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The superhero has magically increased strength but also magically increased weight so that he can utilize his strength effectively and stay planted. The drawback is he has to make sure things can hold this increased weight.

If he was running through an apartment complex and jumping about while fighting someone, would standard floors (built to safety regulations, assuming it's not rotten or poorly put together) generally be able to take this amount of weight or would he fall through a floor? Floors can take a lot more than this weight in furniture but it's a static load. At what level do dynamic loads become an issue for standard flooring?

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    $\begingroup$ @PcMan if you have an answer, please don't put it in the comments. $\endgroup$
    – Trioxidane
    Sep 24 at 7:07
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    $\begingroup$ I don't know the answer to your question exactly, but their downstairs neighbor would answer a resounding "YES". And I'm having fun picturing their neighbor wonder what on earth is going on upstairs $\endgroup$
    – BruceWayne
    Sep 24 at 15:52
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    $\begingroup$ I was really hoping the answer was "yes, he needs to be careful", just so your hero would have to wear clown-snowshoes to fight crime indoors $\endgroup$
    – Cireo
    Sep 25 at 0:38
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    $\begingroup$ @AlexanderNied - Constant destruction of people's chairs because he forgets to sit on the floor. $\endgroup$
    – Axion
    Sep 25 at 9:25
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    $\begingroup$ tbh stuff only breaks when its plot or humour convinient no? $\endgroup$ Sep 25 at 12:57
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In short:

In a wooden floor built up to modern building codes a 750 kg hero is unlikely to break the beams, but he may cause some damage.

In long:

There are two problems with loads on floors and other structures: overall resistance of the floor and punctual resistance. That is, in a wooden flooring, that the beams don't break (making the whole flooring fall down) and that the plank where the hero is resting his foot don't break.

Since most (all?) building codes all over the world have rules to check for those two problems, I am going to take my numbers from the Spanish one (point 3.1.1, page 9) which must be quite similar to the numbers anywhere else. I'm using here kg/m2 although the code is in kN/m2 (1 kN/m2 = 100 kg/m2) for ease of comparison with a 750 kg hero.

Overall strength of floor and beams isn't going to be a big problem because floors are checked for an uniform load ranging from 200 kg/m2 for dwellings and hotel rooms to 500 kg/m2 for supermarkets. If the room isn't full, a 750 kg load is equivalent of the design load of a few square meters. Therefore, a 750 kg hero is only going to be a problem if the room is already full, or if at least the part of the room over the same beam where the hero is, is already loaded. For example, the hero should avoid attending a party in an hotel suit already packed with people.

The point load may be more tricky because structures are checked against loads of 200 to 700 kg on a 5 cm x 5 cm square. Then, in a wooden flooring, his weight might exceed the design load of individual planks. The best advice here would be to walk carefully trying to rest his feet only over the beams. That piece of advice could be hard to follow during a fight, but planks falling and the hero clinging to the beams might make great shoots for an action movie.

In addition to that, there is a relieving fact: strength is not the only condition that leads structure design, and very often the most stringent rule is against excessive bending. Therefore, very often beams can hold several times the design load without breaking, in spite of a large deflection that could be aesthetically unpleasant (or even frightening) and may cause cracks on non structural elements.

And a last cave-at: the OP says that the hero is fighting someone. If "someone" is a team of 750 kg villains or heavier, the situation may be trickier but using villains for load testing could be advisable and could lead to an interesting plot.

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    $\begingroup$ According to hypertextbook.com/facts/1999/SaraBirnbaum.shtml , running causes momentary forces of about three times the body weight, so it is quite likely to break planks. $\endgroup$
    – jpa
    Sep 24 at 15:16
  • $\begingroup$ Yes, dynamic loads can get large but the hero could control them by being careful. For a careless hero, dynamic loads might grow in proportion to his strength. Therefore the corollary of the answer is that an hero can manage not to break the floor, although the hero can break the floor if he wants or if he doesn't care. $\endgroup$
    – Pere
    Sep 24 at 15:24
  • $\begingroup$ "1 kN/m2 = 100 kg/m2" How could these two quantities be equal, when they're not even the same dimension? I assume you are implying some factor to convert mass into force; one such factor is the gravity constant, which converts mass into weight; but if you have speed (such as falling or jumping or running), then the impact on the ground will be much more than your own weight. $\endgroup$
    – Stef
    Sep 24 at 15:28
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    $\begingroup$ 1 kg force also known as kp equals 9,8 N. Previous versions of the same code used to give loads in kg/m2 and using the kg as a force unit is common in the technical field, although it's becoming a bit outdated. $\endgroup$
    – Pere
    Sep 24 at 16:00
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    $\begingroup$ If you're relying on all building to actually perform up to code, you will often be disappointed. TBF, this is fiction so the floor should fail whenever it's most convenient for the plot. $\endgroup$ Sep 25 at 14:32
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Depends on the size of his feet

Load stresses are often mitigated by weight distribution. 3/4 of a ton being constantly imparted by the surface area of the average human foot, really it's half a foot on impact for when you think of how people generally walk and run, is going to be hard to ask the floors to keep intact. Doubly true for a 'running' load, which would be equal to simply dropping that weight on the floor repeatedly, which is very likely to break through, assuming things like fleshy softness and bone brittleness are to be ignored.

