Fall Protection with Wind Loads

[amd13]

I am designing a fall protection system in an outdoor truck unloading area. The fall protection rail system is supported by an open structure with a roof.

I have applied dead load, wind loads per ASCE7-10 and the fall loads. What factored load combinations are used in this scenario? I have considered the fall load as a live load, but it seems unrealistic that the fall protection system will be used during hurricane force winds. Any ideas?

 

[MotorCity]

Loads due to fall restraint are usually considered as extreme events (accidental). As such, they are not combined with wind or seismic loads due to the low probability of someone falling at the exact moment of the design wind or seismic event.

 

[amd13]

Just for my knowledge, is this a generally accepted rule or is this written someplace? I recently read a section in ANSI Z359.6 about the determination of factored load effects but I did not fully understand the code.

Thanks for your help!

 

[MotorCity]

ASCE 7-05 allows some latitude by allowing the engineer to gain the approval of the AHJ. Most AHJ's I have worked with are simply plan reviewers and are not well versed in technical or design issues, so convincing them of your methodology is usually not a problem.

If you accept that a fall restraint load is an extreme event, take a look at the ASCE 7-05 commentary equations C2.5-3 and C2.5-4. These equations do not include any allowance for seismic loads, but they do include 0.2W (which is usually small enough to neglect if the AHJ agrees), 0.5L, and 0.2S in LRFD load combinations.

 

[thaidavid40]

If you are using LRFD analysis procedures, remember to use 2.0 as the live load factor on the pseudo-static fall load event(s), per OSHA, not 1.6 as for normal live loads. However, I would never use less than 2.0 for this in ANY load combination, as there is no justification for a partial fall event load - you either have a full load, or you don't - just like being 100% pregnant. And you would never have a person attached to a lifeline in a high-wind event anyway, so that load combination cannot exist.
Dave

Thaidavid

 

[gwynn]

I agree with Dave, in that you would not have a worker there in a high wind event. Even on the rare chance you did, the peak load from a fall arrest system is an impact load and the peak load from a wind storm is a three second gust. To combine the two is being overly conservative given the chances of the two ever overlapping. And yes, single worker falling is a 2.0 load factor.

As an aside, is there different wording in the area you are? Fall protection can be many things, but what you are describing to me is a fall arrest system. From what it sounds like, you have an overhead rail with workers tied off underneath it with vertical lines. That part is good to hear. Just make sure you also have enough clear distance for the shock pack to fully extend.

 

[amd13]

Hi Gwynn,

Yes, this is a fall arrest system. It is what you described - an overhead rail with workers being tied off with vertical lines. I have checked the fall clearances and the fall envelope checks out.

This question came about since I am trying to reuse foundations and anchor bolt patterns from an existing out of date system. I was having trouble getting the anchor bolt pattern to work out due to concrete breakout per ACI 318 Appendix D. I was trying to eliminate some of the conservative load cases to see if I could get the existing foundation to work.

Thanks for all of your help!