horizontal lifeline - lanyard constant 1

[nongamer12]

I am currently designing a horizontal lifeline (HLL). Engineering Journal (2nd quarter, 2016) recently presented a paper by Thomas S. Dranger called "Design of Horizontal Life Lines in Personal Fall Arrest Systems". I'm comparing my previous HLL designs with this paper and am looking to include a shock absorbing lanyard into the design of the system.

My question is this: what's the spring constant for the lanyard?

Lanyard Information:
capacity: 310 lb (OSHA)
max free-fall: 6 ft
energy absorber: activation > 450 lb
max arrest force: 900 lb
max deceleration distance: 40 inches

If I simply divide the 900 lb by the 40", the constant I get is 0.27 kips/ft.

I've been searching the threads for all sorts of terms, but I can't find the verification I'm looking for. The following thread provided a lot of good information, including denial's spreadsheet, which I have downloaded (thanks!), but have some questions on. but that is not the point of this thread.

http://www.eng-tips.com/viewthread.cfm?qid=393894

Could you all provide me some help? Thanks!

 

[JStephen]

I didn't think there was a spring constant associated with that. I was thinking those relied on a rip-the-stitches effect, not an elastic rebound.

 

[Teguci]

I suggest starting with the book from Nigel Ellis "Introduction to Fall Protection."

Do not try to solve this problem dynamically. Draw a box around the dynamic part a place a question mark on it (not linear, close to infinite variables, not a structural engineer's forte, etc...). You are given the MAF, use it. That is the force you need to design for unless you want to consider more than one person on the system at once (you probably should for most situations in which case I refer you back to the book in order to determine the P design force).

Your first step is to decide on geometry. Anchor spacing (ex. column spacing), initial cable height (ex. 6 ft) and desired sag (ex. 18").

Draw your FBD showing the deflected shape under no load and then under final load. Solve for your reactions and check your cable strength. Check to make sure the worker hasn't hit the floor.

If you need more help, post your FBD's and we'll go from there.

 

[nongamer12]

i'm looking to design the system by balancing the energy equations.

jstephen: yes, it's a stitch-rip system, but i'm just looking to ultimately include the energy the shock absorbing lanyard puts into the system, so i was idealizing the lanyard as a spring as it elongates.

teguci: using a static FBD is how i've done them in the past. i'm just attempting to utilize a more accurate approach.

posts: 25 ft o.c. (max of 3 spans, between anchorage locations)
cable height: 6'-6" tall
initial sag: 12 in
free-fall: 6 ft
lanyard: see original post.

does anyone have any experience in the lanyard portion of it?

 

[EngineeringEric]

Can you just take it in reverse and take those maximum forces which gives you an elongation of the lanyard over a time period....?

 

[theonlynamenottaken]

As JStephen responded most of the lanyards are sacrificial stitch-tearing in design. Shock-absorbing lanyards don't have to have stitch-tearing based designs though. Regardless, would a spring be a good way to simulate a shock-absorbing lanyard? It seems that it may behave something like a bilinear material model where for your elongation-vs-force graph you have a vertical discontinuity line (initial opening force), then either a flat line or a gently sloping line that represents a fairly uniform opening force during the lanyard elongation.

I don't have any specific knowledge of lanyard elongation behavior, though I have watched a few tests where weights were used to test them. That behavior just seems more appropriate than a linear, translational spring.

 

[Teguci]

Journal referenced by OP -

http://media.aisc.org/Files/EJ/EJQ22016.pdf

Dranger is cutting the energy absorption of the lanyard out of the equation with the assumption "The spring constant of lanyards is large enough that strain energy within a lanyard can be neglected." This is conservative, but, considering that the force on the harness (which equals the force on the cable) is not allowed to exceed the MAF, arguably excessive. I assume the reason for cutting the lanyard out of the equation is due to the variability of the different systems under different loadings and conditions. Maybe someone could introduce a dynamic response coefficient for each lanyard similar to our seismic lateral systems. But, until then, I'd just stick to statics. Using just the distance of 40" and assuming a constant reaction force of 900 lbs gets you 3,000 ft-lbs of energy absorption. We know already though that the reaction force is not constant at 900 lbs (it initiates at 450 lbs and probably reduces after initiation).


Another thing. Lowest point on your system will be 5'-6". Freefall will be L-lanyard - (H-system - H-Dring). Once the lanyard hits, the fall is no longer free.

 

[BUGGAR]

Maybe this is sort of relevant: I'm designing a vehicle that will hit a bump at speed. The energy will be absorbed by both the shock and the spring. My spring/shock travel distance is fixed and my allowable force is set by vehicle component strength. I have to spread my energy absorption between the spring and the shock. For a given force, my shock ideally absorbs almost twice as much energy as my spring, so I have to apportion them appropriately. Is this anything like your situation?

 

[thaidavid40]

Why is the design not being based on the simple maximum (unfactored) manufacturer's live load of 900 pounds? I take this load, multiply it by a load factor of 2.0, and apply that against the factored cable ultimate capacity in tension (which governs).

