any additional thoughts on how to put an analysis for this on paper is appreciated.

 

[JSgam]

Regarding buckling.... Many if not most ladders I've seen or engineered are "hung". So buckling under compression isn't really an issue.

Regarding the excessive concern about a single 3/4" round bar being unable to sustain a 1200lb load. You should have a closer look at reality. Steel is quite ductile. If you are concerned about the peak bending moments then look at the plastic behaviour not the elastic behaviour. My quick analysis just now confirms the previous gut feel assertion that 3/4" round bar can sustain 1200lb in a simply supported fashion. Sure there will be plastic deformation, but that is the nature of steel. Plastic deformation is entirely acceptable in fall arrest systems. In fact it often form an integral part of the system as the loads are fundamentally dynamic.


Really?

Make a decision based on engineering knowledge. Controlled testing quite expensive and time consuming. The fall arrest suppliers have done that. The engineer's job is simply to ensure that the fixtures will not suffer complete failure for the required loads. (Deformation is acceptable.)

Of course this is very much my engineering approach. If another engineer wishes to take a more cautious approach then they are welcome to do so.

Fall arrest loads frequently come up in this forum and part of it is because they are so god damn high. But you need to understand WHY they are that high:

• They are brief instantaneous loads and not static loads
• They are ultimate loads (aka they already have all the load magnification factors and material reduction factors included.) Specifically from a manufacturing perspective if the item typically fails completely at 26kN and has a 3sigma deviation of 1.5kN then it is generally considered suitable

My comments and knowledge has been heavily shaped by my climbing experience. I've relied for decades on fall arrest equipment in my recreation. I've taken falls, seen tests and delved deeply into the physics of it all. There are already SIGNIFICANT safety margins built into the ultimate loads required. Don't go overboard in trying to meet them.

Thanks human. I appreciate the input. Deformation is certainly acceptable in fall arrest systems, but should still be considered in my opinion. It wouldn't be great to have a deformed ladder leading to something critical to an operation of a facility.

I also think, from an occupational safety perspective (I can't speak on recreational climbing equipment) that the safety margins are essentially 2:1 looking at the system design requirements in ANSI Z359.16. There are specific test loads in that standard that manufacturers test to, but in the end the manufacturer just tells you loads to the ladder since each ladder is different. Sure a 3/4 round bar rung will reach permanent deformation under 1,200 lbs, but not every manufacturer attaches to three rungs, and most manufacturers design to permanent deformation in their testing of loads based on discussions I've had with them. Load path of these loads to existing structures definitely still needs to be considered.

I think it's an important discussion to have which is I brought it here.