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It is the near-future. Breakthrough, mass produced carbon nanotubes have been invented and are relatively cheap. These nanotubes have the properties that we would expect, and are manufactured in 'ropes' up to 10 cm long with a tensile strength of 200 GPa.

What is the most effective way to use these fibers to build super-tall megastructures in cities?

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  • $\begingroup$ If you can make roped 10cm long, you probably can make them 1 km long. These would go a long way into having proper elevators for archologies. $\endgroup$ – Renan Nov 20 '18 at 2:18
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    $\begingroup$ You need compressive strength not tensile strength to build tall buildings. The space elevator idea works because there is a counter weight at the end of it, being swung around like you would with a hammer throw. So unless your building is held up by a counter weight in space, its not going to work well. $\endgroup$ – Shadowzee Nov 20 '18 at 2:21
  • $\begingroup$ How big you want to go? $\endgroup$ – Artemijs Danilovs Nov 20 '18 at 2:39
  • $\begingroup$ You also need to look at shear strength from e.g. high winds trying to tip your structure over, or twisting it in different directions. $\endgroup$ – Cadence Nov 20 '18 at 3:12
  • $\begingroup$ You'd use them like carbon fibres to reinforced other materials. You have fibre reinforced concrete as well as fibre laminates to produce slender, rigid yet still flexible buildings. Replace the glass with graphene sheets and the building would be almost unbreakable $\endgroup$ – Thorne Nov 20 '18 at 4:31
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The main limiting factors for height in skyscrapers are:

  • Structural: Not in so far we can't create tall towers, but more in that wind causes lateral deflection and oscillation, creating a discernible movement to occupants which can be distressing.
  • Natural light: Most building codes stipulate minimum levels of natural light that must be present in habitable spaces. A taller tower often means a wider tower, and the wider the base the less natural light can penetrate to the interior.
  • Economic: Creating tall skyscrapers creates a lot of lettable floor area to get income from yes - however this is balanced with cost of construction and market demand. The tallest tower in Dubai is funded differently to most market-driven construction in capitals. Flooding the market with lettable floor area is a good way to kill the market, unless the economic purpose of the tower is supplanted with political or other purposes.
  • Lifts: Yes, I went to a seminar on lift design and the consultant's main message is that lifts are a major determinant for height. People only wait for a short while before getting to the floor they need to get to. Keep in mind lifts have height limits, and usually sky lobbies need to be implemented too.

So structure is actually a small part of all the considerations to do with height. However, having said that, to eliminate this limitation it is worth noting nanotubes are good in tension, but not so good in lateral strength or compressive strength.

The best way to use the new technology is to create pretensioned perimeter elements to increase the structural rigidity and prevent lateral movement from wind forces.

A tower with exposed structure that utilises this technique is the fairly modest Central Park tower in Perth, Western Australia:

enter image description here

The architects exposed the structure - but in short a discussion with the structural engineer was that visible column and 'triangle' at the tower top are in tension to reduce this movement. Your nanotubes would enable this principle to create substantially less deflection, and thus a higher tower (how much I cannot say though).

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    $\begingroup$ +1 Your answer convers most of what i care about. For creative points i thought concrete could be reinforced with carbon nanotubes in lieu of steel which may improve tensile strentgh but would reduce maintenace (no rust) and most importantly weight. $\endgroup$ – anon Nov 21 '18 at 4:40
  • $\begingroup$ Lifts - there is a second issue. More height = you want to transport more people = you need more shafts = at some point you will get diminishing returns, and then negative returns, when more floors would cause additional lift shaft take away more volume, than new floors added. The same issue is with water and electric infrastructure, waste disposal, etc... $\endgroup$ – Mołot Nov 21 '18 at 8:19
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Considering that carbon nanotubes as a "rope" fiber would have extremely strong tensile strength but no compressive strength, the question then becomes: How do I use tensile strength to help avoid compression?

There are at least three ways to do this, two of which are already common in modern engineering.

1) Air supported structures - Imagine a giant, vertical dirigible. Now imagine many of these, long and narrow, used as a support structure. A carbon nanotube woven fabric could be made to withstand very high pressures within, creating a very rigid "beam" that is, strictly speaking, a balloon. (Side note, balloon animals for giants!)

2) Pretension - Similar to reinforced concrete, tension added to compression members before loading is already used to minimize flexing, allowing taller construction.

3) Active support structures - This one is my favorite, and incidentally, while requiring the most advanced technology, is capable of the tallest structures. Similar to the air-supported concept, active structures use a non-solid medium to keep a structure's form. Structural support provides thrust. That thrust is usually static, and it is exactly opposite the downward force of gravity. Now, structural supports are not the only way to provide thrust - otherwise it would be very hard to get rockets off the ground. It's possible to use such an active system to support a building as well, but a rocket would not do, as the reaction mass is continuously lost. Instead, the idea for such a structure (commonly called a space fountain) is to force magnetically charged particulate matter in a stream through an evacuated tube, where they will be deflected back downward at the top through a return tube. That deflection provides the thrust to support the building. So far, this is irrelevant to the use of high tensile strength cables such as the carbon nanotube ropes. However, we would want our fountain tower to be rigid, to avoid our penthouse guests losing their lunch. Hmmmm, rigidity? This calls to mind the second method I described. Tension would allow for excess thrust to hold the tower at precisely the wanted height, and by using sufficiently angled stay lines to compensate for sideways thrust (from wind, earthquakes, etc.), just as on the mast of a ship, the tower would be held vertical.

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You could use them to lighten the load of the buildings with vacuum balloons...

The density of air at standard temperature and pressure is 1.28 g/l, so 1 litre of displaced air has sufficient buoyant force to lift 1.28 g. – Wikipedia

...on Venus:

Of all known planets and moons of the Sun system only the Venusian atmosphere has [Lana atmospheric ratio] big enough to surpass [Lana coefficient] for such materials as some composites (below altitude of ca. 15 km) and graphene (below altitude of ca. 40 km). – ibid.

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In addition to Thorne's answer, you can also combine this with active support. The idea is the same as with a sheet of paper above fire - you bounce something off the bottom of the supported structure. You can find a great explanation of this and many more things on Isaac Arthur's youtube channel

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Carbon nanotubes have a great tensile strength. If you want to use them for building a skyscraper, you need to have them work under tension, to properly use their above said tensile strength.

Where in a skyscraper do we have tension?

Not on the pillars: they mostly work in compression, and only when the skyscraper bend under lateral load (mostly due to wind) they have tensional solicitation. For these components you may want to still use steel and concrete.

But if you want to lay the horizontal beams to sustain floors and ceilings, there carbon nanotubes can help you. The bottom side of a loaded beam is subjected to tensile stress.

enter image description here

If you use carbon nanotubes instead of steel to reinforce the bottom side of the beam instead, you can lower the weight of the ceiling for the same surface, thus you will decrease the load on the pillars. This will help you build a taller structure, as you will be able to stack more floors before reaching the load limit of the pillars.

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  • $\begingroup$ It seems to me that looking at the issues on the macroscopic level(for want of a better term) is not the way to go about it? if one knows the directions of applied force beforehand, matrices can be formed from small lengths of cnt into more complex structures than simple rods, converting compression force from one direction against a macro-body into tensile pressure in another. This seems to be a superior approach with materials having one significant strength and several weaknesses, as with cnt. ncbi.nlm.nih.gov/pmc/articles/PMC4362020 explores distension of CNT for filling. $\endgroup$ – Giu Piete Nov 28 '18 at 15:29

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