Well since I have gone crazy in the forums today I thought I would start a new discussion.
My first use of highlines invloved the use of twin 7/16" track lines pretensioned with a flying W (2-2:1 MA's)pulled by one man, and post-tensioned using a 12:1 ratio (4 people on a 3:1).
NFPA comes out with General and light use, and now we use twin 1/2" track lines. Same Flying W pre-tension with a 18:1 post-tenion.
Within the past 2 years I am hearing of FD's using twin 5/8" track lines. Do you pre and post-tension? I have found the Flying W usually puts the right amount of sag and eliminates the need to post-tension. In addition I have been leary of people say they can look at a highline and know how much sag they have.
What do your SOG's say to use for a higline under 400'? Try to support your position with weight calculations or supporting information as to why you use what you do.
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Thread: Highlines - Rope Size/Methods
03-10-2005, 11:55 PM #1
- Join Date
- Apr 2001
Highlines - Rope Size/Methods
03-11-2005, 01:15 PM #2
- Join Date
- Jul 2001
We use 12.5mm(1/2 inch) track lines pretensioned with a factor 2 (one hauler on a 2:1). Further tensioning only needs to be done if obstacles need to be cleared. If we do, we will use a factor 18 at the most. We have placed a load cell at the anchor station during training and after a factor 18 tensioning, the load on the trackline is between 3.5kN and 4kN with two people on the system. The breaking strength of that rope is 40kN, therefore we are still maintaining a safety factor of 10. Now before you jump in and say that NFPA requires a 15:1 safet factor, lets look at what NFPA actually says.
The NFPA 1983 standard no longer mentions the 15:1 safety factor. In fact it no longer even mentions what the safe working load of the rope must be. So if a manufacturer states that the SWL of their rope is 4kN in order to maintain a safet factor of 10 they can do that.
The safe working load of the rope is 2.67kN. If you tie a knot in your 12.5mm rope the breaking strength is now 27kN. 27kN MBS and SWL of 2.67kN = 10:1. The SWL is based on typical vertical rigging acouting for knots which weaken the rope. Tracklines are constructed with maximum strength due to high strength tie-offs where knots aren't loaded, therefore the MBS of the trackline is 40kN. If that is not enough, I would much rather go to a twin track than to have a 16mm rope that is not compatible with the rest of my gear. With a twin track, each rope shares the 4kN load (2kN each). This gives you a safety factor of 20.
So the next argument that the 16mm users will have is that you still only have a single tagline as a safety. For this you must understand that when the mainline fails and the load is caught by the taglines, the load will drop about 1/5 the span distance. On your 400ft highline, that is a drop of up to 80 feet. This drstically reduces the inside angle on the line which brings the force down considerably.
The bottom line is, this technique has been tested thoroughly by Rigging for Rescue and the British Columbia Council for Technical Rescue.Sometimes, in order to make an operation idiot proof, you must remove the idiot!
03-11-2005, 01:54 PM #3
- Join Date
- Apr 2001
My concern with the 16mm rope is what are the forces being put on the anchors during tensioning? It would seem to me that a great more force would be needed to tension a rope of that size and length. For one the shear weight of the rope strung in space would be more compared to 1/2". Now I realize a 16mm rope is stonger and therefore can be tensioned more, but I would be more concerned about the forces on my anchors. Let's face it, a highline usually means a natural anchor is being used. Does anyone know the force calclations for 16mm rope?
In response to NFPA. I have not looked at the standards since their updates (shame on me). From my understanding the 15:1 safety ratio is simply a manufacturing standard and not necessarily to be used as a rigging safety factor. When I teach a course I teach a 10:1 safety factor in the system. I want to say 15:1 is still mentioned in NFPA 1500 but don't hold me to it.
03-11-2005, 07:02 PM #4
- Join Date
- Jul 2001
Nothing in NFPA 1500 about 15:1. As far as the tensioning factors for 16mm rope goes, I don't know. Remember that less tension is always better if you have the clearance required below the span. The BBCCTR did not do any tests on 16mm because they would never consider using the stuff. It is simply too heavy and not compatable with most equipment. The only way for you to find out would be to attach a load cell to your anchor station and gradually increase tansion until you reach your 10:1 for 16mm. Remember that the factor 12 and 18 are based on haulers using a hand over hand technique. Giving the old heave ho and leaning into it will exceed the desired tension. When the rope starts to slip through the hands of the haulers, you have reached the optimum tension.
If you are worried about the weight of the rope pushing you over the desired tension I think you will be alright. The 16mm has a MBS of about 58kN (13000lbs). That is 40-45% stronger than the 12.5mm. The relative difference in weight between the two is not that high so if you went with the factor 18 tension, you will be fine. If you want to find out just how high you can go incase you need to clear obstacles, once again, you will need a load cell.
Hope that is of some helpSometimes, in order to make an operation idiot proof, you must remove the idiot!
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