Making structures "partially out of energy"

General Questions
#1

Recently a friend of mine claimed that it’s possible to make a lighter structure of the same strength out of steel by removing some of the metal and holding it together with magnets. And that it isn’t normally done because the amount of energy needed to power a sufficiently effective electromagnet doesn’t make it economical.

This strikes me as an idea that would come up in a poorly researched sci-fi movie (why would metal be lighter just because it’s being held together with magnets). So I thought I’d ask you guys.

Can merely holding metal together with magnets make it stronger, and if not is there some sort of engineering technique using electromagnets (or some other form of energy) to replace part of a structure? If this is the case, why haven’t more people heard of it?

#2

I don’t see how holding metal together with magnets makes it stronger.

But I can imagine how adding actively controlled electromagnets could. That is, if their power were regulated according to a program that watches the metal structure deforming.

Here’s an example. If you make a tower in the form of a tube, it may fall over because it buckles near the base. Once a dimple starts inward, the forces pushing it further inward suddenly increase, because all the angles change. If you had electromagnet devices all over the thing that could sense a dimple starting and push outward on it to stop it, this failure could be averted.

#3

It’d be more likely that the electromagnets would weigh more than the struts they’re supposedly holding up.

#4

There are three practical reasons I can think of off the top of my head why it’s not done. The first is the reason you gave:

The second is the reason Grey gave:

And the third is reliabilty. Relying on a big hunk of steel for support is pretty reliable: there are relatively few things that could go wrong, and those that could tend to go wrong slowly. With an electromagnet, there are a whole lot more scenarios that could lead to failure, and those tend to happen quickly.

Any one of those three reasons would be enough to discard the idea in all but the most specialized circumstances. That’s not to say that magnetic fields aren’t used in some applications to support things, but I can’t think of any application where the driving reason is “to make a lighter structure” (and I’m skeptical that any such application can practically exist).

#5

Out of interest, what applications are these? When I think “electromagnet” all I can come up with are cranes, school bells and electric motors none of which use them as part of their structure.

#6

The things I was thinking of (again off the top of my head) were maglev trains, magnetic clamps for machining and grinding operations, and magnetic plasma bottles. All of these use magnetic fields to support things, but not in the permanent and static way that you were probably thinking of.

#7

A solid piece of steel is already held together by electromagnetic intermolecular forces. I may be wrong, but I don’t think there’s a commercial electromagnet out there that can even rival the forces generated by those.

#8

I’m having trouble working out what is even being claimed. What is being “held together” here? When we make a large steel structure we don’t use the steel to hold things together, we use it to hold things up. We use bolts or rivets to hold things together (which are made of steel, but what we are hearing about is removing structure, so I assumed it isn’t about removing the rivets out of a bridge or building.)

Two, electromagnets are not very strong. Modern rare earth permanent magnents are stronger, and have the advantage that you they don’t fail when the power goes out. Superconducting magnets are a different issue, but they are not what has been suggested. And wildly impractical. They might stay magnetic without power, but they fail when the liquid helium runs out. But none of these magnets can come close in holding power to a high tensile steel bolt, or even a humble rivet.

So if the magnets are not holding bits together, we assume they must be holding stuff up. Which begs the question, what holds the magnets up?

Is there some weird SciFi idea that magnets makes the steel stronger? Maybe that the steel has improved tensile strength? Which is only one of a large number of metrics needed when considereing a building material. If you want to make a light structure, use aluminium. Just don’t expect it to stay standing even one tenth as long as a steel one. Titanium would be good. Just a weensy bit expensive.

In all it just doesn’t make enough sense to even be wrong.

#9

Is this claim part of a “business or investment opportunity,” by any chance?

#10

To be honest I think that’s what he seems to believe.

Nope, just a computer programming student having to put up with the ravings of a business studies student.

#11

There are materials that become stiffer in the presence of a magnetic field. I have heard that there’s research being done on using those materials in earthquake prone areas. Reduce the magnetic field to increase flexibility during a quake and then power back up to gain rigidity after the movement has passed. I don’t think it’s being used in practice, and that’s not quite what the claim is anyway.

#12

Theoretically, you could use a magnet to suspend one floor above another using repulsion to keep it up there. Then you’d only have to attach the two floors enough to keep them from sliding off of each other, but the magnet would be providing the force to keep them in place. I think that’s the theory the OP’s friend is talking about.

Of course, it’s ridiculous. Just try using a computer or a pacemaker in one of those buildings. And the magnets on the first floor have to be strong enough to support the entire structure.

This idea has also been discussed with antigravity - essentially, each floor floats freely against gravity, with just enough interconnections to keep them from blowing away. At least antigravity has the advantage of being foreign enough to us that we might give it the benefit of the doubt.

#13

Yeah, I was thinking that repelling magnets, not attracting ones, were the idea here. I don’t see any reason why you couldn’t do it. Of course, it wouldn’t be cost-effective, stable, or in any way advantageous, but in fairness that’s not what was posited.

#14

Irrelevant but interesting aside, IIRC, in Kim Stanley Robinson’s Mars trilogy, his space elevator wasn’t physically moored to the Martian surface but held in place by electromagnets at the base. Always thought that was a cool visual (not as cool as the visual of the elevator coming down, and whipping itself around the entire surface of Mars).

#15

I smell a government grant coming !

#16

Maybe he was talking about fastening steel parts together by magnetic attraction rather than using fasteners or adhesives. I suppose in theory there would be a trivial weight gain.

#17

Girders? A fridge magnet can barely hold a piece of paper up no one is going to strap rare earth or electromagnets on either side instead of rivets or bolts.

#18

The idea of the Space Fountain is that ball bearings will be propelled up into the air through a tube. Magnets at the top will curve the bearing to go back down through another tube. This upwards force helps to keep the structure up.

#19

I think people are thinking on the wrong scale - think nanoscale permanent magnetic domains within the material itself, allowing a nano sized honeycomb structure to reduce mass with the magnetic attraction adding rigidity and strength.

Absurd, of course. The magnetic domains could only be aligned North-South for attraction, but any displacement in the structure could shift that in a way that causes the structure to fly apart. And a random alignment of domains would give no benefit.

However, magnetorheological fluids and ferrofluids have interesting properties that allow them to be used for a variety of engineering purposes, utilising the fact that applying a magnetic field will change a physical property of the fluid.

Si

#20

Yeah, for fasteners, bolts or rivets or even glue are lighter and cheaper. If you need to regularly attach and detach, and occasional slipping or outright failure are OK, then magets might possibly be a good solution, but neither condition applies to buildings.

For supports, the problem is that supporting something with a repulsive force is a big pain – you can’t just stick one magnet on top of the other, because the top one would soon just slide off to one side and fall down. So you need lots of magnets, carefully arranged. At this point, you’ve spend way way more money and have more weight than a couple steel beams or concrete pillar, and even then you’re support is only a few inches tall at most. Plus there’s the issue of the building falling down if the power goes out.
The advantage is with nothing touching but air, there’s really low friction, so for high-speed trains, the hassle, weight and cost are worth it, and you can design trains to gracefully sit down onto wheels when the power goes out. Buildings, not so much.