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A group of people from an alternate universe want to extract resources from the remains of a post apocalyptic US. However, a quite large quantity of these resources are locked up in large buildings. The problem is how to turn these large buildings into material that can feasibly be taken away for processing into all the things inside the building.

In real life demolition is very expensive, expensive enough that it's realistically impossible to make a profit off recycling the materials.

However, in real life you also have to worry about things like environmental regulations and damaging surrounding buildings. In this case where large areas are quite simply being torn down for raw materials in an entirely separate world from the one people are living in neither of these are major issues. The only thing that needs to be worried about is taking buildings and turning them into chunks that can be broken down into various metals and construction aggregates.

So what would the cheapest way to destroy large amounts of structures en masse be when the only major concern is turning buildings into raw material?

The technology of this other group coming in to demolish the buildings is only really at that of the 1920s/1930s. They are not from the post apocalyptic world and are from a world that is generally functional and intact. They have access to anything you could expect would be accessible within the funds of a moderately large industrial firm from that time period.

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    $\begingroup$ Is there a reason why Explosives can't be used? $\endgroup$ Mar 7 at 7:43
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    $\begingroup$ I came in like a wrecking ball $\endgroup$
    – user6760
    Mar 7 at 7:53
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    $\begingroup$ What is/are the priority material/s to be recovered? (Noting that concrete is formed by a chemical reaction and cannot be feasibly recycled back into the sand, aggregate, cement and water that it was formed from.) What are they doing with the "useless" bits, especially concrete? What is the maximum size of "chunk" for taking away for processing? (Have to fit in an open-top railway car?) $\endgroup$ Mar 7 at 8:20
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    $\begingroup$ The cheapest way is of course to do nothing at all, and simply wait a few decades or centuries for the buildings to fall over on their own. But I imagine we're working within some time constraint. $\endgroup$ Mar 7 at 15:31
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    $\begingroup$ Just wait a 1000 years. and everything will be on the ground. It's cheaper and cleaner than bombs. $\endgroup$
    – Tony Ennis
    Mar 7 at 20:25

11 Answers 11

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You Ignore the Buildings Almost Entirely

OP says in a comment that the goal is to extract steel and copper. To do that, you leave the buildings standing, and focus on pretty much everything else.

Sky scrapers are mostly made of concrete, but electrical substations are made basically entirely of steel and copper. Find those, and carefully deconstruct them. Find the power lines, and steal those too.

Similarly, cars, buses, trains, train tracks, etc. are all mostly made of useful metals. Now-a-days cars are mostly made of aluminum, which is probably more valuable pound for pound than steel to a 1930s era civilization. But even modern ICE vehicles have steel in the engine block, so you'll get both.

Inside of sky scrapers, there are probably some places you could access high value metals that are far easier to get to before the building falls over. I'm thinking of boilers, air conditioners, pumps and fans, elevator cars, etc. These are all probably easier to find, break apart individually, and remove if the building is still standing.

Leave the concrete buildings alone, and focus on finding and extracting big piles of useful material that already exist.

EDIT: A great example of this is parking garages - most big buildings have multistory parking garages in the bottom floors. Is it easier to scrap all the valuable cars as is, or is it easier to drop a building on them, and then try to scrap them?

EDIT 2: Oh man! Ports! Ships, shipping containers, and heavy machinery - cranes, bobcats, trains, trucks, etc. Even if all the cargo is trashed (and it's probably not, shipping containers are tough) hitting up a good port, like the Port of New York and New Jersey would be a gold mine for your scrappers. Assuming the average container takes a week to process through the port, there would be ~60,000 containers and over 10,000 vehicles at PoNYaNJ.

And bulk cargo often includes... scrap metal and raw ore! Definitely look to the ports.

(Airports too - a 1930s civ would LOVE the aluminum you could get out of all the planes on the ground at a major airport!)

