Fall arrest anchors on wood trusses 2

[RontheRedneck]

I got a set of plans that had a note I've never seen before. It said that the truss manufacturer had to determine the loads from fall arrest anchors, and specify the attachment method for the anchors.

I have never seen any sort of published standards for this. Does any such thing exist? Have any of you dealt with the issue before?

 

[human909]

Absolutely there are plenty of published standards on fall arrest anchors. And plenty of guidelines or manufacturers advice on attachment methods.

But you really need to be more specific on what you are after and what your role is here.

 

[XR250]

Does not seem like this should fall (no pun intended) on the truss manufacturer. Should be the EOR's responsibility.

 

[phamENG]

human909 - Ron is designer for a truss manufacturer.

Ron - I agree with XR. The EOR should be coordinating the installation details and giving you the loads. I'd quote Chapter 2 of TPI-1 to the EOR showing him/her that it is their responsibility. Of course, if you've already accepted the job they could try to argue that it was on the drawings and you agreed, so you have to do it...while I agree there are cases where shifting some of that responsibility around may be warranted, this isn't one of them.

Did they at least tell you what fall arrest anchor they're installing?

 

[LittleInch]

Type in "fall arrest anchor" into the search on this site and forum and loads of answers arrive.

Many quote a 5000 lb load,e.g.

https://www.eng-tips.com/viewthread.cfm?qid=97370

Some seem to be lower.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Aesur]

This sounds like they are purely shifting the problem to the truss manufacturer. Fall arrest are typically 5000 lbs point load at anchor, specialty systems I believe can have less in their ICC reports based on testing. I'm guessing this is how the project went...
Architect: We need fall arrest attached to wood trusses
Engineer: that isn't possible
Architect: make it happen
Engineer: Ok, deferred to truss manufacturer

While I say it's not possible to attach to wood trusses, I'm sure it's possible; but likely to increases costs significantly to make the connection. Additionally, does every truss need this? Typically you need to be able to tie off at many locations, think a long cable running length of building or a rod etc.. so that you cannot fall more than a few feet at any location.

I recommend pushing back to the EOR, but expecting you may need to work with the EOR on how many plies the truss are and making sure the truss can handle the loading.

 

[BSVBD]

See OSHA 1926.502 attachment... (hopefully it worked... let me knoow if not)

+/- once every other year, I need to address a Fall Arrest "situation".

"ALL things considered" (in other words... to "Keep It Simple"... omitting more than we really need / want / decide to know or conclude)... to provide (more than reasonable) assurance of safety, provide the answer and get the job done...

Accompanied with an internet article, entitled "The Myth of the 5,000 lb. Anchor Point", in support of my own interpretion of OSHA 1926.502(d)(15) thru 1926.502(d)(16)(ii), I have "comfortably", with the aforementioned "assurance of safety", applied 1.8K to be the MAF = Maximum Arresting Force necessary to design to support.

FWIW... within the link by "LittleInch", the "theclimber" reply, although contrary to my derived / conclusive opinion, has doubled what OSHA (IMO) is directing. Despite "theclimber" stating being "familiar with OSHAs regs", was the "900 pounds" in OSHA 1926.502(d)(16)(i) overlooked? Am "I" treading on neglect thru my application of the "900 pounds"?

FWIW... within the link by "LittleInch", I respect and agree with the "stewbaby" reply of, "Rule of thumb is...would you put on a harness and jump after the design is complete?"

XR250... I agree... "Should be the EOR's responsibility".

human909... Although I agree there absolutely "... are plenty of published standards on fall arrest anchors. And plenty of guidelines or manufacturers advice on attachment methods", along with the overwhelming additional, open-to-interpretation internet publications, guidelines and advice, can you please provide a credible, respected and beneficial list of such for our use?

I encourage all to thoroughly review "OSHA 1910 Subpart I - Personal Protective Equipment" and "OSHA 1926 Subpart M - Fall Protection" as well as EVERYTHING else advised by Eng-Tips participants! It is topics such as this "Fall Protection" that really make Eng-Tips invaluable!

RontheRedneck and the rest of you... I wish you all well!

Thank you all!

Thank you!

 

[Celt83]

We are seeing the fall arrest anchors needed on almost all of our wood jobs now, we have a typical detail that shows anchorage of the pro-bel or sim tie off post to a minimum of two trusses with loading per truss indicated in the detail. An issue we commonly run into is the locations of the anchors are not determined until well into construction so the best we can do is provide the typical detail and note the coordination requirement for locations in both the roof truss shop drawings as well as the fall arrest anchor shops (if we even receive them).

 

[lexpatrie]

As the truss supplier, you need the loads from the EOR. This is improper delegation. I'd suspect the 5,000 is unrealistic to "spread" to multiple trusses, unless you have specific instructions from the EOR where they've vouched for their design being stiff enough to spread that load to multiple trusses.

Further, for these sort of forces, one would be well-advised to do some form of special inspection, via a third party (this is outside your control, however, from the sound of it).

The engineers who immediately start debating the load down from the maximum arresting force, if you ask me, are going outside the OSHA standard, while it may not fail and may save somebody from a fatal fall, the actual forces involved when they are underdesigned (my word) will probably damage the anchorage to the point it won't be safe for a re-use.

A wood truss does not typically want a 5,000 lb point load somehow wedged into it over a 4" x 4" area (face of the truss top chord).

