I installed a zip line for my 7 YO grandson and his friends. The yard had good trees to pick from on one end, but not the other. The house has a walk-out basement with deck from the main floor. I attached a 6 foot length of treated 4×4 to the double 2×12 structural member of the deck, so that the 4×4 would be perpendicular to the zip line route. I attached the zip line near one end. The configuration amounted to an overhung load on the 4×4. It worked OK for the kids, but an adult sat on the seat to illustrate a concern she had about the zip line, and the 4×4 broke. I had previously said load limit was 90 lbs, but I was just estimating. How many pounds of "pull" should a piece of 4×4 lumber hold, given a 3 foot overhung load? (The 4×4 I used was treated and had been salvaged from another use. We can/should start with a piece of new lumber.)
Wood – Zip line support
wood
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To add stability you could use 2x6 members for the cross pieces along the bottom. Then cover the bottom with a half of a sheet of plywood. Pour concrete into the box thus formed at the base by the 2x6's and plywood.
You could get by without having to go all the way to full angle braces across the sides (which would get in the way of your hanging plants anyway) by installing some gussets made of thin plywood as shown below:
At a suitable size these would probably come right up to where the first hanging pot wires go.
I see that you have each corner made of just one 2x4. I would encourage you to make the corners out of two 2x4's attached in an L shape in each corner. This will go a long ways toward making the unit a good bit sturdier and will also make the cross bars on two of the sides have a whole lot more wood to bolt into.
I would echo that the proper fasteners for this structure are going to be 1/4" or 5/16" diameter carriage bolts as opposed to screws or nails.
I think your two choices are
- make the shelf stiffer by making it thicker
- support the shelf more inventively.
An example of the former would be to make the shelf into a T-beam (or use keshlam's suggestion of a torsion box)
An example of the latter might be support wires up to the top of the wall or to the ceiling, or a glass-brick pillar, or a wooden arch support.
I would think about the myriad of ways bridge builders have tackled the same structural problem over hundreds of years.
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Best Answer
Not at all surprising to me that the 4x4 "mast" broke as you describe. The geometry of a zip line is such that the tension on the line is many times greater than the weight of the rider.
One secondary source suggests installing the cable such that the sag is 5% of the span. If this guideline is followed, the weight of the rider creates a pulling force at each cable support of 5 times the riders weight. The support also needs to take the load created by the cable itself, which is also a multiple of the weight of the cable.
Apparently the Association for Challenge Course Technology (ACCT) and the Professional Ropes Course Association (PRCA) both publish standards on zip line design and installation, which should be considered the authority on such matters.
DISCLAIMER Everything that follows is illustrative only, the above references should be consulted before building a zip line.
So now to attempt to answer your question: How many pounds of "pull" should a piece of 4x4 lumber hold, given a load applied 3 feet from the support point?
You've essentially got what's called a cantilever beam. In a cantilever beam you have both bending and shear stresses to consider, though in this case the bending stresses are the primary concern.
A #2 southern pine 4x4 post has a maximum allowable fiber stress of 850psi.
For a simple beam of square cross-section
Max bending stress = 6*M/(b^3)where M is the bending moment (load in the cable (P) times the overhang of 36 inches) and b is the length of a side of the square (3.5 inches for a 4x4)
Rearranging a bit to solve for the max "pull":
P=(b^3*max allowable stress)/(6*length of overhang)P=(3.5in)^3*850psi/(6*36in)=168lbSo the post can withstand about 168 lb of pull from the cable (really it should be a little bit less when the shear forces are also considered).
But as I stated earlier, the tension in the cable can be around 5 times the weight of the person hanging from the zip line, so the max "safe" weight with this back-of-envelope calculation is only around
168lb/5=34lb, even before applying a typical factor-of-safety to the calculations to account for the dynamic nature of the load, and the fact that a person's (relative) safety depends on it.So based on these calculations, even with just kids on the zip line, you were very near, if not exceeding the design load limits for your 4x4 post.
With a 3 foot overhang, a 4x4 post is nowhere near strong enough to safely support a zip line, even for kids.