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    $\begingroup$ "aluminum, which is probably more valuable pound for pound than steel to a 1930s era civilization". Oh heck, yeah. In our history, aluminum became huge in aviation in the pre-war years, especially the 1920's and 1930's. $\endgroup$ Mar 7 at 20:19
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    $\begingroup$ I think your assumption about skyscrapers is wrong - modern skyscrapers, and even many smaller buildings typically are constructed with a reinforced concrete utility core, connected to a steel frame which supports curtain-walls, cast concrete floor plates, and steel stud interior walls. Even if we ignore the reinforcing rods in the concrete there is actually a good amount of metal that would not be terribly difficult to recover - if you don't mind tossing the glass panels to the ground below (fun!), even a small team with hand tools could make good work of it. $\endgroup$
    – ghostly_s
    Mar 8 at 0:14
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    $\begingroup$ I guess I should clarify, harvesting the steel frame would be non-trivial without demolishing the whole structure in a conventional manner. But the curtain walls and interior walls could be safely stripped and would amount to a wealth of material, potentially in more practical form for re-use than melting down engine blocks, boilers etc. $\endgroup$
    – ghostly_s
    Mar 8 at 0:59
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    $\begingroup$ It's a reasonable challenge, but depending on the apocalypse all the easy to access stuff could already be gone. The reason this corporation can still harvest materials is because they have the resourses to collect them from where roaming scavengers can't. $\endgroup$
    – Separatrix
    Mar 8 at 9:51
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    $\begingroup$ @OT-64SKOT - there are ~280 million vehicles registered in the US, and 120,000 miles of copper transmission lines, and 160,000 miles of train tracks. Assuming 10% of those are easily recoverable after the apocalypse, how long will it take a 1930s era civilization to scrap 30 million vehicles, 12,000 miles of copper wire, and 16,000 miles of iron rail? There's decades worth of easy scrap to go through before you think about knocking over buildings and sifting through the rubble. $\endgroup$
    – codeMonkey
    Mar 8 at 14:29
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Frame challenge: they wouldn't go around demolishing buildings.

It makes no sense to demolish buildings to gather resources. Majority of mass is in concrete which is useless. You would never finance an extra-dimensional expedition for that. If they wanted raw resources they would just loot the warehouses containing those materials: raw steel, cement, chemicals, wire,...

Not that harvesting materials is something you would want to do, except for really valuable stuff. What you would go for instead is knowledge. They are a century behind the post-apocaliptic society. An university library is worth far more than all the gold in the world. If they loot an army base/arms manufacturer they would get a military edge big enough to conquer their world! In any case both their tech level and living standard would increase drastically in the next decade.

Recycling buildings would be literally the last thing any sane salvager would do when having access to a depopulated civilization century ahead of you.

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    $\begingroup$ "It makes no sense to demolish buildings to gather resources. Majority of mass is in concrete which is useless" -- and yet miners routinely dig through large amounts of otherwise useless rock to extract comparatively small amounts of metal ore, which is also of not much use on its own until the metal is extraced from it in turn. Why would it be any different with human-made concrete? At the very least, the rebar inside it already is made of metal. $\endgroup$
    – ihaveideas
    Mar 7 at 10:27
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    $\begingroup$ You could just loot the warehouses but that doesn't really contain all that much in the way of resources vs everything else. While they can take knowledge (and what they do grab puts them ahead quite a bit) what they're allowed to take is heavily limited by the outside group providing the access to interdimensional travel which is a few decades ahead of modern day. $\endgroup$
    – OT-64 SKOT
    Mar 7 at 10:57
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    $\begingroup$ @ihaveideas I would wager it is still more efficient to mine for metal than recylcle skyscrapers. We after all mine, and put demolished buildings into landfills... And OT-64 SKOS, you are underestimate how much resources we have in warehouses, and drastically overestimate how much you can harvest from buildings. Even without counting how much effort is to extract resources from demolished buildings. $\endgroup$
    – Negdo
    Mar 7 at 11:07
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    $\begingroup$ @ihaveideas What make you think that it is easier to mine skyscrapers than a normal mine? And in the 30s they used gas and oil already. We had cars then. Hell, Germans had plans for rockets in 1935. $\endgroup$
    – Negdo
    Mar 7 at 11:50
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    $\begingroup$ @ihaveideas And you should open a history book... The question stated that salvagers have access to 30s technology. Not the technology of isolated subsaharan african tribes. And electric train was invented in the 30s. 1830s... In Italy they had a 100 km long line for an electric train in 1902. Diesel engines were a bit later, but in 1924 they had them in Soviet Russia, at the time one of the least developed European countries. As for cars: there were 7.5 mil registered cars in the USA... in 1919. 75% of them in small (<50k people) cities. $\endgroup$
    – Negdo
    Mar 7 at 14:55
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Selective demolition is already as cheap as it can be. Even if it was to be made unselective, in that the demolition crews were just razing entire districts, demolition wouldn't be all that much cheaper.

