Imagine building a city where previously there was none. Imagine building much of it as a single structure. That's what's being undertaken in China, Dubai, Saudi Arabia, and most recently, Russia. Undertaken doesn't necessarily mean "assured." Real estate can be a good investment though, and the people that tend to live in 1/2 mile high towers do tend to have some cash to spend so such plans aren't necessarily as crazy as they look and sound.
Now that the bar has been set, what would it take for the United States to play this game?
The Burj Dubai will be a science-fiction-flavored 828 meters tall when completed. Let's ask a very serious question. What would it take to build something twice that tall- a mile in height? 1610 meters. Or, while we're up there, what would it take to build something 1776 meters tall? That almost exactly 110% of one mile.
First, let's divest ourselves of the fantasy of doing it the "old fashioned" way. There are no traditional building methods that would allow such an undertaking. The methods used in the Burj Dubai are already maxed out. Otherwise, they would have gone higher.
The answer is to depart from the requirement that the building be "free standing." Instead, you support it in tension, like the third tallest structure in the world, this 2000ft+ tall radio mast.
A radio mast like this is actually an invisible pyramid of high tension lines which extend outward as odd-numbered spokes from the single central compression member of the tower itself. The need for stiffness is all but eliminated using this method, which saves an incredible amount of weight that would otherwise be required. The central mast needs only be stiff enough to resist the forces exerted by the wind operating on the spans between where the tension lines are connected. And it need only be strong enough to support its own weight and the weight of the cables pulling down on it. Such radio masts have almost no payload- just a few hundred pounds of radio antennas.
But the idea is scalable. Take a look at one of the anchors for the above-mentioned radio mast. Notice that three cables come from a single point, attaching to three elevations of the mast itself. At first glance you might think, "that's big." But really, it isn't. It's actually "just big enough." If it needed to be bigger, it would be bigger. There are no physical laws that say that you can't build a bigger anchor.
Here's what I have in mind. First, start with a 450 meter free-standing monolithic base. Most of the salable real estate will be found inside. From there, continue upward with a tapering tower that is with another 850 meters of usable and semi-usable payload- airship apartments that one would be serviced by elevators operating at near freefall speeds. The final 476 meters would be built to support a single observation deck.
The wind would exert hurricane force on the upper reaches. By placing numerous wind turbines on the upper structure, some of that lateral force could be transformed. The added weight of the turbines would cancel most of the benefit of doing this, however, what would be gained in electricity would probably power much of the structure's needs. Individual turbines might be pinwheel sized. Too large and their blade lengths would extend beyond the width of the tower itself which would actually increase the wind load.
Power could also be generated by harnessing the variable tension moments in the high tension lines. This would involve building linear hydraulic pistons into the anchors. The pistons would lift a multi-ton weight. By diverting some of the pressure thus stored, a turbine could be driven, resulting in additional energy gains. The same system could be used to stiffen or loosen one side of the tower's supports, allowing the structure to lean into the wind and thereby remain upright.
The uppermost diameter of such a structure might be only 5 meters. It would be almost entirely made of steel.
Changing the method of construction multiple times from foundation to point would be analogous to a multi-stage rocket. What works close to the ground is different from what works higher up. People buying real estate would be buying the prestige of living in the tallest building in the world, not living in the highest part of it- a situation that would be far from desirable anyway. Above a certain significant height, only the most courageous would feel comfortable anyway. And the amount of space available would diminish such that the daring billionaire might find herself living in something like a lighthouse's arrangement of stacked rooms.
I should say something about it's sensitivity to attack and how it would need to be built to accommodate certain types of failures that would be unlikely except as acts of sabotage.
The first 450 meters would be immune to all but bomb attack. Cutting all the tension lines would not affect it. The next 850 meters would be susceptible only to cutting multiple tension lines and would feature two modes of emergency escape. One mode would involve going down the elevators. Another would involve riding down the tension lines in small enclosed gondolas equipped with speed-activated brakes. The uppermost observation deck would be similarly equipped, only the gondolas would (unfortunately) be even lighter. The structure would need to be built to withstand 200mph winds and the loss of a random assortment of cables. Each anchor would be guarded. Flying an airplane into the tension lines would usually result in simply cutting the aircraft into kites. The resulting shockwave would be absorbed using the active hydraulics mentioned above.
What would it look like?
Imagine something like the Eiffel Tower, only- instead of four sides- it would have an odd-number of sides. The number of sides would gradually decrease as you got higher and higher as features in the outer facade merge. It might have a spiral appearance, much like the Crystal Tower concept. I have a quite different design paradigm in mind that might be applied- I'll elaborate in a future post. In any event, it would have perfectly smooth transitions. You could not tell by looking at it where one construction paradigm begins and another ends. And it would be lit up. The tension lines might be lit with millions of lights. Because of the foreshortening of perspective at such great distances, the top would taper such that, when one is standing withing the tension umbrella, it would appear to extend upward infinitely. The tower's actual vanishing point would be scant arcminutes from the top of the tower.
Where would such a structure be built? California is likely out of the question because of the earthquake risk. In fact, the entire West Coast might be out. Shame that. Not that CA laws would ever allow it. It should also be build away from the most common paths of hurricanes, so the Gulf Coast is also mostly out of the question. Although, it is already being built strong enough to withstand hurricane winds. What's an actual hurricane?
Building too far north involves a reduced building season. Too close to NY City would involve air traffic interference. In terms of real estate value, perhaps the best placee to build would be within sight of Manhattan. Too close to Manhattan and it would do injury to the skyline aesthetic. But looking down on Manhattan would be worth paying for. How much? Between $30k and $150k a square foot.
The location question is something that would take a lot more thought.
What would it cost to build such a structure?
Well, the design and engineering costs would come to between ten and twenty million dollars. To purchase and prepare the building site would require tens or hundreds of millions of dollars. It would incorporate around a million tons of steel (more likely more than less). That's some $1B in raw material costs. Raw construction would cost around $8B. Finishing it would cost at least another $5B. Around $16B in a perfect world. Actual cost is likely to be about twice that. To be profitable, it would need to attract businesses and residents from an area larger than its viewing radius. It would need to compete with Dubai for billionaires.
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