Very tall towers weigh a great deal, and put very high compressive loadings on the structure. The highest stresses are at the bottom, decreasing progressively as you go up.
I propose building a tower where the compression elements of the structure are large diameter inflated tubes, inflated with lighter-than-air gas (perhaps hydrogen – safe enough in a thick-walled tube of suitable material) so that the nett weight of the tube is the same (or roughly the same) as the air it displaces. The pressure in the tube is greater than atmospheric, so the walls of the tube are in tension, keeping the shape of the tube and giving the whole tube considerable compressive strength.
You can't simply use a single unobstructed tube like this for the full height of a very tall building, because the pressure will be greater at the bottom and less at the top, with a much greater pressure difference on the outside of the tube than on the inside – because of the greater density of the air outside than the gas inside. Correspondingly, the tension in the walls of the tube will be very high at the top of the tube, with the whole weight of the tube hanging from the top, and the pressure of gas inside being much higher than the pressure of the air outside.
However, you can instal bulkheads at intervals all the way up the tube. Each bulkhead arches upwards, with a higher pressure gas below than above – at every level, a little higher than the outside atmospheric pressure. The tension in the bulkhead is transferred to the walls of the tube, so the tube doesn't hang all the way from the top, it hangs from the bulkheads all the way up.
This is conceptually a little like supporting a tower with buoyancy balloons all the way up it, but in a rather more integrated fashion, and with reduced susceptibility to wind loading.-- Cosh i Pi, May 29 2007 Inflatable Wind Turbine Inflatable_20Wind_20TurbineInflatable tower and rotor [Cosh i Pi, May 29 2007] Self Erecting Solar Tower Self Erecting Solar Tower...just a bit of cross pollenation... [zen_tom, May 29 2007] Mythbusters: Hindenburg http://mythbusters-.../Hindenburg+MysteryThe first link I found. You'll prolly find it on Youtube as well. [croissantz, Nov 16 2007] I don't know about hydrogen, but Helium supports around 1kg per cubic metre. If you increase the pressure to 2bar (double sea level) it weighs the same as air at sea level.
I just thought of an idea which would be like a stack of conical tubes upside down, which is how banana plants grow.-- marklar, May 29 2007 [tom] I was going to link to that and all. Would have saved you the "..." foot shuffling.-- theleopard, May 29 2007 [Marklar]
You'd have to increase the pressure of helium to 7.2 bar to equal the density of air at sea level - that's an overpressure of 6.2 bar. That's because air is 7.2 times as dense as helium, at the same temperature and pressure. Air at 1 bar weighs about 1.23kg/m³, so helium at 1 bar can lift about 1.23kg/m³ x 6.2/7.2, which is 1.06kg/m³.
Hydrogen is half the density of helium, but more importantly, it's an awful lot cheaper. A similiar calculation shows that hydrogen can lift about 1.20 kg/m³.
You wouldn't want anywhere near 1 bar of overpressure. Aluminium is about 2200 times the density of air (at sea level), so if you want the tube to be neutrally buoyant in air, the wall thickness has to be 1/4400th of the diameter. You're probably looking at a wall thickness of a few millimetres and a diameter of a few tens of metres. At that diameter, you don't need much pressure to carry a huge weight - and for that matter, to stress the tube wall close to its maximum safe working tension.
At high altitudes, the diameter:wall thickness ratio has to be even greater.
(Aluminium just by way of example, of course. Not sure what the best wall material would really be.)-- Cosh i Pi, May 29 2007 A building made of hydrogen? One barbeque and BOOM!!!-- croissantz, May 29 2007 [croissantz] I suppose you'd bone the idea of a flying machine carrying half its weight in hydrocarbon fuel, too? Jet fuel burns even better than hydrogen.
And they keep it in tanks a good deal thinner than the walls of these tubes.
You could always use helium, but it's horribly expensive and really wouldn't make a lot of difference. Breaking the structure would bring the whole thing crashing down; the fire (which would go upwards quite fast) would be a trivial matter by comparison.
It's best to design huge structures in such a way that they don't break too easily.-- Cosh i Pi, May 29 2007 Are you piping fresh hydrogen into the tubes to replace that which leaks out through the solid aluminum walls? Hydrogen can get through pretty much anything.-- Galbinus_Caeli, May 29 2007 [Galbinus_Caeli] I suspect that even hydrogen will only leak extremely slowly through a few millimetres of solid metal (whether it's aluminium or titanium or what it would be, I don't know). On the other hand, if there are any pinholes in welds, or anything like that, then yes, it will leak and it will be essential to monitor the pressure.
