Rope Ratings - Working Load vs. Tensile Strength

[crybdr]

Wondering if anyone can shed some light on rope ratings - working load vs. tensile strength.

I pulled my dock out of the water today - it's a 48' galvanized steel frame with 4 wheels. It has to go up a 15% grade to get it off shore. I tied off to a tree using 5/8" braided polyester? utility rope (Tensile Strenght - 3,500lbs, Working Load 350lbs) and did the pulling with a nylon strap come-along with a 2,000lbs pull rating. I worked in 8' increments and pulled the dock about 60' from it's in-water position.

Everything went fine (the dock is out and the wheels are chocked) - but I'm wondering if I broke some safety rules. I'm guessing that at some point I exceeded the 350lbs Working Load of the rope - that dock frame is wicked-heavy. I bought the thickest rope avalable at the farm supply store - next step up would be steel cable.

I only have to do it once a year - but I'd like to avoid getting 'whipped' by a snapping rope.

Does anyone have any insight into rope ratings? Was I lucky the rope didn't break?

[LCHIEN]

three explanations from the web:

Working Load Limit: The Working Load Limit (WLL) is the maximum load which should ever be applied to the product under any condition. The Working Load Limit is based on a load being uniformly applied in a straight line pull.

Breaking Strength: Do NOT use Breaking Strength for design or rating purposes. Use Working Load Limit instead. Breaking Strength is an average figure at which samples have been found to break under laboratory conditions, in straight line pulls with constantly increasing loads. These conditions are rarely duplicated in actual use.


http://whitewatersolutions.net/Rope_..._Strength.html


One area of misunderstanding that needs to be brought to the surface is the proper interpretation of rope strength, appropriate usage and care. Let's start by defining two important terms: "tensile strength" and "working load". Tensile strength is the average strength of new rope under laboratory conditions. This is determined by wrapping the rope around two large diameter capstans and slowly tensioning the line until it breaks. The manufacturer's recommended working load is determined by taking the tensile strength and dividing it by a factor that more accurately reflects the maximum load that should be applied to a given rope to assure a comfortable safety margin and longevity of the line. Of course that factor varies with the type of fiber and the weaving construction. There are however always exceptions, most notably the fact that rope is susceptible to degradation and damage in numerous ways that are not controllable by the manufacturer.
It may surprise you to find out that the working load for most kinds of rope is between 15% and 25% of the tensile strength. Now consider the fact that any time you tie a knot in a rope you effectively cut the tensile strength in half. The knot when tensioned cuts the line. While certain kinds of knots damage the line less than others, the 50% loss of tensile strength is a good general rule to live by. Research has shown that the figure 8 knot reduces the tensile strength by approximately 35% instead of 50% for other common knots tested.

[LCHIEN]

i don't have any idea of how much your dock weighs but assuming frictionless wheels holding an item on a 15 degree slope is about 25% of the weight of the item. = sin(15 degrees)

I assume it's chocked now so the chocks hold much of the weight and not the rope tied off to the tree.

When you pulled it out with the nylon come-a-long that probably exceeds (due to friction and overcoming inertia) the static load so its more than 25% of the dead weight and you said that this come-a-long was rated at 2000 lbs.

[cabinetman]

The online explanations leave a lot to be desired if trying to understand whether you'll get whipped or not. Seems to me there is a lot of criteria that follows for "tensile strength" "working load", or "breaking strength". Conditions for numbers are dependent on stuff like lab conditions, new rope, etc.

The ratings would differ with changes or differentials in conditions. As luck would have it, the come along might fail. If your 5/8" worked, I would stick with that. You could go with 3/4", but rope that thick gets pricy.

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[crybdr]

Thanks - I've been looking online as well, and re-learning my trigonometry.

My best estimate, I'm going to put the total weight of the dock frame at around 800lbs. One guy delivered it to our shore on a pontoon and set it up by himself on shore(roadside delivery wasn't an option - part of the walk down to the lake is literally a 45 degree incline). Four sections at maybe 150lbs a piece, and associated hardware (legs, supports arms, and an anchor post). Once it was set up - he walked it into the water.

Given 800 pounds (and roughly 25% weight-to-grade estimate) - I probably came close to the working load during the dock's travel up and over some of the lumps and bumps on the shoreline. The lake bottom here is HARD, so no overcoming sunken wheels.

I'm feeling better about the numbers here - given the 25% weight-to-grade equation you provided. It seems I'm closer to the working load than I might have thought. Thanks for the quick reply.

[crybdr]

It's chocked on all four wheels, the support legs are on the ground, the anchor post is driven into shore, and I have some safety lines loosely tied around a big oak - in case the thing wants to get back in the water.

I did see a water skiier out there this afternoon - air temp 42 degrees. Brrrrr.....

[conwaygolfer]

Years ago when I did tree work for a living, the old manilla rope we climbed with was rated at 2650 lbs. (this is for 1/2 inch rope). Then the synthetic climbing ropes came out and what a difference. The average rating for these was around 5-7000 lbs. That is the weight of the log it would hold before breaking. Of course we never got close to that amount.
And like others may have said, it depends on a lot of factors such as knots, abrasions, age, etc.
If you called a local tree service next time and asked where to get climbing rope, they could tell you.

Conwaygolfer

[RAV2]

One thing to note when pulling with a rope is to place something on the rope in case it breaks.

If pulling a car, floor mats work well.

An old section of carpet works also.

If the rope breaks, the weight on the line drops it to the ground quickly.

[cabinetman]

What might take stresses off the ends of the rope is to use a rope thimble.
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