You may recall that last year, we built a dome. The challenge with the dome is that it’s 16 feet high at the tallest part, and building it requires slowly working your way from the bottom to the top, moving a ladder along as you go. We have a 14′ ladder, which makes it possible, just barely, to get the last bolt in at the top of the dome. Let me tell you as the guy who’s usually at the top of that ladder that wrangling dome struts together high above the ground is a little hair-raising. On top of that, only having one ladder means that the construction has to proceed one bolt at a time. No matter how many helpers are available, the ladder is the bottleneck.

If you have a crane, it’s possible to build a dome from the top down. You build the highest part of the dome, attach it to the crane, and then slowly lift it into the air as it’s built. All of the work can be done on the ground, which means nobody has to risk their neck on a ladder, and as many people can help build as you can find wrenches for them to hold. This is, in fact, the only feasible way to build very large domes. Unfortunately, it requires a crane, and they’re expensive.

I’ve often fantasized about building some kind of structure to allow the top-down construction of our dome without having to rent a crane. At 16′, our dome is in a sweet spot where it’s too tall to efficiently build with ladders, but not so tall that you absolutely have to have a crane. At first, I envisioned a pyramid-like structure, with the dome hanging from the point. Unfortunately, this would require legs over 30 feet long in order to contain the entire dome. In addition to being unwieldy to transport and set up, I simply lack the metalworking skills required to build such a structure. The most efficient structure would be a single vertical mast, guyed out, with a winch and pulley on it.

The challenge there is that the dome weighs about 600-700 lbs when complete, and I’d need a mast about 20′ long in order to make things work. I considered building something out of 4×4 lumber, but I couldn’t figure out how to join the individual pieces of lumber into a single mast in a way that would be strong, but also possible to disassemble at the end. Also, 4x4s are heavy.

The epiphany came when I learned that it’s common to use 2″ steel pipe for impromptu antenna masts. Two 10′ long pieces of pipe can be coupled together using a standard fitting to create a 20′ mast. Although steel pipe is not designed to be load bearing, wind load on a parabolic dish antenna can peak around 70 lbs of lateral force, and the steel pipe is strong enough for that. I realized that, as long as the mast remains relatively vertical, the lateral force on it will be minimized, and will stay within the established load limits of the mast.

Consider: If the mast is entirely vertical (0° tilt), then 100% of the weight is parallel to the mast and 0% of the weight is perpendicular to the mast. Weight parallel to the mast is irrelevant because there’s no way that the pipe is going to compress or crush. As the mast moves to the horizontal, more of the weight is perpendicular to the mast. When the  mast is totally horizontal (90° tilt), 100% of the weight is perpendicular to the mast. So we can see that the percentage of weight that is perpendicular to the mast is equal to the angle of the mast divided by 90°.

Based on the antenna example, the mast can tolerate at least 70 lbs lateral loading. If the dome weighs 700 lbs, that means the mast can go up to 10% out of vertical, or 9° without exceeding established safety margins.

Once I figured out that I could use 2″ steel pipe for the mast, the other parts fell into place pretty easily. The mast will be guyed using 3/8″ nylon rope, with a working load limit of around 240 lbs. This is about 30% of the dome’s weight, so the mast would have to tip over to an angle of about 30° before a guy line would break. I will use corkscrew stakes to hold the guy lines. They have a working load of 100 lbs, which corresponds to a mast angle of about 13 degrees.

From a marine supply store, I ordered a boat winch with dual-direction ratchet and automatic brake. This will allow for raising and lowering the dome safely. The winch is rated for 1200 lbs and is geared down to provide mechanical advantage. I purchased a 50′ length of wire rope from the hardware store, with a thimble and loop already present on one end. I don’t recall the diameter, but it was whatever diameter had a working load higher than 700 lbs. Actually, I think there was a rope with working load of about 800 lbs, and I went one size larger. Incidentally, it’s important to make sure that your winch can handle the length and diameter of wire rope you intend to use.

Finding a pulley to go at the top of the mast was difficult. Typical pulleys that you find in hardware stores are rated for around 150 lbs at most. Online, I found pulleys with working loads as high as 400 lbs or so. The breakthrough came when I discovered the correct search term: snatch block. A snatch block is a pulley that is designed for redirecting the pull of a winch. They’re rated from as low as 8,000 lbs to 20,000 lbs and higher.

Another difficulty was figuring out how to attach the pulley to the top of the mast. Standard eye bolts simply would not do. They are rated at a few hundred pounds at most, but that is when you are pulling directly in line with the bolt. If you pull off-axis, the working strength goes down quickly. I ended up buying a 3/8″ carriage bolt and a few links of chain. The carriage bolt runs through the chain, then through a hole drilled in the mast. The head of the carriage bolt holds the chain in place. A quick-link attaches the chain to the snatch-block.

I’m currently waiting for the winch and the snatch block to come in the mail, then I’ll finish assembling the hoist and give it a try. I’m really excited to see if it works. On paper, everything works out, but many an engineering venture has ended in tears when the rubber hit the road. Given that there will be 700 lbs of dome lifted into the air, I plan to work slowly and carefully, and check for signs of stress when I’m done. If it works, I’m pretty excited, because I don’t know of anybody else who has skinned this particular cat in this particular way. This is a common size of dome, so I don’t imagine I’m the only one with this problem.