Z359.6 load combinations for active fall protection systems

[Dave Evans]

Z359.6-2016 lists these load combinations for fall protection systems (simplified a bit for clarity):
(1) 1.4D <--ok
(2) 1.2D + 1.6A + 0.5L <--ok
(3) 1.2D + 1.1(A + L) <--huh? 1.1?
(4) 1.2D + 1.1A + 0.5L + (1.0W or 1.0E) <--WOW. Full W or E.

where A = the fall arrest force, and the others have the usual meaning.

(4) is not consistent with the ASCE 7-2022 combination for "extraordinary (low probability) events":
Eq 2.5-1: 1.2D + extraordinary load + 0.5L + 0.15S, which does not include ANY wind or seismic, much less the full ultimate values.

That's not quite apples-to-apples, but my sense is the joint probability of full ultimate W or E with even a somewhat reduced fall arrest force (1.1/1.6 = 69%) is vanishingly small. However, I do think it would make sense to include "some" wind load simultaneous with the full fall arrest force.

So, is anyone aware of the justification for including 1.0W and 1.0E in (4)? And what is the basis of the 1.1 load factor in (3) and (4)?

Presumably (hopefully) the justifications are "probability based" (using ASCE 7-2022 2.3.5 language), but where would one find the documentation? the calculations? It's not in the Z359.6 appendices. And which structural engineers were involved? (I've contacted ASSE but have not received a response, yet.)

 

[JStephen]

Maybe they're figuring that if you're climbing during a maximum wind or seismic event, that event is likely to cause a fall, so the events are not independent.

 

[Dave Evans]

Safe work procedures would not allow someone to work outdoors during a design-level wind event. And if a person is working indoors, wind would have no effect. And sure, maybe the design earthquake would cause a fall, but the odds of that and simultaneous peak effects have got to be minuscule.