Repelling/Fall arrets

[geoffdale]

Good afternoon-
We are reviewing an existing fire training structure that they want to teach repelling off of the side of the structure. Please see attached sketch.

The literature I am seeing states a fall arrest needs a 5000# anchorage load with limited deflection. From what I read, I see this as a vertical load.
From the sketch, I see this as a horizontal load for the most part on my new beam, which would transfer to an axial load into the existing beam. This creates a slew of other issues for us, as we now need to figure out how to deal with the horizontal loads at the masonry walls.

Does everyone agree that this structure will in fact be seeing horizontal loads? I don't see any other way, however the verbage for fall arrest is confusing.
Thanks!

 

[EngineeringEric]

I would confirm what the fire department requires for load rating. recently we did one that required a factor of two on all fall loads... A Factor on top of the existing factor of 3... so that 5k load turned into 10k in any one direction. Just a heads up and worth confirming not just basing on OSHA Fall Arrest 5K.

 

[XR250]

Seems like you have some eccentricity to deal with. Why not just attach to the roof slab?

 

[charliealphabravo]

The anchor point may also see uplift since they will likely be training with a rappelling tripod which is tied down to the anchor point.

 

[Archie264]

I could be wrong but I think the OSHA figure of 5,000# already has the factor of two embedded within it.

A while back out of curiosity I ran the numbers for stopping a 200# man that has fallen 6' from rest and stopping him over a distance of 6". If I did it right and my memory serves me correctly that came up with around 2,400# of force, which, doubled is around 4,800#, which rounds to 5,000#. And I think those input parameters are what OSHA requires. So the doubling is the FS of 2.

I understand that fall protection is a field unto itself so please don't take my word for it but, again, I think the 5 kip number includes the safety factor.

Per your sketch, yes there is a horizontal component but I would be surprised if it would cause a problem to the structure as shown. Perhaps you could anchor the new beam shown at the bottom of the sketch from being pulled out of the wall but I'm not seeing a falling man pulling down the roof or wall. Let the numbers tell the story, though.

 

[EngineeringEric]

Archie, In our case. since the dept. knew that the anchors would be used they wanted a factor of two ontop of the OSHA factor. You are correct that it includes a FS of 2 (my mistake on the 3). So the end result we were designing an anchor with a FS of 4 on the load and then a factor of safety of 1.5 on most attachments (at least) so the overall system should be 6x any true anticipated load. This was what the owner's safety person wanted, after numerous conversations and showing him the built-in factors in OSHA, I had to design for what they required at the time.

 

[XR250]

@ geoffdale;

There are no overall horizontal loads - only local ones. The tension in the rope will get reacted back thru the roof slab where it crosses over the corner

 

[CANPRO]

I believe the 5000 lbs already has already been factored. In my area the design of the fall arrest anchor must withstand 5000 lbs applied in any direction.

Majority of shock absorbing lanyards limit the arrest force to 900 lbs, giving you a factor of safety of 5.6.

If they're not wearing a shock absorber then the arrest force is much closer to the 5000 lbs as noted by Archie...but at this point you're not protecting the person who as fallen, you're protecting the people below from the body on the end of the line.

 

[Archie264]

Excel,

I'll certainly never fault any one for bumping up a safety factor on something that involves fall protection, particularly an owner, since he's footing the bill. And if it were me on the end of the line I'd like the additional measure of safety.

 

[TomDOT]

I would not start by assuming the largest, fully geared firefighter weighs only 200 lbs. Gear alone is about 70 lbs - if the firefighter doesn't have a hose or some other supplemental gear. 50 feet of empty hose typically weighs about 16 lbs (plus couplings, nozzle, etc) It took me maybe 20 seconds to Google up a firefighter who states he weighs 307 lbs when fully geared (no hose). I'm sure there are ones who weigh more.

Are you also certain that you would only ever see a single firefighter attached to the same anchorage at the same time? In construction, I have frequently seen multiple guys attached to the same anchor point in a manlift (for example)

 

[77JQX]

I think you have the forces pretty well handled, but I want to chime in as a recreational rock climber. Besides the strength and rigidity of the anchor point, there are numerous of suble points in anchor setup that can dramatically affect the safety and usability of the system. Entire books have been written on the subject. I recommend you consult with an expert.

The way the safety lines are passing over the lip of the building is a big red flag to climbers. We try to avoid that setup whenever possible because pulling over a sharp corner dramatically decreases the strength of the lines, and then the action of rappelling results in a sawing action over the edge through stretch of the safety lines and sideways movement of the rappeller. If this was my design, I would move the anchor point closer to the edge to reduce the angle the safety lines are deflected through, and install a piece of steel pipe on the lip to make sure there is no sharp edge the lines pass over.

If you'll have beginners doing the rappelling, then you'll also want the anchor to be at least head height, so the anchor can begin to take body weight before lowering begins. The lower the anchor is, the spookier the first step is. A well thought out anchor will also have attachement point for backup belayers, not just the rappeller.

That's just a couple of quick points I get from glancing at the proposed design. Again, this is a big subject, and subtle errors in rappelling anchors have killed experienced people. For instance, google "american death triangle" for an example of a common error. Rappelling anchor design is an area where long and serious study pays off. I recommend you consult with an expert.

 

[charliealphabravo]

Since there is no way to know/control how the anchor point will be loaded I think it is best to design the maximum load applied in any direction.

Also since the anchor point will be used to support a rope rescue system it will certainly be supporting two trainees.

In any event, your drawings should state that any attached fall arrest or travel restraint system is to be designed and be the responsibility of others. The drawings should require signage stating the rated capacity of your permanent anchor point. Any smaller print on the sign regarding allowed direction of load or maximum number and size of adults, would certainly be ignored/abused at some point and just attract liability.

In my opinion the anchor point as shown is well placed for anchoring a rescue tripod which is commonly used to provide overhead clearance above the edge of the roof.