Thaidavid

 

[Lomarandil]

Thaidavid, for horizontal lifelines, the geometry of the lifeline cable amplifies the cable and anchorage loads significantly beyond 2*MAF.

The quickest way to visualize this is drawing a static FBD with a worker suspended from the cable at your design sag.

For anything that maintains OSHA's maximum free fall distance of 6 feet, the horizontal cable angle is pretty shallow, so it takes a lot of tension along the length of that cable to resist the vertical MAF.

 

[thaidavid40]

@Lomarandil,
I never said that the anchor loads are equal to 2*MAF. I said that was the load effect to be applied to the cable. Statics then takes over from there. Please reread my post again for clarification, and kindly don't put words in my mouth which I didn't say.
Thanks,
Dave

Thaidavid

 

[Lomarandil]

Glad to hear we're on the same page. I was worried when you said you apply 2*MAF against the cable ultimate capacity -- it wasn't clear to me whether you were also accounting for the geometry.

And the truth is I'm a stickler about horizontal lifelines, because of how many I've seen designed improperly (or not at all) over the years -- for a direct life safety application at that!

 

[zcp]

You have the max arrest force. Use that to design the system with FS>2.

How many people on the line at a time? Use Nigel if you have more that one.

Reminder to check for swing clearance

.... Took a look through the paper, seems like the conclusion is an energy absorber is needed somewhere. The lanyard, the end of the cable, or the support. Otherwise the user's spine is in for a bad day.

We often put energy absorbers at the end of each line (which may go over multiple stanchions). Then they can have one on their lanyard also, covered either way.

ZCP

http://www.phoenix-engineer.com

 

[jayrod12]

Thaler has lifeline systems that have energy absorbers included, I believe it is rated for 2 workers, and the design load at the anchor points is 5500lbs I think.

 

[nongamer12]

this thread has gotten a bit off topic from my original posted question. thanks for all input so far, but i'll try to get it back on track yet again:

if one has experience accounting for a shock-absorbing lanyard on a cable horizontal lifeline design utilizing an energy approach, please post some guidance based on the information provided:

posts: 25 ft o.c. (max of 3 spans, between anchorage locations)
cable height: 6'-6" tall
initial sag: 12 in
free-fall: 6 ft

Lanyard Information:
capacity: 310 lb (OSHA)
max free-fall: 6 ft
energy absorber: activation > 450 lb
max arrest force: 900 lb
max deceleration distance: 40 inches

lomarandil, if you've seen many of these designed incorrectly, have you seen shock absorbing lanyards accounted for in the design?

currently, i've designed it using a static final sag conditions that satisfy cable and MAF forces, and then back-calculating initial sag conditions prior to cable elongation.

 

[Lomarandil]

Negative, as Teguci has been saying the variability of each shock absorbing lanyard is too significant.

What you describe in the last sentence (also checking the resultant design against OSHA requirements, not just the structural criteria) is what I've been doing, and is the best practice I'm aware of.

 

[Teguci]

Ah! Much better question and you have all the information you need to solve it.

The energy absorbed by a fully failed lanyard must be (at least) equal to the potential energy of the initial mass calculated at an initial height relative to the final resting height (6 ft free fall + 40 inches).

PE = m g h; PE-0 = (310 lbs/g) slugs x g x (6 ft + 40/12 ft) = 2,900 ftxlbs, PE-f = 0. Work performed against the system must be 2,900 ftxlbs. For a rigid tie-off, about 100% of this work must come from the lanyard which is what they are rated for. (As a side note, this averages 870 lbs which tells us the lanyard force must stay pretty flat near the MAF of 900 lbs - still not a spring though and shouldn't be assumed constant)

For an HLL, we could certainly absorb some of this energy in the stretching of the cable and any elastic action available at the supports (note - this energy absorption is from elastic action and will rebound from the MAF back to the final weight). But, in the end, the design of the supports and cable will still statically be designed for the MAF with any excess energy being dumped into the shock absorber.

Now, if I had to rate a system for a 350 lb person, I might be more interested in an energy equation. For this weight, the PE-0 = 3,300 ftxlbs and a rated lanyard will only absorb 2,900 ftxlbs. In the world of numbers, I need an HLL system that can absorb the extra 400 ftxlbs of energy. However, in the world of lawyers, I wouldn't entertain the idea any further without a substantial fee and field testing (or just go right to a self retracting system that limits falls to 18 inches).

 

[Denial]

NonGamer12

Your above problem is probably history by now, but you might be interested to know that I have just released a new version of my spreadsheet.[ ] The main change from the version that you would have downloaded a month ago is that I have attempted to include energy absorbers in the dynamic analysis.[ ] You will find it on my website.

 

[Lomarandil]

Since this discussion, I also found this article in my stack of "I'm going to get around to reading these someday"

http://media.aisc.org/Files/EJ/EJQ22016.pdf

In which the author at least talks about energy methods and lanyards as energy absorbing devices.