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Roughly 120 years after a nuclear apocalypse, an external group that didn't experience the apocalypse (the why of it isn't going to be explained here) came to the land that did, to pull apart their cities for scrap metal. How much steel could be recovered from those cities?

The area in question that they're pulling apart for steel is the Northeast US and some of Canada. The civilization that blew itself up was at a similar technological level to our modern day although with more advanced robotics, the people coming to pull the city apart for scrap are only at a roughly ~1920s-early 1940s level of technology although with a greater understanding of radiation and it's dangers and the equipment to mitigate it. They are from an unaffected large, industrial state that got to earth through means that would distract from the main point of the question.

The nuclear war that caused the apocalypse mostly involved weapons around the same size to slightly smaller than what we have now, but most of the bombs were cobalt salted.

The main cities being pulled apart by this outside organization are the cities of the north east US the Golden Horseshoe of Canada, Montreal, and Los Angeles, however the population of the US is closer to 410 than 330 million. The notable exceptions are Washington DC and NYC in the Northeast US as they were too heavily targeted (although the outskirts of NYC are still able to be pulled apart) and Pittsburg as being squished between a bunch of mountains makes to hard to recover resources from it.

The problem is how much steel would there be in those cities, and how much would still be able to be recycled with interwar technology after rusting away for 120 years?

Edit: a big part of the question is how much steel is in a city to begin with, not just how much would be lost

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  • $\begingroup$ related: worldbuilding.stackexchange.com/q/91810/30492 $\endgroup$
    – L.Dutch
    Commented Dec 4, 2023 at 5:24
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    $\begingroup$ We've answered a fair number of questions like this over the years. VTC:Duplicate. Are you suggesting the population of the U.S. in your world is ~410 million? Post apocalypse? And only 1920 tech? Unless you're commenting on the population before the apocalypse, that's too many people for such low tech. 99.9% of today's tech was invented in the last 150 years with fewer than that many people. And rust doesn't ruin metal - it simply needs smelting. The metal atoms are still metal atoms. $\endgroup$
    – JBH
    Commented Dec 4, 2023 at 11:00
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    $\begingroup$ And per all those other questions... the amount of metal is vast. You're just mining cities and landfills rather than mountains. The only metal missing is whatever was converted to energy at the center of the nuclear explosions. It won't be a significant fraction. $\endgroup$
    – JBH
    Commented Dec 4, 2023 at 11:02
  • $\begingroup$ @JBH the entire situation is compicated. The US in this world was at a similar level to or own if a bit more advanced, historical factors lead to the larger population. The people pulling the cities apart with a 1920s level of tech are from an outside large, industrial civilization not effected by the destruction of earth in nuclear hellfire. There's also the problem of how much steel is in a city to begin with. $\endgroup$
    – OT-64 SKOT
    Commented Dec 4, 2023 at 13:10
  • $\begingroup$ @JBH how much steel *specifically is in a city because that's important for determining how much can be extracted for how long and how viable this would be compared to just mining ore normally $\endgroup$
    – OT-64 SKOT
    Commented Dec 4, 2023 at 13:37

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Rusted steel is not a problem for metallurgy. Smelting is precisely the process of turning iron oxide into metallic iron. In principle 100% of the available rusted iron alloy is recyclable.

The limiting factor will be having the tools to cut the available structures into manageable chunks and the means and infrastructure to transport them: think of construction beams used in bridges or buildings, which might be too large and heavy to be transported and smelted.

