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In my setting the entire world is a vertical wall, so I've been thinking about what might pass for a watchtower in such a setting, and come to the idea of a watchcantilever, a "horizontal tower" of sorts, a protruding (most likely triangular or half-arch styled building) structure with a small outlook post on the end, where one or two people are stationed to watch the surrounding cliff sides for travelers or enemies.

But I'm unsure that one with a sufficient length can be built to be used as an observation post. There's The Leaning Tower of Lire problem that gives some insight, but it focuses on an independently laid unsecured stack of blocks.

So, using anything that predates the invention of steel construction (steel cables and girders can allow you to reach pretty far I'd imagine), what's the longest structure protruding from a cliff wall can be achieved? Assume no fantasy elements are involved.

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  • $\begingroup$ Have you checked bridge building in general? They are large protruding structures and many are large cantilevers or simply hanging bridges, even though they are often fixed to the other side. The weight of a lot of road on many large bridges is carried often carried from a side. $\endgroup$
    – Trioxidane
    May 11 '21 at 13:29
  • $\begingroup$ Made out of what? Just saying "not steel" is rather vague. $\endgroup$
    – mwarren
    May 11 '21 at 13:31
  • $\begingroup$ I looked up the definition of cantilever, and it says it's only attached/supported "at one end"...but that still leaves room for interpretation. How close in to the one end are supporting elements allowed? Perhaps more importantly, are trusses allowed? $\endgroup$
    – Qami
    May 11 '21 at 13:39
  • $\begingroup$ @mwarren anything that was invented before steel $\endgroup$ May 11 '21 at 13:41
  • $\begingroup$ Also, in the interest of all the wood-based answers you'll be getting, are there any restrictions on the types of wood available in your cliffworld? $\endgroup$
    – Qami
    May 11 '21 at 13:41
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You need supporting structure.

You need to understand structure.

A Cantilever is not just a structure supported on one end as this forms a weak joint around which it would pivot and collapse, it needs either of the following:

  1. A 'Bracket', to support a portion of it in compression
  2. A balancing part of the beam to support an equal amount of weight
  3. A suspension rod, to support a portion of it in tension

So I know a professional structural engineer who once said to me "You should never cantilever a beam more than 1/3 its length". ie. 2/3 of its length is on the other side of the pivot point and this allows for loads. This is 'Option 2' of the above options. I would expect at 20m long timber beam to then be able to cantilever 6m, and hold the weight of 1 person.

Alternatively - use a rope to increase this length, or another timber under to form a bracket, and this can be increased. I would expect then this length could be effectively tripled (roughly) depending on the timber used.

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I have sought guidance from Thang Tong Gyalpo and I have another approach for you.

https://en.wikipedia.org/wiki/Thang_Tong_Gyalpo

Thangtong Gyalpo (Tibetan: ཐང་སྟོང་རྒྱལ་པོ་, Wylie: thang stong rgyal po) (1385 CE–1464 CE1 or 1361 CE–1485 CE[2]), also known as Chakzampa, the "Iron Bridge Maker"... and the King of the Empty Plain. He was a great Buddhist adept, a Chöd master,[5] yogi, physician, blacksmith, architect, and a pioneering civil engineer. Thangtong Gyalpo is said to have built 58 iron chain suspension bridges around Tibet and Bhutan, several of which are still in use today. He also designed and built several large stupas of unusual design including the great Kumbum at Chung Riwoche...

Thang tong bridge

The horizontal rectangles are stone. They are moored by chains each of which is anchored in a stupa on the cliff. Between the stones are planks of wood as shock absorbers so the stones do not rattle against each other. I am sorry I could not make 3d chains that faded into the background; imagine those black triangles as chains.

Stone is strong in compression, weak in tension. The stones here are essentially stacked on top of each other, their weight being transmitted in part through the anchor chains and in part compressing the stone behind it.

The benefit of this is that it is fantastic yet plausible. A great web of blessed chains extends back from the bridge to nowhere, with the anchor to each in its stupa guarded by three statues.


The best thing about this idea is finding out about Thang Tong Gyalpo, who really did use iron chains to make bridges that have lasted 500 years. I like the combination of Chod master and civil engineer. Not all Renaissance men were from Europe!

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  • $\begingroup$ and how are they making chains that can support the hundreds of tons this thing will weigh. $\endgroup$
    – John
    May 11 '21 at 19:27
  • $\begingroup$ @John - many hands make light work. Many chains can carry heavy loads and do, in modern suspension bridges. If you want to get into the weeds about 15th century Tibetan iron tech you can start with the link in the post. $\endgroup$
    – Willk
    May 11 '21 at 19:46
  • $\begingroup$ @John - alternatively you could build this with wooden beams and tarred ropes. $\endgroup$
    – Willk
    May 11 '21 at 20:17
  • $\begingroup$ Modern steel chains are very very different than medieval iron chain, I am not asking about labor I am asking how they are making chains strong enough. The chain in your link basically only has to support its own weight, not exponentially more by handing stone off of it. The hanging bridge you reference had a span of 140 meters supported by chain and wood, stone would make it exponentially shorter. $\endgroup$
    – John
    May 11 '21 at 20:27
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    $\begingroup$ I feel like there's some "pull yourself by the bootstraps" trickery going on that should make this construction be impossible in real life, but can't quite point at what exactly the point of failure would be. $\endgroup$ May 12 '21 at 7:47
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Before steel was invented the only available material for making beams was wood.

The length of the longest beam you can make out of wood is limited by the height of the tallest, straight tree you can find around.

Stretching it over the tallest tree in the world, you would get to about 100 meter, using as reference Hyperion

Hyperion is a coast redwood (Sequoia sempervirens) in California that was measured at 115.85 m (380.1 ft), which ranks it as the world's tallest known living tree.

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    $\begingroup$ Sure, but how much weight could that hundred meter wood beam (presumably near full trunk thickness) support before either breaking at the anchor point or flexing so much you'd need to tie in to stay in the basket? $\endgroup$
    – Zeiss Ikon
    May 11 '21 at 13:35
  • $\begingroup$ So stone is a non-starter? That's actually surprising. $\endgroup$ May 11 '21 at 13:42
  • $\begingroup$ Would iron or other metals not work? $\endgroup$ May 11 '21 at 13:42
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    $\begingroup$ @DarthBiomech, stone works well in compression and poorly in traction, while with a cantilever the material work part in compression, part in traction $\endgroup$
    – L.Dutch
    May 11 '21 at 13:52
  • $\begingroup$ @DarthBiomech not a non-starter, there are natural overhangs on a large scale, but a fairly early finisher. $\endgroup$
    – Separatrix
    May 11 '21 at 14:07

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