Fall Arrest Anchor Support Framing

[drewpe]

I am looking at providing fall arrest anchor supports for new fall arrest anchor systems going into an existing steel building will steel roof joists. The new fall arrest anchor system is designed for an ultimate load of 1,800 lbs (900 lbs w/ safety factor of 2) and they are looking to support it off the bottom of the existing joists. My question is a two part question:

1) Is it reasonable to reduce the load by a factor of 1.6 to account for the fact that I am checking the joists for allowable loads?
2) Is there an acceptable over-stressed limit that these joists can be under to account for the fact that these are impact loads and not sustained loads?

Thanks in advanced.

 

[Lomarandil]

1). When designed by a qualified design professional (presumably you), it is reasonable to check the resultant ultimate anchorage load against the nominal capacity Rn (either against an LRFD capacity, where the load has already been "factored", or compare against the allowable stress capacity without a factor of safety).

2). No, when checked as above, the joists should not be over-stressed. You can look at what failure mode is happening to see if that gives you some leeway (maybe a ductile mode like flexural LTB doesn't constitute "failure" if it doesn't cause problems with your fall distance), but (timber aside), you can't and shouldn't try to "outrun a failure" just because it's not sustained. Not unless you're running a full dynamic and energy-based analysis. And on fall protection fees, you aren't going to do that.

Fall protection is literally life-safety stuff. Leading cause of death in construction. And with most contractors, systems end up getting abused. Don't go looking to cut corners from the start.

----
just call me Lo.

 

[Geoff13]

If this is a permanent anchor point then I wouldn't design for 1,800 lb. There's a chance that over the lifetime of the anchor point another fall protection system could be attached to it. Even today there are personal energy absorbers (often referred to as 12' free fall absorbers) that have a MAF of 1,800 lb, and thus 2xMAF would be 3,600 lb. If it's temporary and I felt I had sufficient control over the use I would design for 1,800 lb and have lots of warnings and controls in the User's Manual.

I would suggest reading ANSI Z359.6 (Design Requirements for Fall Protection Systems) Chapter 5. It includes load combination factors when combining fall arrest loads with other loads (live, wind, etc).

 

[miningman]

There are numerous postings on this subject within ET. Unfortunately , different regulatory bodies list different acceptable loadings. I believe in the USA.. OSHA presently insists on 5000 lbs. In Canada, we do not use OSHA standard s but we also design to 5000# As Lommadil alludes to , this is real life and death stuff. Do you really want to be on the wrong end of a coroners inspection???

 

[ldeem]

I have done several of these and I second earlier posters that you don't want to cut corners. It is an infrequent event but failure consequences are very high.

Some points to consider.

Check for yourself the anchor loads recommended by the fall arrest system vendor. Don't accept the interpretation from anyone else.

Life lines need to be installed with some slack, this almost never happens in the field. I always call out on drawing to install per manufacturers instructions and make special note about required slack.

Anchor loads must be applied in all directions. You are probably safe to ignore a load that would put the life line in compression if you can be sure the configuration would prevent such a load. So don't forget lateral loads in every direction which could be the case if the person falls and they are next to the anchor.

You mentioned checks in joists - if attaching to joist bottom chord you will probably need to look at the lateral load on the bottom chord. I am not sure how well that would work unless you are on a bottom chord bracing line.