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  • $\begingroup$ You might also want to expand this to include infrastructure. Post-apoc the road network is going to be trashed, especially if people are coming in and taking bridges apart for their steel beams. Unless they have cheap flight with VTOL any large scale extraction is going to have problems. $\endgroup$
    – aslum
    Commented Dec 4, 2023 at 15:16
  • $\begingroup$ Rusted steel /is/ a problem: it crumbles and blows away. Leave things long enough and the entire surface of the planet will be red-tinged. $\endgroup$ Commented Dec 4, 2023 at 15:47
  • $\begingroup$ @aslum the people taking things apart are on outside, well developed industrial society. They can build their own transport infrastructure to move the steel to port, (mostly railways in this case) they don't have to rely on what there already. Their in this not out of necessity (their world still has enough iron ore) but profit motive. $\endgroup$
    – OT-64 SKOT
    Commented Dec 5, 2023 at 0:52
  • $\begingroup$ Are you saying that if you have 1 tonne of steel, then it all rusts without blowing away, resulting in 1 tonne of rusted steel, you could smelt it to re-produce the original 1 tonne of steel? So in other words - rusting metal does not reduce the amount of material available after recycling? $\endgroup$
    – Web Dev
    Commented Dec 5, 2023 at 3:54
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    $\begingroup$ @WebDev If one tonne of steel rusts completely, you now have something that weighs considerably more than one tonne, as the iron in the steel has oxidised, typically to the ratio of adding 3 oxygen atoms to every two iron atoms. But smelt it, and you remove the oxygen and get back to the iron (some of the impurities that made it steel, like carbon, may also have been removed). $\endgroup$ Commented Dec 5, 2023 at 10:05
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Let's look at ore grades to see if they would bother.

So, according to wikipedia, to be viable, an iron ore mine needs ore with 25% iron. To produce this, they expect 3 tonnes of regular stuff will need to be removed for every 1 tonne of iron ore. So, dividing 25% by 3, we can expect ground with 8.33% iron ore to be worth mining. So, is a city worth it, with iron, as Vesper's answer points out, making up 9% of the mass of the cities rubble?

Yes - but maybe not with 1920s techniques. We've got better at mining, but the deposits we're working with have got less rich. A new civilization might find it easier to open existing mines, and not deal with the radiation.

But wait! Iron is probably the least valuable thing to come out of a strip mined city. Instead, we're getting a tonne of hard to refine resources.

Aluminium - skyscrapers use a bunch, in window frames and similar. It takes a long time to degrade, and is insanely expensive to refine. Your new civilization might find it easier to raid recycling plants for cans, smash up cars, and drag old skyscraper window frames out of piles of rubble than grind up concrete for rebar.

Copper - copper is rare, we've mined a whole bunch of it, and it's everywhere in cities, from building pipes to electrical systems. It's very useful for a 1920s era civilization - any time you need two super precise connections in the 1920s, like a piston in ships or engines, you need brass. For brass you need copper.

Lead - lots of lead on roofs, still, and it doesn't break down fast. If you're dealing with radiation, lead is very valuable.

Glass - glass takes effort, clear glass takes relatively rare resources. But picking over a ruined skyscraper will give you tonnes of glass, which can be remelted.

In conclusion - strip mining for iron or steel, probably not worth it. Strip mining for everything in a city? Probably extremely useful. But you might find better iron sources elsewhere.

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    $\begingroup$ The 25% ore grade is for iron oxide. The steel in cities, even with some surface rust, will be far more reduced. $\endgroup$
    – MSalters
    Commented Dec 4, 2023 at 13:57
  • $\begingroup$ @MSalters useful! I wasn't sure on that point - but if we're refining it anyway, do we care? I guess you need more stuff to reduce it if it is completely iron oxide, making it harder $\endgroup$
    – lupe
    Commented Dec 4, 2023 at 14:31
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    $\begingroup$ The reason is that you typically need coal to reduce iron oxide. Basically, you heat up iron oxide with something that wants to burn, but don't let air in. Now the only source of oxygen left is the oxide. Since we've used a lot of the readily available coal in the Northeast US/Canada, this is quite annoying. And with 1940's technology, the protagonists aren't going to set up a hydrogen reduction plant. $\endgroup$
    – MSalters
    Commented Dec 4, 2023 at 14:36
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    $\begingroup$ @OT-64SKOT: Iron is dirt common in the universe (literally!). Coal is very rare. Even if they can't bring iron for some contrived reason, bringing a lot less coal still makes sense. Aliens or not, what makes iron valuable is its chemical composition, not its scarcity. $\endgroup$
    – MSalters
    Commented Dec 4, 2023 at 14:54
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    $\begingroup$ @MSalters Note that you generally need more coal than iron ore to in an iron smelter. IIrc it is something like 3 to 4 tons of coal per ton of iron ore. That is why the early iron smelters where build close to the coal mines not close to the iron mines. You need to work less if you transport the iron ore to the coal source and not vice versa. $\endgroup$
    – quarague
    Commented Dec 5, 2023 at 12:19
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They can get all the steel they can reach and cart away. Rust makes no difference. At that level of tech they'd probably have to process a lot of the large stuff onsite before transportation.

