HELP!!! Forces in cable for safety rigging
HELP!!! Forces in cable for safety rigging
(OP)
I have been asked by a client to analyze installing a 1/2" steel cable that spans 40 feet to existing roof beams. Before I analyze the roof beams, I need to know the lateral force caused by the cable. The client asked if the cable could sag 2". I take a 250# person falling at midpoint of cable (multiply by 2 for impact for total of 500#) and neglect weight of cable. I sum moments at support to find downward reaction (naturally half of total) and then sum moments at center of cable to determine lateral reaction at support and I get 29.4 kips? So your telling me if a 500# load is hung from a cable that is only sagging 2" the lateral force at the ends is 29.4 kips??? If I let cable sag 24" the load becomes more manageable. Why is this? Does anyone know? I have read up a little on the catenary curve but I still cant grasp how 500# turns to over 29000#.
Calcs:
Cable span = 40 feet, sag is 2" (.17'). Supports are "A" and "B" and midpoint of cable is "C"
Sum moments at "B" = Ax(0 FT)-Ay(40 FT) + 0.5 kips(20 FT) => Ay = 0.25K
Sum moments at "C" = -Ax(.17 FT) - Ay(20 FT) + 0.5 kips(0 FT) => 0.17Ax = -Ay (20 FT)
Substitute 0.25 kips for Ay, Ax = -0.25 kips(20 FT) / 0.17 FT = 29.4 kips
If cable sage 2 FT, force is only 2.5 kips
Calcs:
Cable span = 40 feet, sag is 2" (.17'). Supports are "A" and "B" and midpoint of cable is "C"
Sum moments at "B" = Ax(0 FT)-Ay(40 FT) + 0.5 kips(20 FT) => Ay = 0.25K
Sum moments at "C" = -Ax(.17 FT) - Ay(20 FT) + 0.5 kips(0 FT) => 0.17Ax = -Ay (20 FT)
Substitute 0.25 kips for Ay, Ax = -0.25 kips(20 FT) / 0.17 FT = 29.4 kips
If cable sage 2 FT, force is only 2.5 kips






RE: HELP!!! Forces in cable for safety rigging
Regards,
Mike
RE: HELP!!! Forces in cable for safety rigging
BA
RE: HELP!!! Forces in cable for safety rigging
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: HELP!!! Forces in cable for safety rigging
I have a spreadsheet that calculates the forces and deflections in a cable under the actions the cable's self weight plus a point load anywhere along the cable. It allows for the extensibility of the cable, which might be important in your case because your cable seems quite taut. You might well find it useful, given the way you state you intend to solve the problem.
Recent changes in my employment circumstances mean that I have recently decided to make this spreadsheet generally available. You can download it from the "downloads" page under
http://rmniall.com
Your particular problem is a dynamic one, but perhaps not really an impact one. The factor of 2 that you propose using is, strictly speaking, applicable only to problems of a "suddenly applied" load. It might not be appropriate here.
My initial thoughts on your problem are that it is an energy balancing exercise. Your man falls a known distance, thereby acquiring a known amount of kinetic energy. His fall is then "gradually" arrested by the cable. In this process his kinetic energy is absorbed by the cable and manifests itself as strain energy in the cable. Because of its initial tension, the cable already has some strain energy before the man hits it. How much more can it absorb before it yields? Before it breaks?
If the roof beams can move laterally as the tension in the cable increases, that will help you by being another absorber of the man's kinetic energy. And if the harness that the man is wearing (??) has an ability to stretch under load, it will be another potential absorber.
RE: HELP!!! Forces in cable for safety rigging
SnTMan said it well - draw the cable to scale. The lengths of the sides of the triangle are proportional to the forces. So if the vertical distance is 2 units and the diagonal distance is 20 units then the force in the cable is 10x the vertical load. Look at the extremes - if the cable were vertical it would have 1x the load in it, if it were horizontal it would have infinite load. Sag is necessary to prevent over-stressing the cable and the anchorage. In your case the horizontal is 240 inches and the vertical is 2 inches. Our Greek friend Pythagorea tells us that the diagonal is 240.008 inches. The ratio is therefore 2 to 240. If the load is 200 pounds then the cable force is 24000 pounds. For a center load, that is.
RE: HELP!!! Forces in cable for safety rigging
2":240" = 500/2 lbs:30,000 lbs
RE: HELP!!! Forces in cable for safety rigging
RE: HELP!!! Forces in cable for safety rigging
Don't miss what IFRs pointed out. If your project is in the US, and if it's intended to stop a falling person, per OSHA it has to risist 5,000#, not 500#.