If we're talking about individual houses, excavators are probably the quickest and cheapest way to demolish them, though excavators don't necessarily result in the greatest amount of recoverable materials.

However, if we're talking about skyscrapers, demolishing them safely is a far different matter. Simply excavating their foundations will eventually bring them down, but that would be extremely unsafe for the workers. Explosives could be used, but large buildings are designed to resist collapse, and there is a lot of preparation work involved in the explosive demolition of a large building, weakening and demolishing smaller parts of the structure, or a lot of no doubt expensive explosives could be used.

Of course, if explosive demolition is used, the buildings could be collapsed onto neighbouring buildings, but there is no guarantee that a building struck by a falling building would collapse itself, and failure to collapse would make demolition of the damaged building less safe.

So, unless these alternate universe people have nukes (which they probably shouldn't with a 1920's-30's tech level), there is probably no quick, easy, cheap and safe way to demolish large numbers of buildings for reclamation... not that even nukes would make demolition safe.

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    $\begingroup$ Agree (+1) except for the comment about nukes - even with an airburst there will be radiation concerns and there will be more buildings in the damaged-and-now-unsafe-to-finish-demolishing category than get knocked down cleanly. $\endgroup$ Mar 7 at 9:44
  • $\begingroup$ @KerrAvon2055 I agree with the lack of safety with nukes. $\endgroup$
    – Monty Wild
    Mar 7 at 9:46
  • $\begingroup$ Most of the difficulty with demolishing a skyscraper comes from the fact that we don't want the skyscraper to take out its neighbors... If that isn't a concern then knocking a skyscraper down with explosives is trivial. $\endgroup$
    – Questor
    Mar 7 at 17:36
  • $\begingroup$ Maybe safety isn't much of a concern for them. Perhaps they are like ants or bees, capable of producing workers in large quantities. Or they may be using slave labour. $\endgroup$
    – Abigail
    Mar 8 at 17:10
  • $\begingroup$ Skyscrapers are designed to resist extremely powerful winds. When they fall, they fall inward, not sideways. (Exceptions are rare.) @Questor $\endgroup$
    – Zautech
    Mar 8 at 18:05
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The cheapest way would probably be to divert the nearest river into the buildings that need to be demolished. It would take a while, but the technical skill needed to divert even a moderate sized river for irrigation has been around since ancient times, and any city is bound to be somewhat near a source of freshwater. For a single building, this would be very inefficient time-wise, but with proper diversion you can take out several city blocks with a single, fast flowing stream of water sustained over several days.

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    $\begingroup$ Water is unpredictable, and concrete can resist flowing water for quite some time, perhaps years. This seems quite unreliable. $\endgroup$
    – Monty Wild
    Mar 7 at 8:58
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    $\begingroup$ The question states that damage to the surrounding area isn't a concern, so I don't see the unpredictability as a problem. And concrete supports designed to resist flowing water, like that used for bridges, are very different from the relatively much smaller concrete supports used in most high-rises. Just look at the complete collapse of buildings during a flash flood. $\endgroup$ Mar 7 at 10:08
  • $\begingroup$ Then you have to un-divert the river unless you want to train underwater welders to harvest your materials. Doesn't seem at all efficient to me. $\endgroup$
    – ghostly_s
    Mar 8 at 0:16
  • $\begingroup$ @AnthonyKhodanian Concrete will withstand water for decades, not years. Floods destroy buildings through loss of foundations in muddy climates, but you have to be there to begin with. $\endgroup$
    – Therac
    Mar 8 at 1:37
  • $\begingroup$ If the goal is to smash the buildings, then undermining the foundations with water would do quite a bit of the destruction for you. Miners in the 19th century used hoses from streams to demolish entire mountains to extract the gold. en.wikipedia.org/wiki/…. $\endgroup$
    – DWKraus
    Mar 8 at 3:51
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As others have pointed out, wrecking balls and explosives are still used because they're the most efficient solutions. Most construction waste is in fact "recycled" (more properly, the concrete is downcycled into aggregate), even if not profitably, because you have to put it somewhere.

