Fall Arrest Load Cases 1

[JohnnnyBoy]

I am looking into a fall arrest anchor design and I could not find any documents speaking to load cases for fall arrest. Lets say under 1.0DL + 1.5 LL + 0.4 WL a roof truss is at 95% utilization. Given the additional 5000lbs arrest load added to the load case the truss would be overstressed and fail. Now the chances of a full live load, dead load and partial wind load being perfectly timed with a fall is extremely unlikely. Does anyone have any documents or experience with what load cases should be used for an existing building?

 

[CrabbyT]

Building on pham's post

So would you apply these loads as extreme events? Or concurrently with other live/wind/dead considerations?

If it's only during construction/maintenance, and as long as you don't anticipate that someone will be utilizing a fall arrest system in a 115 mph wind (or whatever your wind speed is), I would use the load combinations specified in the latest edition of ASCE 37 (Design Loads During Construction).

Personally, I would probably look at the 1.2DL + 1.6LL case and call it a day (there's always fringe scenarios, I'd consider other loads if required).

For example, if the anchor point exists in an industrial setting, for the live load I would include whatever applicable area live load exists in that area (e.g. if you're in a heavy manufacturing setting, I'd model a 250 psf unfactored live load + 3600 lb unfactored concentrated live load).

 

[Greenalleycat]

We deal with a lot of fall arrest here in NZ, working under our national code and ISO 22whatever it is (22846?)

Some thoughts as to why I wouldn't get too worked up:

1) Stuff is generally oversized for gravity due to lateral considerations (wind or EQ here) so the extra loading from fall arrest is often not going to govern anything as long as your fixing is adequate
2) In the case of stuff that is gravity dominated (e.g. roof trusses) I don't agree that you have a realistic chance of overload concurrent with fall arrest loads (1.2G + 1.5Q along a full truss at the same time someone falls off?)
3) Even if you do, the fall arrest loads generally manifest as shears in the plane of the roof more so than tension, and certainly unlikely to be a downward load to actually act concurrent wit your gravity load, so they're not really going to be problematic - they're resisted by different mechanisms
4) The design load under ISO is 15kN but in reality the harnesses/dissipators/ropes etc they use limit loads to 4-6kN at the support - so the actual load is much smaller which is a great 'sleep at night' factor
5) There are huge factors of safety inbuilt into our design - material 5% strengths, overstrengths ignored, beneficial fixities, strength reduction factors etc - quantify any of these and you will easily get the capacity you need to resist the fall arrest loading under most situations. This seems appropriate to me given the absurdity of the load case we are considering, which is effectively 1.2G + 1.5Q + 2.5F (where F is the baseline code-required fall arrest live load of 6kN and 2.5 is the code-required load combination factor to give 15kN)
6) The fall arrest max load only exists for a tiny period of time (and even then, actually only exists as a small fraction of the design load) so a static analysis can cloud the real story

 

[human909]

Excellent summary Greenalleycat. As an engineer across the Tasman from you I'm in full agreement. While I have limited direct fall arrest experience in engineering as a climber I've dealt with it often, different requirements but the physics is the same. The only time I've directly dealt with engineering fall arrest was an addition to the floor of the item shown above, the documentation specified a moment requirement of 21kNm which was very onerous on a relatively small connection. But I followed the requirement for the fixing. But it didn't extend that load into other load combinations of the entire floor.

Fall arrest and lifeline anchorages.
In addition to any other applicable live loads, fall arrest and lifeline anchorages and structural elements that support these anchorages shall be esigned for a live load of not less than 3,100 pounds (13.8 kN) for each attached lifeline, in every direction that a fall arrest load can be applied.

That is an absurd code requirement. The notion that fall arrest loads should bit directly additive is crazy. As Greenalleycat said the peak load only exists for a tiny fraction of a time. If you have 6 fall arrest points along one long truss there is no way in hell I'd consider adding them together.

 

[Greenalleycat]

Agree with you, even though I am currently in a stage of grief against your country following the cricket last night...

The clauses that we work under give diminishing loads for each additional user e.g. 1 user is 15kN but if you have 2 (say, on a rail system) the load requirement is 21kN

 

[RontheRedneck]

IMHO this is a solution looking for a problem.

If workers are on a roof, there's no way the trusses will be at full design load.

The diaphragm action of the plywood would help distribute the shock load.

Given the duration of the shock load, the stress increase would be 1.6 or more.

And to the best of my knowledge, this has never been a problem. So I have no reason to be concerned about it.