What is this architectural feature?

March 19, 2006 5:07 PM

What purpose does this architectural feature serve?

I saw these structures in the Detroit airport. Criscrossing steel arches support a roof, and at the point where these arches meet, a steel column points down, more or less vertically, and is supported at its bottom by steel rods that attach almost to the base of the arches.

Based on my layperson's understanding of engineering, I don't see how these things could support the roof in any meaningful way. Are they just for looks?

I have seen similar structures before on tents, which are apparently called "flying peaks." I'm not sure I understand the point of those things, either.

What are these things called, and how do they work?

posted by Brian James to science & nature (9 comments total)

I'm not an architect, and I don't know what they're called. I can tell you how they work, though.

Those steel rods (they may be cables) running from the column to the base of the arches are tensioning devices. They hold the base of the arch stable in relation to the other end(s) of the arches. They're not supporting the column at all. The column is also a tensioning device, keeping the upper (roof) arch and the lower (cable) arch at the optimum distance for stability. Dome tents work the same way, with the tent floor serving the same purpose as the horizontal cables. The vertical columns are a way to keep the intersections properly aligned.

Why? By using these (and other) tensioning devices, an architect can make an arch longer and wider without needing to use heavy support posts.
posted by jlkr at 5:39 PM on March 19, 2006

The flying peaks on the tent are to control drainage of the roof. Absent of them, the weight of the water would pull the roof fabric down below the level of the edges preventing the roof from shedding its accumulated water.

To elaborate on jlkr's explanation, the crisscross of cable/rods in the center forms a tension structure that has two effects; they pull the wall supports inward, thereby thrusting the roof upward, and this is augmented by the post running up from the "floor" created by the crisscross. The post further supports the roof at its center.
posted by RMALCOLM at 5:59 PM on March 19, 2006

If it was just an arch the entire thing would be in compression. Natural materials like stone and concrete like to be under compression, but most modern materials like steel have much more strength in tension. (Though note here that I am not a structural engineer!) So by adding those extra members and making it into more of a truss than an arch you can distribute the load with things like those steel cables that are thin and light.
posted by Rhomboid at 8:01 PM on March 19, 2006

They remind me of the spines on an umbrella. From the answers so far, it sounds like maybe that's their function. They help hold the ends of the arches out where they belong, so the arches don't "fold up" under the stresses they're subjected to. And the vertical rods keeps the arches and the lower thingys the proper distance apart.

(Of course, they may be entirely decorative - it's hard to jusge the scale from the picture, they may be far too small to be load-bearing.)
posted by attercoppe at 8:45 PM on March 19, 2006

Hard to see from your photo, but these may be, as others have indicated up thread, tensegrity devices. The Georgia Dome was one of the first big roof supported this way.
posted by paulsc at 9:31 PM on March 19, 2006

These work like a bowstring holding a bow in an arch. Without the cables, the roof arches would exert an outwards force. This way, the walls don't have to take as much horizontal forces, giving the architect more options.

A small demonstration:
Take a small piece of paper and fold it in half. Put it on the desk so it form an sort of an arch. When you push on the top, the ends move outwards and the paper goes flat. Now, try supporting the ends of the paper so that they can't move outwards and you will see the paper taking the force of your finger. Now, imagine (unless you are crafty) a wire attached to both ends of the paper. When you push on top, the wire takes the outwards force.
posted by insomnus at 10:02 PM on March 19, 2006

I'd have to agree with paul, I'm pretty sure they have to do with Tensegrity, which was brought to you by Buckminster Fuller.
posted by 517 at 10:06 PM on March 19, 2006

They are essentially acting as a king-post in a truss, where the upper chords of the truss have been shaped into a curve. Whether a king-post is in tension or compression varies depending on the truss geometry and the loading of the truss, but you can typically tell -- if it is slender (as in the case of the detroit image), it is probably designed to be in tension. There may be additional depth of the truss above the ceiling plane -- it has been a while since I have flown through Detroit.

The reason that trusses are such a common structural solution ( as well as allowing for a great deal of architectural expression, space-making and such), is that they are extremely efficient structures, which means that they can support a given load using much less material than a simply-spanning beam (where different portions of the beam are in tension or compression). The Detroit roof truss can have very slender members because each chord is largely only in tension or only in compression, which reduces the need for excess material, which saves cost.

You lose a little structural efficiency by shaping the top chords into an curved (arch-like) profile, because there will be some bending forces in those chords, but it is a trade-off between the architect and the structural engineer.
posted by misterbrandt at 10:26 PM on March 19, 2006

Ah, excellent. Nice to see all the answers basically align with one another!

I was thinking of the problem only in terms of vertical force, I wasn't accounting for the horizontal (outward) force exerted by the arches. Now it all makes sense.
posted by Brian James at 10:44 PM on March 19, 2006