What keeps tearing down buildings to sell for scrap from being a profitable enterprise isn't environmental regulations or concern about damaging the surroundings, it is the economic conditions in which a standing building is more valuable than its raw materials.

So assuming those raw materials are so valuable to your dimensional travelers as to change that equation, as others have pointed out there are more readily-available caches of these materials to harvest first: warehouse stock, electrical infrastructure, steel-frame warehouses and the industrial equipment within them, equipment for the various mechanical systems within buildings, often conveniently located near ground level, maybe even digging up sewer pipe.

But if we suppose they exhaust those first, and have a preference for intact metal elements suited for re-use rather than re-forging scrap (not sure if the particular steel and aluminum alloys used today were known in the 1930's? or perhaps energy is incredibly expensive for them?), they might then start stripping the curtain walls and interior steel stud partition walls of high-rises, which could be done easily enough with a small crew and hand tools. Most modern high-rises and even many low buildings are constructed such that these elements are non-structural.

But now I'm curious, if that's not enough and they want to start extracting steel frame elements with minimal damage too, perhaps a sufficiently reckless demo team could pull all the bolts connecting the outer colonnade of a hull and core high-rise to the poured concrete floor slabs, tie some ropes to the loose posts at the bottom, and start yanking them out until the whole thing topples like a Jenga tower, hopefully ejecting many of the girders outward? So long as you're not on the unbolting crew it would at least be fun to try!

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Overheat them

Remember 9/11? If what has actually happened was direct overheat of concrete at strongpoints, this is what should actually do the job for tall structures. Select a point at about 80% height, and light a bonfire out of anthracite encompassing several floors. After about half a day the strongpoint would give in, and the top would drop into the building collapsing it.

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    $\begingroup$ That would only work (potentially) for buildings sharing the same structural build than the twin towers, which is not the majority of high rise buildings. Besides, a large number of structural engineers are still not convinced that this is what happened exactly/only. For the other types of buildings, just google large buidling fires and count how many actually collapsed ... you'll have enough fingers on one hand. Fire in buildings is very dangerous for humans, but not really for the building structure. $\endgroup$
    – Hoki
    Mar 8 at 12:41
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In addition to explosives, you could equip some workers with oxygen lances, which can cut through thick steel beams and concrete like butter, with showers of sparks and flames to add to the excitement.

It requires a supply of metal rods and oxygen (preferably liquid oxygen) in quantity however that should be well within reach of your 1920-30s technology.

The same technology is used in South Asia to break giant ships by workers with minimal equipment (including minimal PPE).

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You can make use of a lot of seemingly useless equipment.

I'd actually go with 'careful' salvage of materials you can't easily make. Something like copper and aluminium's probably easier to strip pre destruction, and electronics and such is probably easier/more portable to remove, then process alone.

Even a post apocalyptic wasteland might have things you can't trivially make with 20s tech - large windows (if unbroken)

Depending on the size of the buildings, explosives to do implosions, or undermining the base of the buildings and making them fall over will let gravity to do the bulk of the work.

Electromagnets would pick up ferrous materials. You can use trains to move anything non ferrous, and break it up with a ballmill to sort out anything you didn't salvage. This incidentally lets the materials do a lot of the breaking up, and you can sort out chunks manually.

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The unfortunate answer is to use forced labor. This removes several categories of cost from the company that commissioned the job. Wages are a non-issue; so is workplace safety; and you no longer have to worry about your employees leaving for a different job. (Yes, I am aware that these costs are not 'removed' but simply dumped onto the forced laborers; the company saves money all the same and needs not care.) There is in fact a historical precedent for major engineering projects primarily using forced labor in the very 1930, and therefore using that period's technology by definition. The results were of questionable quality to be sure, but for a demolition project that's not really a consideration... ;)