As far as rust goes you just need to remove the oxygen from it by smelting and it's metal again. You can shovel rust into a furnace if you want. This is tech they have and understand well. Iron ore itself is basically a rusty mess.

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Count former cities as a source for mining

Whatever did not instantly vaporize upon being hit by a nuke would be radioactive but will still be metal or oxide available for the taking. While "mining waste" would be a lot more durable than most minerals, as it'll be concrete which is pretty much solid stone, the steel under that concrete would not deteriorate enough to not justify breaking that stone up in order to dig the metal. And the metal would still best get smelted into new form than used as is.

Currently, metal in a skyscraper varies in form from thick supporting rails or columns to relatively thin wall filler wires (if bounded cement flowing is used in the process of building that particular house, the wires are about 10mm thick while the main carcass is 25-40mm thick, both types are expected to remain largely intact after being in a wreck state over 120 years), allowing relatively easy transportation after being cut into transportable chunks, and acetylene welding (which can also be used for cutting) and airpump hammers for separating concrete off steel are techs available to the proposed scavengers. Therefore, if the scavengers would find a piece of reinforced concrete, they will be able to retrieve almost all of the metal hidden within.

The amount of metal would be modest

A single house made of reinforced concrete is estimated to contain about 3-10 tons (depending on exact construction) of steel per 100 m^2 per floor, plus extra per main frame which might be absent in case of it being not too high. A skyscraper is an unique building most of the time, most of them have a very hard central core worth about 100 tons of steel per floor (together with literal floors), a city the size of New York would then have thousands of houses 2-30 floors tall and about two hundred skyscrapers remaining after a nuke (give or take a half). Since for mining steel we don't need them standing, but would be happy to find them already fallen, to lessen the risk of cave-in, we count all houses that didn't turn into vapor as available for mining. New York itself is said to be unavailable, but LA is, so assuming 90% of steel-containing houses would remain solid, we might take the current LA as base for calculations.

But, LA looks like being relatively small for the purpose of steel mining. Wiki states it only has 816 buildings of 30 meters tall or above, and 54 skyscrapers (120m or taller) out of them. Wilshire Grand Center wiki page provides some interesting data on its basement's mass - it's said 21600 yd^3 (16500 m^3) of concrete was used to form a 6-yard thick basement, which is solid, totaling to 16500*2400 = 39.6e6 kg or about 40000 tons (only concrete). Elsewhere Wiki states that cross section of reinforced concrete (vertical only tho) contains 1% to 6% steel, since steel density is about 7800 kg/m^3 or triple the concrete density, the mass of steel in reinforced concrete varies from 3% to 18%, thus only that basement is expected to contain up to 7200 tons of steel. However we might not assume that all the concrete used in either the Wilshire or all LA is that rich, taking the more average value of 3% volume or 9% mass would be better estimation for this source's metal richness.

The Wilshire's roof height is 283 meters and it has 73 floors, meaning 73 layers of full concrete over the basement, each no less than 15 cm thick, and its core should account for at least the same amount of mass, and by its basement each floor is 3600 yd^2 (3010 m^2) wide (technically less as the building is not brick-shaped, but we don't use the exact data for the core's volume so this should come close to being evened out if we use lower estimate for core and higher one for floors), so the volume of reinforced concrete in the Wilshire is about 2*3010*0.15*73 = 65900 m^3, so there is about 65900*0.03*7800 = 15.4e6 kg or 15400 tons of steel in the main structure of the skyscraper. This totals to the Wilshire alone having 15400 tons of metal above ground and 7200 tons below ground.

Now, for the other 53 skyscrapers of LA - digging hard for their construction data doesn't seem to be needed, but a skyscraper has to have a modest ground area, otherwise it's a needle and is susceptible to earthquakes which should not be the case for LA's buildings. Thus we can assume that each of them has at least half the ground area of the Wilshire, and since only 19 are below 500ft, we might safely assume that each of them has mass at least a quarter of the Wilshire's, giving their total steel content of 53\0.25*22600 = 299450 tons of steel. High-rises might estimate to each have a tenth of an average skyscraper's worth of steel, or 2.5% of the Wilshire, totaling to 22600*0.025*(816-54) = 430000 tons. Overall, the high-rises of LA currently contain about 752000 tons of steel, this number can be used to estimate how long should the scavengers bother with LA's remains until exhaustion.

