What are the best materials for an organic skyscraper one kilometer high?

Question

The tallest structures on the surface of Earth are trees, which can reach 120-130m already. But I want something much bigger. In the setting of my magic-powered sci-fi story, humans use bioengineered creatures to build organic skyscrapers: the Spires.

Initially, Spires comes as human-sized 'seeds' which, one planted in an appropriate ground, are supplied by pumps with a steady flow of nutrients. The Spires have no branches or leaves, it is mostly a vertical pole supported by strong roots. Spires have multiple hearts to propel the flow of nutrients up to the latest stages, hence overcoming the limitation of our IRL trees.

After its colossal growth over a decade, the Spire is emptied out and human habitations installed within.

Geometry of the Spire would be like a thin cone with its base on the ground. The habitable section is considered as any portion with a width larger than 3 meters. I would like the diameter of Spire's base to be between 50-100 meters.

What organic materials would be strong enough to support the growth of the habitable section of such a structure up to a kilometer?

Answers involving really organic materials/structures, such as wood or mother-of-pearl, would be preferred over the ones with yet-artificial compounds such as carbone nanotubes or graphene.

Answer 1

Bone

Cortical bone has a compressive strength in excess of 200 MPa, while concrete is closer to 20-80 MPa. Of course, trees don't make bone, but since they are bioengineering, perhaps they can make a frankencreature that crosses a tree and some large animals so that it incorporates calcium and other minerals for the high strength.

Wood would have higher compressive strength if there were a biological reason for it. A stupid simple way to get this naturally is to simply attach weights to the tops of trees so that they are always in more compression than an unburdened tree. Of course, it would take many generations for this to result in trees that naturally make stronger wood, but perhaps this tribe has been grooming trees to make "ironwood" over thousands of years. And maybe the even seed the soil around the trees with extra minerals to give them the raw materials they need to incorporate them into their trunks.

Note that you want to always size the weight relative to size of the trunk. So you put small weights on saplings, and you increase the weight as the tree grows. Perhaps you add on another layer of weight each year. There could even be a tradition for this, and this method could be incorporated into the culture that produces these behemoths.

Answer 2

Red Oakwood, but very unlikely

Although your organic Spire skyscrapers are not impossible, they are very, very hard to create. Red Oak seems a really likely choice, but again, it has its own drawbacks.

According to this article from Oklahoma University, Red oak has a compressive strength of 6540 PSI, which in MPa, translates to roughly 45 MPa. In case you are new to physics, compressive strength is basically how much load a substance can withstand without breaking apart. Concrete and steel have really high compressive strengths, that's why they are used in building buildings and of course money wasting useless tall skycrapers, that extend to over half a mile (Burj Khalifa) and stuff like that.

45 MPa isn't bad. Granite in contrast has a compressive strength of over 100-300 MPa. Granite is basically the stuff which comprises most of Mt. Everest.

The summit and the topmost part is basically just marine limestone and stuff.

Basically, you can get 1km tall organic skyscrapers, but the problem is, you need a really wide base for the building. I cannot calculate the exact width of a 1km tall red oak skyscraper, but something tells me it would be far in excess of the 50-100m diameter of the base.

TL;DR Although your "organic skyscrapers" are indeed possible, they would be a pain in the a- really hard to make them, considering any organic material crushes down if its very tall, unless it has a wide base.

Note: Spider-silk or silk does NOT SUFFICE. Sure it does have a strong tensile strength, meaning that it is hard to tear apart. But creating a multi-story skyscraper out of silk or SS, would be akin to building a Burj Khalifa out of jelly.

Answer 3

I did some reading.

• Concrete tops out at 40Mpa
• Granite tops out at 300MPa
• Hydroxyapatite (enamel) tops out at 400Mpa
• Nacre (Mother of Pearl) exceeds 500Mpa

So in theory a Nacre skyscraper would be very workable. Possibly you'd want some additional internal tethers to balance it for wind shear.

Answer 4

I'm sure you could handwave something with cellulose, spiderwebs, and the other materials people have mentioned. Maybe have it grow akin to those somewhat organic looking AI designed structures, and use that as a justification for it to be relatively much stronger than a regular building of the same material. You could also have people perform 'surgery' on the building at certain points in its growth cycle, adding steel rebar to aid in its strength.

Answer 5

Use tension and compression for a yurt-style shape Surround the high-compression-strength base with an unbroken band of high-tensile strength material. Perfect graphene is ideal for the bands, but making that at low temperatures and pressures in a biological structure is unlikely.

Hydrogen (or helium) Provide lift to your lightweight structure by filling it with gas. Helium is naturally produced at a very slow rate by uranium deposits, but can be found in pockets in natural gas chambers. Let's suppose these Seeds only succeed over natural gas chambers. This is also a handy source of energy in an oxygenated atmosphere.

Active pumped water and nutrients Trees and plants are able to pump water and nutrients up their stalks easily when they are short due to capillary action and the pressure from the water table. However, your very tall plants would need another mechanism, such as actively spending energy to pump upward. Gathering rainwater or dew at high elevation is fine, but it contains no nutrients if the water doesn't come out of the ground.

Answer 6

Sedimentary Rock

. . . is an organic substance formed from billions of tiny sea creatures, compacted over millions of years to make a hard and rigid building material. Use that.

Answer 7

Coral reef

Make a pool, fill it with seawater and let the corals grow. Add and remove some mud in certain points to create some voids. While it grows shape a little bit the walls to let it form arches and vaults.

When one layer has grown put some temporary panels to retain the water on the top of that layer and create another pool to grow the second layer. Like the movable box used for slip forming.

After it is grown the inhabitants can improve it limestone is easy to carve. You could have stairs and tunnels built inside the walls, but they will have to be very thick.

Drawback, inhabitants would have to use some kind of cement to plaster the surfaces to prevent weather erosion.

Plot twist: many smaller building where already built in this manner, this tower is special because it is the result of the work of many generations.

Answer 8

In addition to other good answers before me, I'd suggest bio-engineering the tree to include copious amounts of Osmium crystals, a naturally occurring element that is the densest element on Earth. More so than gold, platinum, and diamonds.

That density can make it nearly incompressible. In fact, compressibility is measured in gigapascals (GPa), and diamond rates at 442 GPa, while Osmium rates at 462 GPa, slightly higher.

That in turn makes osmium a great building material, a matrix of osmium in the outermost layer of wood, combined with the designs in previous answers to mine, can provide the outer scaffold for the spire.

Since you are engineering the plant anyway, I'd grow it already hollow: Some variety of giant bamboo seems like the obvious starting point. they are completely hollow, with a very tough outside, and internally (for structural integrity and nutrient transport) are already segmented by periodic "nodes" (the official name), which are analogous to floors in a building.

Combine that with osmium crystals (or microscopic diamond dust), engineer some gigantism, and bingo, you've got a skyscraper.

On Earth, Osmium is much more rare than diamond, but obviously that does not have to be the case for a scifi planet. Maybe their planet got struck billions of years ago with a huge Osmium meteorite shower or something.

And bamboo grows fast, and proliferates fast, so much so it is considered an invasive species that out-competes native vegetation, destroying biodiversity and causing ecological problems, like replacing the food plants and grasses of the local wild animals.

In places where bamboo is native, the wildlife is already adapted to it, and it provides food and shelter, so not considered invasive there.

Overcome the engineering problem of that much height, and bamboo is a great starting point.