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    $\begingroup$ Forced labour using what tools, though? You can't knock down a skyscraper with picks and shovels no matter how many people you throw at the job. $\endgroup$ Mar 7 at 11:21
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    $\begingroup$ @KerrAvon2055, thank you. You don't need even that just to knock down a skyscraper; you just pile up a lot of fuel at the bottom and set it on fire. This will eventually heat the metal supports to the point that they are overwhelmed by the weight of the building above and the skyscraper will collapse, conveniently breaking into smaller chunks in the process. Then once the worst of the fire dies down, you break it all down with picks, shovels and perhaps an occasional acetylene torch. $\endgroup$
    – ihaveideas
    Mar 7 at 11:54
  • $\begingroup$ @KerrAvon2055 You know what they say about brute force... $\endgroup$
    – DKNguyen
    Mar 7 at 21:20
  • $\begingroup$ @ihaveideas Doesn't concrete support the weight, but steel protects from stress? Like if you only had steel it would collapse, but with only concrete it would just crack if shook? $\endgroup$
    – Zautech
    Mar 8 at 18:11
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We build the cities, we can make them go away too. No nukes of course, cause that would be cheating. While we are at it- lets stay civilized in scale of destruction all together- no planetary bombardment, thats the spirit.

Cheap: Recombinable drones

Artillery, drones, rockets are expensive and artisanal. Why not instead - standardize all the components into 1 Legobrick size- and then make them re-combinable? So you can destroy your opponents, with ever changing drones, tactics and implements. Slice a sentry turret into a thousand pieces, fly it in as drones, click it together. I added that to my game and it basically is able to replace most military equipment if you do it properly. For additional stability, add girder bricks, fill hollow bricks with PU-foam, and wrap the whole affair in foils. Best part is, if the parts are designed properly, robots can handle them and assemble your killer-robots. Universal constructor for universal destruction has go. A doglike robot build from standardized pieces

Cheaper: Remote aka Artillery:

You need very cheap artillery, as in no barrel attrition with precise ammunition. You can have that- with coilguns and basically glide bombs to lob. If that is to uncontrolled for you, you can launch actual small gliders with the coil guns, that glide to a target area and hover in large clouds over them. If they get a target designation, they simply fold there wings and come whistling down. For optimal destruction, you want the gun-cotton gliders to explode in two stages, one basically drilling a hole, into which the rest of the bird slams, the tail section sealing it up and blowing all things out of proportion.

Cheapest: Accelerated steel rusting

An alternative is- artificial rusting. Many concrete buildings have steel and moisture inside them- all it takes is one crack and the falling apart by rust expansion begins. Now if you take two small drones and drill down towards the steel, you can provide air via electrolysis (O + 2H) - and then reversing the effect, speed rust the whole structure from within, doing one final refreshing crack from which nothing ever came back. As this takes time (and eventually water), you might want to conceal the drones as infrastructure boxes.

https://en.wikipedia.org/wiki/Galvanic_anode

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  • $\begingroup$ The technology available for taking the cities down in this case is only 1920s/1930s. So no advanced computers. $\endgroup$
    – OT-64 SKOT
    Mar 10 at 11:29
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so, how precise do you want to be? Because I can see a solution that's totally great:

Bagger 288

It's a large-scale bucket wheel excavator. Currently in use to dig up brown coal in an open-pit mine this massive steel leviathan would be very good at turning buildings into easily siftable rubble. bagger 288, a long boom with a rotating bucket wheel on a large track assembly

at several hundred metres long this machine could roll down wide american streets wreaking utter devastation.

Add an artificial mind and let's roll.

For more information see this documentary from the ancient internet

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    $\begingroup$ What happens when the buckets get caught in the reinforcing steel in concrete, or if it's a steel-framed building? It'd probably become jammed at best. $\endgroup$
    – Monty Wild
    Mar 7 at 8:56
  • $\begingroup$ I don't think you get how absolutely massive the Bagger is. the bucket wheel is 21 metres diameter and the machine runs on 16 megawatts of power. Sure there are some walls you can't get through but that will only be when demolishing very large skyscrapers or heavy industial factories. $\endgroup$
    – Borgh
    Mar 7 at 9:03
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    $\begingroup$ It also dates from the 1970s... a bit anachronistic for people with 1920s-30s tech. $\endgroup$
    – Monty Wild
    Mar 7 at 9:04
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    $\begingroup$ @MontyWild It is technically feasible with 1930's tech. Nothing in those things requires post WWII technology. (No advanced electronics needed.) $\endgroup$
    – Tonny
    Mar 7 at 14:11
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    $\begingroup$ The size isn't really the relevant variable, it's designed to dig through soil. No matter how large a spoon you build it will never be able to take a scoop out of a block of reinforced concrete. $\endgroup$
    – ghostly_s
    Mar 8 at 1:06

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