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Arcologies are never defined by shape. One of the options to consider is constructing "dam arcologies".

Envision the Hoover Dam with most of its non-support structure given over to living and arcology space. The biggest advantage of this scheme is that the arcology would not only have its own, artificial body of water to draw upon, but would also have a ready-made power provided by the dam itself.

Now in this alternate history scenario, dam arcologies are a reality, and they come in three different dimensions. To get an idea, let's take a peek into the three chosen dams from back home:

  • Gavins Point Dam, 23 meters tall, 11 meters wide at the crest and 259 meters wide at the base
  • Hoover Dam, 221.4 meters tall, 14 meters wide at the crest and 200 meters wide at the base
  • Three Gorges Dam, 181 meters tall, 40 meters wide at the crest and 115 meters wide at the base

In this AH, the three listed dams' height and proportion between support and non-support is identical, but the width is greater:

  • Gavins Point Dam, 35 meters wide at the crest and 850 meters wide at the base
  • Hoover Dam, 45 meters wide at the crest and 660 meters wide at the base
  • Three Gorges Dam, 131 meters wide at the crest and 377 meters wide at the base

As quoted above, all non-support structure has been converted into living space. So taking into account the proportion, the height and the width, how many people can reside per dam?

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  • $\begingroup$ Seems like one would need some pretty specific knowledge about the inner workings of each dam to answer this. $\endgroup$ – apaul Jun 8 '17 at 1:14
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    $\begingroup$ What is the blockquote from? It reads like it's part of your narrative, not a quote at all, so I wonder if you’re just applying the markup inappropriately. $\endgroup$ – JDługosz Jun 8 '17 at 10:09
  • $\begingroup$ @JDługosz orbitalvector.com/Megastructures/Arcologies/Arcologies.htm $\endgroup$ – JohnWDailey Jun 8 '17 at 12:17
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    $\begingroup$ The accompanied sketch he draws inspiration from seems to indicate most of the habitats are on the outside, which make sense as there is little space inside the dam. $\endgroup$ – Mormacil Jun 8 '17 at 12:52
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    $\begingroup$ That belons in the post and I see someone else already incorporated it for you. In general, don’t (just) answer review comments with a comment in reply. They are meant to direct you to things that can be improved in the post. $\endgroup$ – JDługosz Jun 8 '17 at 13:35
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There isn't any space in an active dam to convert to living space.

Dams are designed to support an immense mass of water. They aren't hollow. Their entire purpose is to support that load. With the exception of a control room and maintenance access ways, the only parts of a dam that aren't support structure are filled with water and machinery for generating power.

In your world dams are over-engineered to be wider. This doesn't get you any extra living space as is, since as a rule dams aren't hollow. If you relax the proportionality constraint and allow for living space to be built on the downstream side we can do some basic maths to determine how many people could live within. Take the difference, in volume, between your version and their real world equivalents. Then divide by the volume per resident, to determine how many people can live in a particular dam.

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  • $\begingroup$ There are some rather large rooms to house machinery and turbines in the Hoover Dam: en.m.wikipedia.org/wiki/Hoover_Dam#/media/… $\endgroup$ – apaul Jun 8 '17 at 1:22
  • $\begingroup$ @apaul34208 As you can see in this map, that's in the powerplant downstream of the dam. It's not part of the dam proper. $\endgroup$ – sphennings Jun 8 '17 at 1:25
  • $\begingroup$ Ah, my mistake. $\endgroup$ – apaul Jun 8 '17 at 1:26
  • $\begingroup$ @sphennings Then why are there talks of dam arcologies? $\endgroup$ – JohnWDailey Jun 8 '17 at 2:20
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    $\begingroup$ @JohnWDailey I've never heard the term before. Perhaps because it sounds cool. It would be more practical to build conventional buildings near the dam yet outside the flood plain than put apartments on the downstream side of the dam. $\endgroup$ – sphennings Jun 8 '17 at 3:24
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WARNING: Back of the envelope calculation, based on assumptions and heavy guessing, done by a non-engineer.

Typical Forces for a Gravity Dam

As you can see from the picture of a gravity dam, one of its necessary features is being tough and heavy to balance the force of the headwater. In construction, where weight is premium, toughness can be achieved by a sandwich composite structure.

Different compositions

So it would be more or less realistic to assume that one can build a dam using a sandwich composite structure to achieve proper toughness and achieve proper mass through heavy unrelated stuff used in arcology.

OK, but how big?

Presumably a well designed sandwich construction should achieve better toughness than a simple thick dam. Nevertheless, when designing rooms in arcology one would presumably have different priorities than just toughness of the walls. So let's assume that mass would be similar.

I would suggest just to calculate how many people could be housed in a building built of the same amount of concrete.

Hooverdam weights 6,600,000 tons, while Sears tower weight 222,500 tons so roughly counting one could construct a sandwich composite structure having an internal volume of roughly 30 Sears Towers.