If he's going to be traversing floors I'd suggest he trains himself to both adjust how he puts his feet down as well as adjust his method of imparting his weight in similar manner as to how one would be careful across ice that would be fine to walk across but not run.

I'd also suggest he get himself some shoes that increases the surface area of his feet, in a similar way as what snowshoes would do.

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    $\begingroup$ +1. Accepted answer says, "The point load may be more tricky" and then fails to calculate it, because it can't be calculated without this (and a degree in structural engineering). I looked up how to calculate a point load and I gave up when I saw it required knowing 10 variables. $\endgroup$
    – Mazura
    Sep 25 at 1:43
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    $\begingroup$ I think we should advise the superhero not to wear stilettos. $\endgroup$
    – abligh
    Sep 25 at 12:42
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It is likely Ok

At least in the US, 750 kg should be easily supportable. Building code requires support of at least 40 pounds per square foot, so even a small 100 sq ft room is designed to carry at least 4000 lbs (1,818 kg) of load. So unless a room already has heavy load in it, 750 kg is perfectly fine.

What is making the outcome iffy is that his load is not static - he's running and probably jumping. His feet are working like sledgehammers wrapped in shoes. The floor as a whole can support his static weight, but if it is momentarily applied at a weak spot, for example a joist below his feet has a hidden crack in the middle - then yes, this joist may fail, although this would not cause the entire floor to collapse.

P.S. The stress on the floor can be classified as "highly localized", as limited to individual foot, or just the its heel or ball, and "locally distributed", as spread into the area as large as 10 sq ft/1 m2. My "Likely Ok" verdict was based on the latter, "locally distributed" stress estimates. The way the floors are constructed, the former (highly localized) stress strength has a much higher tolerance - it's always the supporting joists, rather than subfloor (typically plywood) that are failing first. To damage the subfloor, you need to hit it with something hard, like unwrapped sledgehammer, or damage its support (joists).

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    $\begingroup$ Static it's about 50% more than a piano, which is clearly a reasonable thing to put on a floor, and to move around (though hopefully not drop). Even a tall bookcase is getting close if very full. $\endgroup$
    – Chris H
    Sep 24 at 12:50
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It's possible

Maybe a not so scientific answer, but we can assume much from current footwear. High heels. From several research sources I gather that a 50kg woman in high heels is exerting more pressure per square centimetre than a 4000kg elephant on all it's feet. Quick math tells us that each heel is then exerting more force than 1000kg on one elephants foot. The foot is between 40 to 50cm in diameter. The pressure of a high heel is about 15 times higher per square meter than an elephants foot.

The feet of your guy might be closer to the surface area of high heels, but in all cases are still several times bigger than a high heel. The difference in weight is coincidentally 15 times the weight of the woman (the weight of a 50kg woman vs 750 of your protagonist).That would mean your protagonists feet always exert less force per square meter than high heels. As I've seen woman who, despite their claims, are more than 50kg run in impressively high heels without breaking floors, I would say it's entirely feasible to fight on 'normal' floors for your protagonist.

As for the carrying weight of carrying beams, I think it won't be much of a problem. In my experience with both parties and moving (heavy) equipment, it is relatively easy to exceed 750kg in small areas. Of course this is temporarily much higher when pushing off, but as the floors would spread the weight over multiple beams it's unlikely to be a big factor overall for these carrying beams.

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  • $\begingroup$ The comparison to high heels addresses the failure of floorboards or equivalent, but the spanning beams are more likely to be the vulnerability (and the load has been spread by the floorboards so that the difference in contact area between high heels and my big shoes is irrelevant) $\endgroup$
    – Chris H
    Sep 24 at 12:46
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    $\begingroup$ @ChrisH in an earlier draft I had something about that. Forgot to rewrite that. I think they aren't a vulnerability at all. In my experience with transport and parties the weight can easily exceed 750kg on small areas. The weight of the protagonist is spread thanks to the floor to the beams, making this for me the least worrying issue. Even when pushing off. $\endgroup$
    – Trioxidane
    Sep 24 at 13:40
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I think the answer is that he would have to be very careful. The point of failure is going to be the plywood subfloor that you then put carpet, tile, or other flooring over.

Typical flooring is constructed of 2x8 or 2x10 joists spaced 16" apart with 3/4" or less plywood on top. The plywood distributes the load to the joists which distribute the load to columns or walls. The problem is that your hero is a highly localized phenomenon. A piano may way 800 pounds but it has 4 legs and distributes its weight in four different places on the floor so each one has only 200 pounds.

I found a couple of interesting links. This one has a strength calculator which gives 960 pounds as the max total load for 1" plywood floor with 16" spacing. Typical 3/4" plywood gives only 384 pounds as the max.

Then there's this study on concentrated loads. Not a single flooring system tested would support your hero standing on one foot, except in a couple of tests where the joists were spaced at 6". Most of the tests had failure at less than 1/2 the weight of your hero.

But your hero is not going to be a static load. He is going to be 'running and jumping'. An Olympic sprinter can push off the ground with a force of 1000 pounds (average people are about 500-600 pounds). If your hero is running, he will be temporarily applying 5,000 pounds of force on the flooring. If he steps between the joists his foot will go right through the floor.

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