Fall Arrest Anchorage

[reubz]

thread507-374163

Hi, So I am designing a permanent fall arrest monorail style system. The design criteria includes 2 workers so 5000lbs each 10,000lbs as the design anchor load from Worksafe BC in my mind.
Should this load be applied all the way through for the design of the structure, beam, column to the foundation etc. or is it just the anchor location?
For a cantilevered structure over a rail line, 10,000lbs will induce significant overturning loads etc.

Thanks.
Reuben

 

[Agent666]

Yes, you need to ensure there is a loadpath to resist the load.

For a normal building structure this usually isn't an issue and you can kind of stop chasing the load once it reaches the main structural system capable of transferring/distributing the loads further.

However for your case of a cantilever over a railway line, if you don't resist the significant loads then what is the purpose of the fall restraint? i.e. someone falls and entire structure collapses. So yes you need to consider the loads to ensure the system is stable when its required.

 

[CANPRO]

I don't know where you're located, but this recent thread about fall arrest in Ontario might be worth a read - thread507-446868. I think designing for 10,000 lbs is excessive, should be able to use 2x(2xMAF) = 2x2x900lbs = 3600 lbs. You also may not have to design for two impact loads acting simultaneously (at least according to CSA standards) unless there is a reasonable probability that both workers could fall at the same time (swing stage letting go for example).

Another load case that should be looked at is what is required during a rescue. This is something that is often overlooked. If both workers need rescuing, and the rescue plan involves the monorail, is there a 3rd or 4th trolley available? A single trolley may have to suspend the weight of two people. Designing for that scenario can be difficult, and maybe a rescue plan that doesn't involve that fall arrest system is required. Its worth having the discussion with the owner/user.

 

[reubz]

Thankyou Agent666. Yes I am in agreement and that was my initial plan. I was second guessing myself when I got through to the fairly large size of the foundation (3mx2mx1.5m deep) compared to some of the existing foundations on site(2mx2mx1.2m).
The thought was if it was an impact load of such short duration then the load will reduce due to the flexibility of the system. Therefore I should be using more like the 2 x MAF number etc. once I was further away from the "anchor point".

I think I'm just going to follow the load through at the 2 x 5000lbs and just size it accordingly.

 

[Agent666]

I think in reality with fall arresters (energy dissipation devices attached to the harness) the loads will be much lower as they are designed to lessen the impact on the person falling, but who is to say those using the attachment point will always have one of those available.

I guess if there is a time vs force relationship for the impact load being applied then the dynamics of the situation could be looked at as the load is not a constant load as you note. Codes I work with don't have this, so its just design it for a static force of so many kN's that represents the fall, I suspect the effort involved in getting into the dynamics to justify a lower load or at least demonstrating that the structure does not become unstable simply doesn't stack up when a larger foundation might work fine and take a fraction of the time to design.

Converting your load to kN just now, it does seem high. I think in this part of the world if you have 2 people attached then I seem to recall that the design load is something in the order of 15-21kN (3370-4700lbs).

 

[BMart006]

I would echo the advice of CANPRO with a few modifications.
The maximum arresting force (MAF) for body belts is 900 lbs., however those are not permitted anymore in the US. Any arrest system should utilize full body harnesses, which has a permissible MAF of 1800 lb. OSHA requires a safety factor of 2, so that would put you at 3600 lb. per user (you can from there have the debate of whether or not to include both users simultaneously; at the very least include the MAF from one user + the weight of the second user (and equipment). The 5000 lb. number comes from a "non-qualified" person checking the system. If you consider yourself "qualified", then 3600 lb. is sufficient per user.
A few other things to note:
[ul]
[li]It is best to specify the entire system, or at least put some constraints on it. For example, dictate that the lanyard meets ANSI Z359 so that your MAF is ensured to be under 1800 lb.[/li]
[li]Check both deflection of the monorail and overall fall clearance of the user. You don't want them to be caught but break their legs at the bottom.[/li]
[li]As CANPRO said, rescue needs to also be considered. It only takes a few minutes of hanging to cause some serious problems.[/li]
[/ul]
Hope that helps.