Additional remarks:

1) There already is a problem in developed countries that all really good places for big dams are taken. So any arcology would presumably be built in no place, which by standards of early XXIst century is not so good (like example not so much height difference or need to build really big structure for meager amount of water)

2) One of nowadays complains concerning huge dams is that they produce plenty of methane from decomposing organic remnants. By the time you build any arcologies that methane should be harvested and treated as extra gain.

3) All over the world people tend to live in bigger and bigger homes. (Not only US, just that country is leading ;) ) So unless it's a clear distopia, assume that they have plenty of space inside.

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  • $\begingroup$ I'm curious how all this extra effort and cost of creating a complex superstructure can be justified vs building a conventional dam with a collection of conventional apartments adjacent to it. $\endgroup$ – sphennings Jun 8 '17 at 12:01
  • $\begingroup$ Additional remark #1 is moot because this is alternate history. $\endgroup$ – JohnWDailey Jun 8 '17 at 12:18
  • $\begingroup$ You missed a point: Current dams draw, at least part of, their stability from their own weight. You can build a much lighter structure with the same mechanical strength, but that would be swept away literally floating over the outrushing river. You could anchor the dam on the sides, but that would be a completely different design, strongly depending on their geology (almost unfeasible in a lot of current dam locations). $\endgroup$ – ZioByte Jun 8 '17 at 16:21
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The politics and purpose of your arco would significantly impact the number of residents that would be packed into it.

In my mind, the default picture for arcos is the hippie utopian 1970s Arcosanti, and I immediately imagined a clean city with apartment balconies over a lake. Any ugly industry gets tucked away inside the dam and the residential population is limited by air/light exposure…

Paolo Soleri

No, wait. That is an architect pipedream.... An actual arco is going to be constructed for a purpose. It's a machine with a built-in power source. Industry would need the outside wall for venting CO2 or whatever byproduct it produces. It probably has a large industrial conveyor lift to bridge cargo from the lake above to the river below. The population is limited to only the required workers, managers, and their families. The people serve the machine, like an oil rig. The costs to provide for them will push the number of residents to a minimum.

Moloch from Metropolis (1927)

You could take the fragile ecology/minimum human footprint scenario to opposite extremes. If the goal is to gather up all the people into one building there would be pressure to pack as many people as physically possible into every cubic meter of the structure. Workers live in crowded dorms. Only the elite would have private apartments, but even those would be tiny. If the purpose of the arco is to keep as many people as possible alive in one concentrated space, I think we have not seen the limits of where we can go.

enter image description here

Even in a utopian/socialist future where a residential megastructure also serves a utilitarian function, there is a negotiated balance between space devoted to industry and space devoted to community housing. Those needs might change over time leading to overcrowding or reclaiming of industrial space. Was the dam financed by luxury condos, or is this low-income project housing in exchange for pollution credits?

Hugh Ferris – Metropolis of Tomorrow

It's not volume or surface area that defines the population, but the purpose of the arco, and politics of the society who built it. You mention an "alternate history scenario" but you don't offer any hint how it is different from today. In reality humans are not like logs that can be stacked into a defined volume. Humans will adapt to overcrowding, and "personal space" is based on income and real estate values.

You asked for reality check. You should decide WHY the arco was built (presumably to accommodate a denser population than "an apartment building adjacent to a dam". Zone your arco's industrial areas and subtract them from the volume. The space left over is available for residents and public areas. Depending on where in the world your dam is, it will have different legal definitions of Overcrowding. If your goal is maximum number of dwellers start with existing overcrowding/zoning regulations and make it more dense. Kowloon City for example was unregulated but obv not designed as an efficient arco. Cruise ships can be very dense and still comfortable, but they are almost the opposite spectrum of a self-sustainable arcology. Also look at Chungking Mansions and Mirador Mansions budget hotels in HK for examples of very small private living quarters. After that you have SRO hotels with shared bathrooms, and then hostels and dormitories. Your arco might have any mix of these with the ability to reconfigure as housing needs fluctuate.

Without some idea of what you consider suitable space for your population there isn't a hard answer. Social and legal pressures would decide the limits, not architects.

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    $\begingroup$ That does not answer the question. $\endgroup$ – JohnWDailey Jun 8 '17 at 16:34
  • $\begingroup$ @JohnWDailey I'm saying you have not provided the information necessary to answer the question. As I've shown, the number is extremely flexible. It is not a simple math problem of volumes. Define how much personal space you are providing for each person and what kind of living conditions you are allowing them. $\endgroup$ – wetcircuit Jun 8 '17 at 16:41
  • $\begingroup$ This is a fascinating analysis and critique of arcologies. The crux of the question, however, is asking about population. Could you highlight that more prominently in your answer? $\endgroup$ – sphennings Jun 8 '17 at 16:43
  • $\begingroup$ If this is a math problem, not a worldbuilding discussion, may I suggest math.stackexchange.com $\endgroup$ – wetcircuit Jun 8 '17 at 17:26
  • $\begingroup$ @wetcircuit Worlds do not get built out of a vacuum. $\endgroup$ – JohnWDailey Jun 8 '17 at 18:03

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