steel mill in the sky?

March 12, 2007 9:16 AM

Let's say you want to build an extremely tall really tall building with giant metal rods supporting it. Would it be feasible to "forge" the rods in place with some sort of self-raising crane/steel mill?

Also, how thick and tall can metal rods get before they are no longer able to support themselves (and a building I guess)?

posted by delmoi to Technology (12 answers total)

Do you really mean casting?
posted by popechunk at 9:29 AM on March 12, 2007

Why would you want to? The problem with tall structures isn't the joints, it's the physics of the structure. Like, the reason your Lego house falls down isn't because plastic is weak, it's because you didn't stagger the bricks when you made the wall. (That's a bad example, because a continuous form process would actually help that....)

Your question about self-supporting columns was answered by Euler.

Some very accessible books on these topics:

- Structures: Or Why Things Don't Fall Down
- Why Buildings Fall Down: How Structures Fail
- Why Buildings Stand Up: The Strength of Architecture
posted by DU at 9:36 AM on March 12, 2007

You could probably do it, although it would be insanely difficult and unimaginably expensive. Modern electric-arc furnace mills roll continuously, and just chop the red-hot steel into lengths as it comes out of the form.

So, just don't chop it into lengths (no, it wouldn't really be *that* easy).

Some selected difficulties you'd encounter:
1) cooling would be tricky, and could seriously limit the speed of your mill. I'm not sure how you'd handle the differential cooling, or if in fact that would be a problem at all.
2) getting raw materials to the top of your rig, and melting them there for the forming process, would be difficult. It would be far easier to leave the mill on the ground and extend the rod up out of it.
3) avoiding blowouts would be tricky; this is when the forming process fails and the still-molten core of your extrusion breaks out and spills out all over the place.

There are more problems, of course. They're probably not insurmountable, but there's zero reasons why anyone would ever bother trying to surmount them, so they're effectively insurmountable.

For one thing, going with a guyed design and cables is a lot easier. More economical too. If the designer insisted on using rods, it would be simpler to just use long rods connected by coupling nuts rather than one continuous casting. Keep in mind the casting would need to have a variable width (thicker at the base, thinner at the top).
posted by aramaic at 9:39 AM on March 12, 2007

Rods are not the ideal steel structural support. Columns in steel construction are usually rolled W sections due to their ideal bending properties and to their easier ability to be connected.
posted by JJ86 at 9:47 AM on March 12, 2007

But that's not the point. The point is that you can achieve arbitrary building height today, given a sufficient budget, using standard steel beam construction. There is no need to make special purpose pieces.
posted by Rhomboid at 10:50 AM on March 12, 2007

Steel must be rolled in order for it to be strong. If it's simply cast in shape, it is brittle.

As mentioned rods are great when you need tensile strength but lousy for compressive strength. They're not acceptable for load bearing structures.
posted by Steven C. Den Beste at 11:42 AM on March 12, 2007

Forging

Steel mill (casting and rolling)
posted by Kirth Gerson at 12:10 PM on March 12, 2007

If you are thinking that a single piece of steel is significantly stronger than many steel columns or beams bolted together, you are mistaken. Im also unclear as to what the benefit of these idea would be.

Oddly enought I remember my structural engineer professor saying that it is somewhere near a mile in length before a steel rod would fail under its own weight.
posted by comatose at 12:14 PM on March 12, 2007

For the second part of your question a simple analytical approach may yield the answer comatose's professor gave. In the real world you have lateral wind loads and shear loads for which you need quite a bit of bracing.
posted by JJ86 at 2:40 PM on March 12, 2007

1. What everyone else said about the pointlessness of having a forge in the sky. Building height is currently held more to the limitations in conveyance technology (plumbing, elevators) than structural feasibility. Plus, if you're going to have to haul all the iron ingots or whatever to the top of the structure the whole time to supply the forge, what's really the difference between that and just taking up finished pieces of steel?

2. If you're really interested in self-raising cranes, like, that go up with the structure as it's being built, that's pretty much how they built the Gateway Arch in St. Louis.
posted by LionIndex at 3:14 PM on March 12, 2007

Columns in steel construction are usually rolled W sections

To clarify for the laeity here: a W section is not something shaped like a W. In fact, what most people would normally call an "I-beam" is in fact a W section. The W stands for "wide flange". A real "I beam" is actually a similar shape, but with narrower flanges on the top and bottom, and isn't specified very often in steel construction.

Also, to add on to my own comment, it's not just conveyance technology that limits building height--it's that plus not having enough leasable floor area to make building that high worth the expense. Once you get to a certain height, you start losing more and more floor area to elevator shafts and things like that. As far as plumbing, it takes a pretty thick pipe to contain a 1/4-mile high (roughly WTC height) column of water.
posted by LionIndex at 4:48 PM on March 12, 2007

Perhaps if you twisted the giant rods into giant springs (forming a massive shock absorber) you'd have more luck. The building would simply bounce and sway.
posted by DenOfSizer at 5:39 PM on March 12, 2007