Michael Heggen 💯🐸 ❌👑

Disclaimer: This should not be taken as a treatise on building shearlegs. Consult a competent expert and/or a reputable reference work before attempting—especially if there is potential risk to human life.

Here's the link to Part 7:
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My friend Mark shows us how to begin construct shearlegs. He is holding a length of 10,000 lb. 5/8" static (low-stretch) rope. First, put two same-length poles side by side, with the butt ends close to where you want the base of the shearlegs to end up. The smaller ends of the logs will be at the top of the shearlegs. Choose the point that you want the shearlegs to cross. This will be your lashing point. For extra security, attach cleats to the poles just below your lashing point to eliminate any possibility of the lashing creeping up or down. Put a piece of 2x__ lumber between the poles a few feet down from the lashing point to serve as a spacer. Tie one end of your lashing rope to one of the logs with a clove hitch. The tail of the clove hitch should point away from the shearlegs.

We then took eight full wraps around both poles as shown, then brought the free end of the rope up in between the poles, over the outside of the wraps, and below the wraps to start the frapping turns. The free end is in Mark's left hand. Why, yes, as a matter of fact, Mark is wearing an 1860s U.S. Army kepi with a Corps of Engineers insignia.

We completed four frapping turns around the wraps, pulling them tight as we went.

Finally, we make another clove hitch on the opposite pole from the first clove hitch, with this second clove hitch above the wrapping turns. As with the first clove hitch, the tail of the clove hitch points away from the shearlegs.

For the west side shearlegs, we dug two holes about ten feet apart, a foot deep, and just a bit bigger around than the butt ends of the logs.

My buddy Mark showed us how to lash the tops of the logs. See alt text for captions.

Classic scouting fieldcraft!

All for today. Gotta go feed the horses.

The view in this sketch is looking south (downstream). The west side (where we are currently in the story) is on the right. The skyline will be suspended across the creek by two sets of shearlegs (the angled poles on either side of the creek). The base of the shearlegs form a rectangle if viewed from above that puts the center of the skyline as close as possible to where the center of the main span of the bridge will be.

Here is a working sketch that I made at the time of what we would be building. We will be referring to this sketch multiple times in this story.

For now, just consider it as an overview. See alt text.

Next we determined where to set the western shearlegs. Ideally, our skyline would have been directly in line with the bridge. However, the trees suitable for anchoring the skyline were not where we needed them, so we had to set the skyline at about 15 degrees off the line of the bridge.

We now had four straight, tight-grained Douglas-fir logs weighing 350-500 pounds apiece.

Using hand tools, we cleared a narrow trail down to the bridge site. Six of us (as I recall) spent most of a day with ropes and lifting straps dragging the logs the 150 yards through brush to the bridge site.

That meant that they would have closely spaced growth rings and few knots—just what we needed.

We chose trees that were beefier than we needed, felled them, found the point at which they were five inches in diameter, made a cut, then measured down the trunk from there 34 feet and made a second cut.

We poked around in the woods until we found four likely contenders in a stand of natural (not planted) second-growth Douglas-firs about 150 yards west of the bridge site.

These trees grew up partly shaded by their ancestors and siblings, so they lost their lower branches early due to low light.

🧵 How We Built Our Bridge, part 7

Link to Part 6:
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Now, with our terms defined, on to the shearlegs....

We sized our shearlegs based on the size of the arches. We figured that 34-foot logs that were 5 inches in diameter at the top would be just about right.

#SmallFarmLife

I, too, have known some cats like this.

This took place about two hours from where I live.

Having been around police K9 units, this violates pretty much every set of use-of-force rules for dogs out there.

#Resist #NoFascism

Chick-a-dee-dee-dee!

Our native trailing blackberries are like that. Small, low-growing, polite plants—for the most part (although they do like to catch the toes of yer boots when ya walk through them).

It’s just the invasives that are… rude. :-)

A few years before we bought our place, the blackberries covered parts of the farmhouse. We found a few dead runners up in the attic where there had poked their way through little gaps in the siding.

You win.

Like misplaced tools.

Or a small car, in the case of Himalayan blackberries.

I am in season 3 of Homeland. Great television!

Meta-gaming is a force to be reckoned with.

I have become hypersensitive to the presence of Chekov’s _____ in television and film. I loves me some writing that is not mired in such spoonfeeding.

We went a couple of steps up from Harbor Freight, but yeah.

And funny that you should mention that, because a previous project with my buddy Mark was a pulley/leverage machine for dumping manure into a compost bin that we dubbed the Trebushit. It was effectively the reverse of a trebuchet. :-)

Link to Part 6:

bsky.app/profile/mich...

It turns out that 135-ish feet of 3/8" swaged wire rope does indeed have a non-trivial amount of catenary sag—even with nearly 12,000 lbs. of tension.

But that telling will have to wait until the next chapter, because now I have to go feed the horses.

Because you need to know how tall to make the shearlegs. You just need a vague idea of how much sag you will have so that your skyline will be high enough to move the load.

Unfortunately, we didn't take catenary sag into account because we thought it would be trivial on this short of a span.

No matter how much tension you apply, you will not be able to eliminate all of the sag. Eventually the line will break or the anchor will fail.

But, you say, you aren't building a suspension bridge, so why do you care about catenary sag?

You can experiment with this using twine, rope, and chain—all of the same length (at least 20 feet to make things easy to see). Attach one end to a fixed object; apply the same horizontal tension to the other end of each; observe how the amount of sag varies.

There is Some Math to calculate this catenary sag, but it is mainly a function of the distance between supports, the tension on the cable, and the weight of the cable itself. If you look at photos of the Golden Gate during construction, the sag is there even with no roadbed in place.

There are towers with massive cables hanging between them, which in turn have smaller cables holding up the roadbed. The main cables typically swoop all the way down to roadbed. The reason for the swoop and the height of the towers is because gravity makes the cables sag.

Come-alongs allow a single person to easily move multi-ton loads. They are also a excellent way to apply tension to rigging.

And that brings us to another bit of jargon: catenary sag. To explain this, imagine a suspension bridge like the iconic Golden Gate Bridge in San Francisco.

A come-along (or cable puller) is a hand-operated winch. A lever handle is attached to a drum with a length of cable wrapped around it. One end of the cable goes to the load. The lever arm and the drum creates mechanical advantage. A ratchet mechanism holds the load when no force is being applied.

A Dutchman block is a pulley (a block) suspended from the skyline; a line connects the pulley to an anchor on the ground at roughly a right angle to the skyline. By pulling on the Dutchman block, the skyline can be shifted laterally, allowing a limited lateral movement of a load on the skyline.

So, a set of shearlegs supporting a skyline with a trolley would enable us to move the arches into position with the addition of two additional bits: a Dutchman block and some come-alongs.

I'm not making these names up, I swear.