What would be the cheapest way to demolish large amounts of buildings when damage to the surrounding area isn't a concern?

Question

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.

Answer 1

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!)

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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!

Answer 6

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.

Answer 7

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).

Answer 8

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.

Answer 9

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... ;)

Answer 10

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.

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

Answer 11

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.

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