HLL problem 1

[JaouadS]

Hey everyone, I'm trying to design an horizontal lifeline with Robot Structural Analysis and I have a lot of shadow zones of understanding:
1) When falling,the energy absorber stops the fall and the dynamic load applied to my cable is equivalent to the static maximal arrest force? (MAF=Mg(1+(1+2*K*f/Mg)^(1/2))
2) My lifeline is expected for multi tie-off use (7 users, is there any simultaneity falls studies on the matter (something like the diminishing return on economics) or should I design it for 7*MAF (I wish I won't have to do so)
3)How would you model the wind effect on a structure like this? (almost linear and no surfaces; a prestressed cable connected to two columns)
4) I guess that the effect of wind on the 7 people is something to take account of, even more than on the stucture itslef my intuition says, should I take some equivalent surface for each person and calculate the wind load on it?
5) Someone has any ideas of how to simulate the resonance that would arrive due to wind or parasismic effects?
Thank you advanced

 

[CANPRO]

1) You should check local Occupation and Health and Safety regulations and also National Standards (depending where you are). You likely have a specified arrest force you need to design for. There is a provision in the Canadian Standards which allows you to design for 2x the MAF...the MAF is often given on the specific fall arrest equipment. In my experience this number is typically 900 lbs.
2) I think this depends on the usage. If all 7 people are standing on a single platform that could potentially fail, then I would feel it necessary to design for 7 simultaneous falls. If they're working from a fixed surface (like an existing roof) then I would have to take a closer look. My initial thought on this would be to design for at least 2 falls, and possibly for 3. As you have indicated, the MAF is a dynamic load which will reduce to the self weight of the worker once the fall is over...with that said, I would probably design for a scenario where all but 2-3 workers have fallen and the remaining 2-3 fall and produce a dynamic load.
3) Assuming this isn't being setup and used in a hurricane, I think your wind loads are going to be insignificant compared to you MAF loads.
4) I'm not sure why this is a concern for you. Are you concerned about how the cable will behave when not in use? If you feel this is a necessary check, I suggest waiting until you have designed the cable and its anchors for 7 workers...the forces involved are going to be large.

You may want to consider running more than one cable. I'm not sure what your spans are like or your exact usage, but you may find this difficult to design for 7 workers. I hope this helps.

 

[JaouadS]

Hey CANPRO, thank you for your reply,
1) I think that for us, MAF= 6 kN as specified by EN 355, the security factor is 2 aswell, so I think that the combination induced by the dynamic load + wind's load + self+weight should be <= the resistance of the cable/2, isn't designing for 2 MAF a neglection of wind's effect and the cable's self-weight? (I can understand it for the self weight, but a wind of 60 km/h velocity can increase the cable's diameter, no?)
2,3 & 4) Well, the lifeline should stand in a coastal area and for a wind's velocity equal to 60 km/h, the structure already has gaurd-rails, so I don't expect an hazardous fall to occur, I was willing to design the HLL for a fall caused by wind's effect on the users. I guess I was wrong and that it doesn't matter that the wind causes the fall, the result of it will be MAF anyways, right?

Concerning the lenght, it's about 20 meters, I'm willing to put an intermediate anchorage so as to have two ten-meter spans. I guess that running more than one cable would reduce the diameter, but it will change nothing for the anchores, unless you're suggesting a two HLL solution (2 cables+ 6 anchors) or more!

Thank you again!

 

[jayrod12]

If there are guardrails why the HLL?

I wouldn't be designing for wind, what's the area of a cable (hint, it's next to nothing).

I also couldn't see the requirement for designing for all 7 falling at the same time unless as canpro said they were all working on the same platform that could give way and cause all 7 to fall simultaneously, but then I'd also be worried about them hitting each other during the falls.

I was always told that the MAF of 5kN (6 in your case apparently) was the ultimate load, i.e. check it against the strength directly with no further reductions.