This is really possibly the Achilles heel of the whole proposal: a very tall structure that collapses if you neglect the maintenance isn't very nice.
Of course you have to maintain conventional skyscraper structures as well. Corroding steel frames will fail in the end, too. The timescales of maintenance might be different - but I'm not certain about that. You're looking at huge volumes of hydrogen at pressures not much above atmospheric, with leaks only through hopefully very few, very small holes.-- Cosh i Pi, May 29 2007 I sketched a pressurized structure tube, and came up with something that bulged in the middle and tapered towards the ends: () sort of.-- baconbrain, May 29 2007 [Cosh i Pi]
Ever heard of the Hindenburg? Hydrogen, even as a non-jet fuel form, is extremely flammable.
Also, jet engines are built to withstand the stress and heat of combustion, while skyscrapers...aren't.-- croissantz, May 29 2007 [croissantz] Of course I've heard of the Hindenburg. Hydrogen is indeed extremely flammable. It ignites more easily than kerosene, but produces much less heat when it burns than kerosene does. Kerosene ignites easily enough that you avoid releasing large quantities of it into the environment.
The problem with the Hindenburg wasn't the hydrogen - it was the fact that the envelope was flammable. The hydrogen made that worse - but not actually all that much worse, because the burning hydrogen rose rapidly away from the scene - spectacular from a distance, but not actually the big problem. The deaths were caused by the burning envelope, not the burning hydrogen.
It's not the engines of a jet plane that I'm concerned about - it's the fuel tanks. They can stand a bit of a fire - despite the fact they've got kerosene inside, and aren't more than a thin skin of aluminium. If the Hindenburg had had a fire-resistant skin, the fact it was full of hydrogen would have been no more important than the fact that jet planes have tanks full of kerosene.
My towers would have their hydrogen contained in much thicker skins.
Yes, if someone flew an airliner into them, they might collapse if they weren't tough enough - just like any other tall building. The fact that high above them there'd be a hydrogen fire wouldn't be a significant contribution to the disaster. The burning kerosene pouring DOWN on the destruction would be a far greater concern.
But if you really must, you could fill them with helium. A collapsing tower would still be a disaster, of course. Such events are.-- Cosh i Pi, May 30 2007 No matter whether this idea is practical or not, you have to admit it brings up some fantastic imagery---especially the parts about it exploding and showering the rest of the city with burning debris, etc. I say we should all build one, move into it, and find out what happens. You have to live on the edge.-- Ander, Jun 01 2007 I think it's a great idea. It would probably work a lot better with some kind of carbon fiber walls, or perhaps a material that resembles bullet-proof vests, except air-tight.-- mikaman, Jun 01 2007 //if someone flew an airliner into them, they might collapse if they weren't tough enough//
Maybe things would just bounce off...-- Ling, Jun 01 2007 Very bad idea. Look up "Hydrogen embrittlement". That's where the hydrogen slowly reacts with the metal until it fails. Not to mention that the hydrogen would escape from the metal tubes unless they were thicker and heavier than the bouyant force of the hydrogen could support.
Another issue would be that hydrogen burns or explodes over a far wider range of gas mixes than anything else known. Even as little as 3% oxygen mixed in with the hydrogen will support combustion, so even the slightest leak would set up lethal conditions very rapidly. Something like kerosene can only burn only in the vapour phase, and only at a much narrower range of mixtures, and, further, is far harder to ignite even when properly mixed!
[croissantz], the hydrogen wasn't the cause of the Hindenburg disaster. The pure hydrogen inside didn't burn that fast, it was the nitrated material used for the outer skin that was the main issue, as it was essentially a nitro propellant (as used in firearms cartridges) and so it didn't even need air to burn!-- Soapy, Jun 01 2007 [Soapy] I know about hydrogen embrittlement, and diffusion.
Hydrogn embrittlement is a problem with most steels (possibly all, I don't know) and with some other metals, but it's not a problem with all alloys of all metals; and there's nothing in this idea that requires the tubes to be metal anyway - carbon fibre reinforced plastics or even ceramics would also be possible (although diffusion does become more of a problem with composites or plastics - and you certainly couldn't use unreinforced ceramics).
As to diffusion: whatever material you use, avoiding pinholes is of course vital, but most dense, pinhole-free materials don't allow hydrogen to diffuse through them at any significant rate once they reach a thickness of a few millimetres, even when the hydrogen is at quite a high pressure. In this case, the pressure would only be slightly above that outside the tube.
Finally: getting 3% oxygen into such a tube would take a very long time through any but a large hole. You could easily detect ingress of air long before it reached 0.1%, never mind 3%.
