horizontal lifeline - lanyard constant
horizontal lifeline - lanyard constant
(OP)
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!
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!






RE: horizontal lifeline - lanyard constant
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
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?
RE: horizontal lifeline - lanyard constant
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
RE: horizontal lifeline - lanyard constant
Thaidavid
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
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
RE: horizontal lifeline - lanyard constant
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!
RE: horizontal lifeline - lanyard constant
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
www.phoenix-engineer.com
RE: horizontal lifeline - lanyard constant
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
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).
RE: horizontal lifeline - lanyard constant
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.
RE: horizontal lifeline - lanyard constant
http://media.aisc.org/Files/EJ/EJQ22016.pdf
In which the author at least talks about energy methods and lanyards as energy absorbing devices.