Steel joist supporting a fall arrest system 1

[upjengnr04]

I have a client who want to use an existing 28LH07 steel joist as the tie off location for his fall arrest system. Although I personally would design a separate tie off, the client wants the joist analyzed for the 5000lb concentrated load. I realize there are methods to analyze a new concentrated load on the existing joist, but I really do not want to go thru the process if I can find a code reference to back me up.

My question to you.....Is there a code reference I can take to them to support my advice NOT to use the steel joist? I have not found one yet.

Thanks!

 

[msquared48]

How about the provision by the joist manufacturer that if the joist is loaded in any other way than what it was originally designed to take, the warranty is voided.

Mike McCann
MMC Engineering

 

[steellion]

It shouldn't take much work to show by analysis that a 5,000# load will probably overstress a joist that wasn't specifically designed for it.

 

[upjengnr04]

Just finished the calc. The analysis was easy, especially considering there is no lateral support to resist the 300lb mechanic swinging around after he falls. I decided to use common sense and basic engineering to back up my opinion.

 

[dhengr]

Upjengnr04:
Actually, the OSHA 5000lb. criteria is an ultimate strength criteria (it can fail at 5001lbs.), their requirement for an attachment point/fixture, for an unknown loading, direction of loading, type of load, etc.etc. If the loading is controlled and understood, and designed for by a registered engineer, you can get by with a lesser loading. Energy absorbing lanyards, distance of fall, etc. can be taken into account in the analysis, and made part of the system. I believe they do require a 250 or 300lb. man with 50lbs. of added equipment. You just have to dig through the OSHA requirements to dig this stuff out, and that’s not an easy matter either. Talk with your local OSHA office, they are instructed to be helpful, when you come to them with a reasonable question, during the design phase, and before the accident. Then, also, the local office interpretation is what you have to deal with first during an inspection or if anything does go wrong.

It’s a crazy thing these days that we don’t want to do anything (can’t do anything) which is not explicitly covered by a paragraph in a code or std. You say... “ I decided to use common sense and basic engineering to back up my opinion.” Can a falling guy really exert 5000lbs. of loading on an attachment point, only 16g, or is the 5000lb. requirement intended to make non-engineered attachments and fixtures unwise or difficult to do? There are a thousand questions unanswered in your OP...: is the guy working above or below the jst., length of fall? Is his attachment point to the top chord or bot. chord? Are these attachment points at a panel points on the jsts., or otherwise? As part of this system can they apply some temporary bracing or load distribution to several other jsts.? Is this system for erection of the jsts. or being used on a completed roof system? I’m not saying it is a simple half page calc. problem, but you are trying to help your client, aren’t you? And, certainly he should willing to pay you for your effort in helping him.

 

[upjengnr04]

dh: I appreciate your response. With more thought, I too questioned the 5000lbs, and after speaking with a colleague we recognized that additional info and analysis was required to properly evaluate.

I replied to the client indicating this and expressed more time is required for a proper analysis and am awaiting his reply.

 

[Archie264]

I once ran the numbers for a 200# man falling 6' and stopping over a distance of 6", the distance some fall arrest systems use, I think. In any case it worked out to something like 2,400#, which, if multiplied by a safety factor of 2 gets you close to the 5,000#. I *think* a safety officer had told me those criteria, which is why I used them in the first place. But it seems to me that might be the derivation of the 5,000# criteria.

So...if you could run a channel or angle or something between joists enough to share the load between two or three joists would that change matters? As dhengr pointed out, it's an ultimate strength issue, not a service one. You would have to make sure the load-sharing member wouldn't pull out or damage the joists. And check for rupture, tear-out, crippling, etc. And make sure your client knows about the warranty issue that msquared48 brought up.

But if there's a way to responsibly justify it and make it work you might earn yourself a loyal client.

Disclaimer: I am NOT a specialist in this field; take the above with a grain of salt.