 

[CANPRO]

BMart006, thank you for backing my statements. I was not aware of the change in the minimum MAF in the US - in Canada, most lanyards limit the MAF to 900 lbs...the change you referenced, I assume that you can still use a lanyard with a 900 MAF, but the design of the system has to assume 1800 lbs?

reubz, you referenced Worksafe BC in your OP...so I assume you're in Canada. You should really read through CSA Z259.16, it has a checklist of everything that should be on the fall arrest drawings (including the intended rescue plan)...this may also open up a list of items you need to consider in your design that you may not have considered previously.

 

[BMart006]

CANPRO,
Off of the top of my head, I believe the criteria for a standard 6ft lanyard for the US is 450 lb. to activate the energy dissipation, and the total arresting must average a force less than 900 lb. with a peak not to exceed 1800 lb. A search of ANSI or OSHA could confirm those.

 

[darthsoilsguy2]

OSHA anchor points for single human fall arrest is 5000 lbs. IF you are in USA, that is it.

 

[Agent666]

What are you protecting from falling in the US, elephants or something?

 

[reubz]

Hi All, Thankyou for your comments. You are correct I'm based on Canada. I will get a copy of CSA Z259.16 and review.

Regards,
Reuben

 

[BMart006]

darthsoils,
Not necessarily. Its 5,000 lb unless you decide to get more refined with it based on the system being used (e.g. lanyard with MAF of 1800 lb). See below from OSHA.

1910.140(c)(13)
Anchorages, except window cleaners' belt anchors covered by paragraph (e) of this section, must be:
1910.140(c)(13)(i)
Capable of supporting at least 5,000 pounds (22.2 kN) for each employee attached; or
1910.140(c)(13)(ii)
Designed, installed, and used, under the supervision of qualified person, as part of a complete personal fall protection system that maintains a safety factor of at least two.

 

[chicopee]

5000LBS is not excessive for anchorage. Consider the lanyard to be six feet with the older fall protection requirement instead of the current 2 feet drop, the impact force will be much greater than the weight of an average let's say ironworker with his tool belt and spud and whatever ealse he may carry such as rivet busters. Best bet for such worker so as to minimize internal injuries from an impact at the end of lanyards would be either breakable stitches or the shock absorbing feature incorporated in these lanyards.

 

[Geoff13]

I presume when you reference Worksafe BC this will end up pointing you to BC OHS Reg 296/97 Part 11 for Fall Protection. This section doesn't give much clarity about what meets the rules, but using CSA Z259.16 will give you a solid basis for saying you've designed an acceptable system.

You original posting says the requirement is for a fall arrest system over a rail line. Thus I'm going to guess that the monorail beam as well as the support posts exist for no other purpose. In that case I would argue that this is a "fall arrest system" all the way to the ground, and there is no "anchor" that has to meet the 22 kN requirement in 11.6(4). I understand an anchor to be the connection from a fall arrest system to a part of the world that is primarily there for some other reason.

In that case the monorail and support posts fall completely within the rules in Z259.16. It will give you the load factor for the MAF load(s) for your CSA S16 structural design of the monorail and posts (which should be significantly lower than 22 kN per worker, which should keep your rail and posts sizes smaller). Z259.16 uses a lumping factor for multiple worker falls, but I suspect you'll have to use 2 as your system won't be flexible enough to use the Table 7.2 factor.

If you're using a monorail over a rail car I assume the workers will be connected by SRL's attached to trolleys. For the SRL's I'm familiar with the MAF will be only 4 kN per worker. In the post linked to by CANPRO you can see that a rail car should be high enough to arrest the fall for a worker on a partially extended SRL. Both the MAF and clearance would change if you are thinking of lanyards, but that would be odd (poor??) choice for an overhead monorail in my mind.

Depending on how long the monorail is the thermal expansion/contraction of the monorail may get to be the most significant load on your posts, unless you can incorporate some form of expansion joint.