RE: HELP!!! Forces in cable for safety rigging
OSHA 29 CFR Appendix C (c)(10) allows for a factor of safety of 2, if designed, installed, and used under the supervision of a qualified person. I think that gtg477f is trying to do that, but will need to apply the factor of safety to the maximum load including impact. [note: impact load is defined in paragraph (d)(1)(iv)]. I believe it was specifically intended for cases such as this where the 5,000 lb criteria would cause problems.
RE: HELP!!! Forces in cable for safety rigging
Using those equations that gave me so much trouble in calculus, physics, diff. eq., and dynamics...
For an object falling from rest...
d^2x/dt^2 = g
dx/dt = gt
x = .5gt^2
...and...
kenetic energy = .5mv^2
Work = Force x distance
...so, using imperial units...
An object falling 6' falls for
t = sqrt(2x/g) = sqrt [(2)(6)/32.2] = .610468 seconds
...which yeilds a velocity of...
v = dx/dt = gt = (32.2)(.610468) = 19.657 ft/sec = 13.40 m/hr
...a 200# man that has fallen this distance has generated...
E = .5mv^2 = (.5)(200/32.2)(19.657^2) = 1,200 ft-#
...setting energy equal to the work done on the system by stoping this in 6" yields a force of...
F = E/d = 1,200/.5 = 2,400#
Using a factor of safety of 2 gets you to 4,800#, which appears to have been rounded to 5,000# for the regulation.
These calculations are probably easier to do in metric, and our mechanical brethern can correct whatever mistakes are made above, but that's my understanding of where OSHA's requirement came from. Regardless, the point remains: the anchorage required for fall arrest in the US is legally required to resist 5,000#. So my suggestion is to proceed with that in mind.
RE: HELP!!! Forces in cable for safety rigging
Whoops, it looks like our posts crossed in the mail. It sounds like you have a handle on OSHA specific requirments. I was listing what I understood them to be. I don't have their requirements in front of me and I could be wrong.
RE: HELP!!! Forces in cable for safety rigging
it's tied to the requirement to keep the 40' for all intents straight, so the component of cable tension reacting the applied load is very small.
and you've got to deal with 10x the load !
RE: HELP!!! Forces in cable for safety rigging
I think what the OP doesn't understand is ,that a 40' 1/2" cable with 2" of sag is ALREADY a highly tensioned cable...
RE: HELP!!! Forces in cable for safety rigging
1. Tighten the winch on your jeep to a tree - you can then use your little finger to move the truck
2. When your mom told you to quite fooling around with the clothes line becasue your weight would pull the poles inward.
Both good examples of what everyone is trying to tell you!!
RE: HELP!!! Forces in cable for safety rigging
RE: HELP!!! Forces in cable for safety rigging
BA
RE: HELP!!! Forces in cable for safety rigging
I believe you can use the principles of cable barrier design to get what you're after:
http://pti.mountaininternet.com/reportFiles/28-Tec...
My final suggestion would be to not make your beam the final anchor point. Carry the cable past the beam and perhaps downward, and anchor to something else that can take the tensile force in the cable, and your beam sees only a vertical load.
RE: HELP!!! Forces in cable for safety rigging
What do you think takes the horiz. reaction from the cable at the beam, where the cable now makes a 90° turn to an anchor point below and adds a new vert. load to the beam?
RE: HELP!!! Forces in cable for safety rigging
RE: HELP!!! Forces in cable for safety rigging
Suggestion, take this to some of the fall protection vendors (DBI / Miller / etc.) and see if you can get a standard HLL system w/ absorber to lower the end reactions to an acceptable level. Also see Nigel Ellis's textbook on fall protection which includes discussions about multiple spans and multiple people (don't forget the rescuer!). At the very least, find a qualified professional engineer (P.E.) experienced in fall protection to help you with this.
ZCP
www.phoenix-engineer.com
RE: HELP!!! Forces in cable for safety rigging
RE: HELP!!! Forces in cable for safety rigging
That would be approximately correct for the horizontal component of cable force due to a uniform load 'w' if the sag is small. It assumes a parabola instead of a catenary.
It is not applicable in the above situation as the primary load is concentrated, the weight of cable being neglected.
BA
RE: HELP!!! Forces in cable for safety rigging
RE: HELP!!! Forces in cable for safety rigging
BA