 

[sundale]

You may use a 2,500 lb point load if the person has a backpack decelerator fall arrest device. Joist still hay not pencil out for that in shear or bending.

Welding in additional web members at the truss panel point and having cross bracing to force partitipation of multiple joists would not be bad detailing.

Would also have to think about the timing of the fall arrest usage. There is typically very little "fat" in a bar joist design; joist would have little reserve strength for a point load with transient snow/live load present.

 

[dhengr]

Upjengnr04:
From OSHA’s standpoint, they don’t want you killing people or seriously injuring them. They don’t care if you bend a couple bar jsts. saving a life in a fall. And this has to be figured into your design solution and discussed with the client, as to its likelihood of ever happening. Would his insurance cover something like this to save a life? These falls don’t happen every day, if the client has a good safety program, so he probably won’t be fixing/replacing a bunch of bent bar jsts. every month. How often does this system have to be moved, does it take an hour to move every half hour? Is it fixed, to service a piece of equip. once a week? Alternatively, you could prop a W14x730 up under the load point to protect the bar joist. Look at what OSHA has to say about steel erection, what is that industry doing? I’ve seen things as simple as some sort of posts/vert. elements clamped to the top flg. of a pl. girder, with wire rope attached to the top of the posts, and the worker clips-off to the wire rope and reclips at each post. I don’t speak for them, but I’d think OSHA would give an employer a couple points for designing a thoughtful safety system and training/controlling his workers to be safe. Members yielding significantly without really collapsing absorb a lot of energy, and that’s good. Alternatively, you see/hear about guys throwing a rope loop over a vent stack (plumbing stack) sticking up out of the roof and assuming the are o.k. You’ve really got to protect some of them from themselves.

 

[JLNJ]

I believe we are justified in using a service load of 1800#. This is the max force allowed to be exerted on person using a body harness. My opinion is that the 5000# is for the pieces parts of the system, not the whole of the resisting structure. Additionally, the 5000# is an ultimate, not service, load.

It is not crazy to think that it might be possible to get the joist to work. I would be liberal when combining an instantaneous fall restraint load with other live, dead, wind, and snow loads. You might also be able to spread the load out to multiple joists.

The structural engineer's job is to find a way to legally and safely make things work.

 

[Agent666]

In New Zealand and Australia we are required to design for 15kN (3370lbs) for a single person attachment, and 22kN (4945 lbs) for two people attachment for reference. There is no serviceability requirement, just an ultimate requirement. These loads are ultimate load factored loads. I will try dig out the table from our code as it's a really good summary of the various conditions, for example if there is no risk of a fall, such as limiting the length of any lanyards so the person can't even reach the edge then you can design the fixing and supporting structure for a smaller load.

 

[steellion]

We support tie-offs from joists all the time... when the joist was specifically designed for that load. Maybe you can modify the joist to get it to work, but the existing joist likely will not meet load requirements and likely would not pass a pull-test. IMO, the OP's original instinct is good. Let's not be too "liberal" with components critical to life safety without any inherent redundancy. The maintenance guy's life is just as valuable as the office worker underneath the roof.

 

[Agent666]

The summary I referenced is attached.

 

[Duwe6]

" They don’t care if you bend a couple bar joists saving a life in a fall." Exactly [and bar joists do fail 'benignly' i.e. they don't fracture.

"I believe we are justified in using a service load of 1800#." Or even less [but I wouldn't]. Just takes a little detailed design.

Will a 'positioner' - retractable lanyard - be part of the assy? If so, the fall will be << 12-inches.

Will an Energy Absorbing Device be part of the assy? If so, 150% - 400% of the rated energy is appropriate; i.e. device limits to 600#, so use 900 - 2400# for the design criteria - ONCE. This isn't a building that will stand for many decades. This is an emergency ststem that only has to work one time, then be 'renewed'. Try to imagine the design criteria for automobile air-bags that would work at least once weekly, for the life of the car ?!!?