(Also note that broken concrete makes good gravel, and there would be some 22 million tons of bravel as steel mining byproduct, to be probably used in new buildings or roads.)

PS: this SO answer states that the total weight of all buildings in Manhattan is 125 million tons. If 9% of that is steel, then Manhattan, even bombed heavily, would still be a better mining site yield wise. Yet I expect your scavengers would not be able to mill over even the LA, due to overall lack of energy to break concrete in the process.

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  • $\begingroup$ 3-10 tons of steel per 100m2 of floor seems high. It's possible for steel-framed buildings, but not for RC, and definitely not for wooden construction. Where's that figure from? $\endgroup$
    – Therac
    Commented Dec 5, 2023 at 22:01
  • $\begingroup$ @Therac-PeaceforPalestine no one ever said wooden here. No high-rises have wooden floors (and IIRC never had). I've witnessed a couple of RC buildings being built and I estimated how much steel did the builders layer to form the floor+ceiling steel frame. There was quite a lot of steel in form of two sets of parallel bars, and some minor wiring to tie them together at intersections, but I couldn't see what was the cross section of those bars to find out the exact weight. So this is an estimate on what I've seen. $\endgroup$
    – Vesper
    Commented Dec 6, 2023 at 5:56
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  1. How much steel do they need, and in what time frame (how urgent)?
    • Is it an emergency, or is someone running a business?
    • Or is it more like older civilizations carting away stone from abandoned structures, over hundreds of years?
    • After the quantity and time are decided, one can make better decisions about which sources to exploit, and in which order.
  2. A few examples of low-hanging fruit:
    • sewer and storm drain covers are ~100-200lbs each; a city the size of Toronto will have more than a hundred thousand of them on its 5,600 km of streets; New York will have more.
    • rails from trains, street cars, subways
    • steel from bridges and other open-framework girder structures
    • buried pipes; less work than sifting through millions of tons of demolished buildings
    • [edit] My son suggested cars as a source, not just the constructed infrastructure. Cars and trucks (and trains!) are highly concentrated, easily moved, sources of steel. And that steel is easier to recover than eg. rebar from concrete or girders from rubble. However it might be better to retain some advanced vehicles for eventual repair.
  3. Do they need any particular type of steel, ie. can they only re-work it rather than smelting their own? Or do they really only need iron in the steel?
  4. re: the dismantling of buildings: This may not be economical. What are their budgets for time needed and energy expended for demolition, for separation, for rework/resmelting?
  5. re: the dismantling of buildings: What level of risk to human lives can they tolerate during demolition?

I'll stop there for now. If I'm not 'way off the mark, I'll edit this answer after your responses.

--klode

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  • $\begingroup$ Just a heads-up: questions for the OP should be asked in comments (which you don't have sufficient reputation for yet) and not in answers. $\endgroup$
    – Joachim
    Commented Dec 4, 2023 at 22:06
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    $\begingroup$ Thanks for the heads up, Joachim :-). That's why I posted an answer, that I'll edit if they respond. I have reputation elsewhere on Stack Exchange, just not yet here. If this is too egregious an exception, let me know and I'll delete my "prototype" answer; perhaps you could comment on my behalf if you think it's worthwhile. $\endgroup$
    – klode
    Commented Dec 5, 2023 at 1:46
  • $\begingroup$ Your son was right: there are about 8 millions vehicules registered in the L.A. County according to the DMV. If we consider a car to weigth a ton, half of it being steel, you get 4 million tons of steel. (this is a back of the envelope estimate, not all vehicules are cars, old cars weight less than new ones ...) $\endgroup$
    – user38304
    Commented Jan 14 at 14:00
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It's an economical question: as the steel fetches more per kg, more effort can be put into refining it from a starting material that is only X percent steel (scrap).

E.g. here on planet Earth, the element Al is omnipresent, but only where it has accumulated (Bauxite) do we mine it.

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