You could potentially design for the 5,000 lb load on a truss top chord and then ship the truss back "early" to the EOR so they can design the actual guts of their connection. Cloud it and indicate "truss is designed for 5,000 lb reaction." Be quite clear that the EOR has to provide the connection TO the truss for that load. You could also indicate what duration of load was used for this element (I believe 2.0 for impact is not allowed for connections, but you aren't designing the connection, unless your truss plates somehow get involved), and the load case for the arresting force is another question. It would seem like it would most likely happen during "normal use" (i.e. minimal live load), or construction (so roof live load) and not during design snow events.... that might be wise to hold the EOR feet to the fire on the appropriate load cases. "This design is based on fall arrest force and roof live load applied simultaneously, and 1.6 (2.0?) duration of load, bottom chord live load has been omitted for this load case. EOR please confirm."

 

[ChorasDen]

Bit ancillary, but still relevant to the conversation, WIJMA (Wood I-Joist Manufacturing Association) has a white paper on this topic, see link below.

[URL unfurl="true"]https://i-joist.org/wp-content/uploads/2020/05/WIJMA-Fall-arrest-Anchorage-Guide-2020.pdf[/url]

Summary of the document recommends not to exceed 1.6 for LDF, determination of connection requirements from the induced moment couple will need to come from the project EOR, and that after engagement of the fall arrest system, removal of the the fall arrest system from service will be necessary until the underlying framing has been inspected and recertified for continued fall arrest use.

 

[Brad805]

Fall Arrest Anchors on a wood truss, wow. This type of problem is far from a simple note on a drawing. I have never seen a decent layout for these and they are afterthoughts as Aseur suggests.

 

[dvd]

Fall arrest systems on wood trusses also suggest roof penetrations. It seems like design for roofing materials and flashing is important in this. Having the truss designer come up with a loading and attachment does not seem particularly thorough.

 

[Rabbit12]

IMO this isn't Ron's problem. The EOR needs to define the location of the load and design the attachment. There are likely dozens of different fall arrest systems that could be used. If the EOR needs additional plys or a certain wood type to get the connection to work he needs to stipulate that.

 

[jayrod12]

Ron,

Is this a pitched roof, or flat?

I have some interesting stories about people trying to do wood framed fall arrest anchor supports and it failing testing. Long story short, we now detail a steel H-Frame that turns the moment at the base of the anchor into T/C couples on the adjacent framing. It's not great, but it gets the job done.

Pitched roof, never seen that before.

 

[BSVBD]

I believe, as as typical, we've wandered far beyond the OP intent and, also, answered the OP inquiry?

RontheRedneck... is that correct? Have we assisted in providing an answer for your situation?

jayrod12... what difference is the roof pitch? ARe you referring to construction application only?

The reason i ask, is because, EVERY Fall Arrest "situation" that "I" have addressed, was for elevated manufacturing platforms in existing buildings, for which I, then, had to reinforce the existing OWSJ (Open Web Steel Joists / aka Bar Joists) or in new, typically, steel structures with OWSJ, for which I could then incorporate the necessary Fall Protection loads into the new design.

It sounds like most replies on this post are referring to construction applications, for which I have never had an inquiry! Thankfully!

 

[EZBuilding]

Note that in addition to the lateral force of 5 kips, there is an associated moment resulting from the lateral force and the distance to the tie-off attachment point. Depending on the type of roofing assembly, this could be a significant component of the attachment and structure design.

I recall hearing about a Cold-Formed Steel Truss project that was very challenged in incorporating tie-off anchorage attachments - after truss fabrication. I concur with the other posters who have mentioned their concern about a reasonable solution being possible with a wood truss.

 

[RontheRedneck]

Dang - Never thought this subject would stir up so much of a response. To address some things that were said -


phamENG said: "Did they at least tell you what fall arrest anchor they're installing?"

No. Absolutely no info at all.


LittleInch - Thanks for posting that link. Very informative.

ChorasDen - The link you posted was also very helpful.


BSVBD - If OSHA requires a design load of 5,000# per person, that could be daunting. It's rare that one person works alone when framing a roof.



jayrod12 asked "Is this a pitched roof, or flat?"

Sloping flat. (1/4" per foot)



BSVBD asked "Have we assisted in providing an answer for your situation?"

There's certainly a lot of information here. But it raises as many questions as it answers.



EZBuilding said "I concur with the other posters who have mentioned their concern about a reasonable solution being possible with a wood truss."

I'm kind of thinking the same thing.

Such as - When does the load occur? What if the trusses are in the process of being set, and they're pulled sideways? (out of plane) No way can a truss be designed for that.

How many locations do you need attachment points at? If I designed a girder truss capable of handing a 10,000# shock load (Assuming 2 people attached) it might cost over $1,000. Throw 3 or 4 of them in a house and you're adding a hell of a lot of money. No one will want to pay for that.


BTW - When I ran across this, I was only quoting the job. In the quote I noted that we could not design fall anchors, their attachments, or loads. That may cover us, or it may not.

 

[jerseyshore]

This has come up on a lot of the apartment buildings we've done the last 5 years or so. Washing windows on a 5 or 6 story building is a challenge from the ground on a lot of the buildings we've done so they've added these anchors to the roof to make it easier.

5 kips in any direction is typical.

This is how we've done it:

[ul]
[li]Arch & owner come up with preliminary anchor layout on the roof plan.[/li]
[li]They supply us (EOR) with product they want to use. Manufacturer of product gave us the loads (typically 5k in any direction).[/li]
[li]We (EOR) are responsible for designing blocking/ reinforcing roof around anchor locations. Actual anchor product is by others.[/li]
[li]On the first job we sketched up a nice load diagram for truss manufacturer to design from. Truss manufacturer suggested some layout changes if the anchors fell in the middle of long span trusses or created connection/ deflection issues.[/li]
[/ul]