Likewise, only a fairly substantial hole would let hydrogen out at a rate that would create flammable mixtures outside the tube. You'd obviously have to prevent such holes occurring for structural reasons anyway.-- Cosh i Pi, Jun 01 2007 I'll bun this for now, but:
- what would the tower be useful for?
- what about the wind issue?-- TheLightsAreOnBut, Jun 01 2007 So the tubes are going to be a few mm or more thick? Then what's the point of the hydrogen?
Also, I agree that it would take a long time to mix in even 0.1% air with the hydrogen in bulk. However, any small hole would cause local mixing to a combustion sustainable ratio in seconds or less. Then the slightest spark, a bit of heat, or whatever, and you have a heck of a fire. And this would happen long before anyone could find the leak, let alone do anything about it.
It might be better to leave it a few years, then use diamond-like towers instead, or carefully grown diamond spheres holding nothing at all (a vacuum sphere would be lighter than one with any gas in it).-- Soapy, Jun 01 2007 [Soapy] "...and you have a heck of a fire..." No - not as long as the tube wall isn't further damaged by the fire. A small hydrogen flame won't raise the temperature sufficiently to damage the wall for some time; you could easily have systems in place to detect the heat before that happened.
This is a much less vulnerable piece of engineering than an airship, and the design flaws that led to the Hindenburg disaster (and a couple of others) could have been quite easily avoided even for airships.
The point of the hydrogen is that it's lighter than air. We're talking about tubes tens of metres in diameter here; the weight of air in them would be greater than the weight of the tube. This means that the pressure in the tube need only be slightly more than atmospheric to keep the tube walls in circumferential tension, and in the vertical direction they're in tension, hanging from the bulkheads.-- Cosh i Pi, Jun 01 2007 [To CiP]
The theory that the Hindenburg's semi-thermite skin was the cause of the rapid fire is a total myth, and all the tests done on it prove this: although it played a small factor in the burn, the hydrogen was the main cause of the rapid expansion of the flame and probably would have decimated the craft without the thermite skin involved. [see Mythbusters linky]-- croissantz, Nov 16 2007 i like this. expensive and impractical, sure, but many things are.
wind could be dealt with by making the tube large enough to carry some stronger material on the outside.
the hydrogen-leakage problem could be solved by constantly making more hydrogen from a plant at the base of the tower, and pumping it into the balloons. leaking hydrogen could be continuously flared off to avoid any explosive buildups.-- sninctown, May 29 2008 //the hydrogen was the main cause of the rapid expansion of the flame and probably would have decimated the craft without the thermite skin involved//If the Hindenberg had been decimated there wouldn't have been such a disaster - 90% of the craft would have survived.-- coprocephalous, May 30 2008 Incredible idea. Doubt it will be built due to the pressure maintenance issue, but the idea has beauty. (+)-- MisterQED, May 30 2008 For the Mythbusters test to be meaningful, they would have had to fill with 100% hydrogen a model airship with a fire-resistant skin. Pumping hydrogen into a model airship that's pre-filled with air is obviously going to cause the hydrogen to ignite and burn very well, but I don't think anyone's disputing that.-- supercat, May 30 2008 This is really a wonderful idea if [Cosh I Pi] would just leave the hollow interior filled up with high-pressure air rather than with costly hydrogen or helium that would eventually leaks over time. I have just remembered how rigid yet lightweight my Shimano bicycle cranks with their air-filled HollowTech model are.-- rotary, May 31 2008 No smoking-- pashute, Mar 01 2011 It seems clear to me that this tower should be built at sea, at the equator, in the regions known as the "doldrums". There is little wind there, wind being the bane of a tower such as this. The ample sunlight will heat airfilled rings on the tower, making them lighter. Building it at sea decreases the chance it will hit someone if it falls over. An equatorial tower of great height is just what the space elevator requires.-- bungston, Mar 04 2011 This is undoubtedly a good way of making a very tall tower. However, in order to use the tower habitation/equipment would need to be attached. The structure requires walls which are very thin in relation to the overall size. This would impose upper limits on the weight of anything mounted on the walls.... this could be overcome by making the walls thick enough, or at least with spare capacity to mount large frameworks. To do this you'd have to make it HUGE. Which would require a fairly major purpose.... How big does it need to be before you can make a large portion of it out of something cheap like steel?-- bs0u0155, Mar 06 2011 I can not believe this has not been mentioned yet- gravity as a load is but a joke in structural design loads for skyscapers.
WIND load controls and this design scheme fails. Skyscrapers are gigantic cantilevers.
They don't fall down from flesh wounds when designed for hurricanes.-- Zimmy, Mar 14 2011 random, halfbakery