Snow on the roof

Why did I tell you such a weird thing in the last post? The reason was the KAIT Kobo building you can see in the photos below.

External view. Photo from naoyafuji.

Internal view. Photo from brandon shigeta.

Is there anything remarkable in this building? Since the previous post had something to do with columns, you may make an educated guess...

There are many, many columns, exactly 305 steel columns in total. Careful, don't let them fool you. In fact, there are only 42 columns. But wait, didn't I write 305... where did the other 263 columns go?

They're still there, of course, they didn't disappear in the air. But they're not columns, they just look like columns. A column is not only a vertical constructive element. There's an additional feature it must fulfill: it has to be compressed. And these 263 ain't compressed at all. They're tensioned: the roof pulls them.

Such high number of supports was intended for architectural reasons. They limit the different spaces, and resemble a natural bamboo forest (which happen to be in Japan, like this building). Almost every single column (290 of them, to be accurate) has a different quadrangular shape, ranging from 16-by-145 mm, to 63-by-90 mm. If the thinnest of them (which is not even 2 cm thick!) was a real column, it would buckle. So, the question behind the structural concept was the one I asked you in the previous post: how to prevent such a thin column from buckling?

The "columns" would buckle when the roof is loaded...

And the answer is the same: it won't buckle if it doesn't buckle.

To achieve our slogan, the steel structure was built in two phases. First, the 42 real columns and the roof steel frame (made of 20 cm deep steel beams arranged in a 1.5 by 0.9 m grid). Then, the 263 fake columns were hanged from the girders, the roof was loaded with weights up to the snow load (the maximum vertical load the building has to stand), and then (and only then) the lower part of these 263 supports was fixed.

When the roof was unloaded afterwards, the roof structure went up (as you already know, with the previously applied "snow" load it was deformed), and consequently the 263 false "columns" were tensioned. By means of this peculiar strategy, these supports are supposed to be never ever compressed and consequently they will never ever buckle, they will always stand straight and resemble real columns to the profane eye, but no longer for you, right?

Details

• Location: Kanagawa, Japan
• Architect: Junya Ishigami + associates
• Structural engineer: Konishi Structural Engineers
• Year: 2008
• Material: steel
• Built area: 1898 m²
• Height: 5 m
• Weight (approximate)
• columns: 47 ton
• roof: 1432 ton
• ratio: 1.35 ton/m²

It won't if it doesn't

How can you prevent a column from buckling when it's compressed? Making it stiffer (that is, bigger and "fatter") is a possible solution. However, what if you wanted a really slim column?

Stop. Think on it, and keep on reading afterwards.

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There's another possible solution: it won't buckle if it doesn't buckle. Yes, I know, it may sound like a trick. In fact, it is: the solution is not to compress the column. But... how can you avoid compression if it must be compressed (so that it is an actual column)? Easy: you just have to stretch it before. Never wondered why the thin spokes of your bike's wheels don't buckle? Because in fact, they're never compressed, they're always tensioned by means of the nipples.

That's a way to design a slim column: apply as much tension as it does not get compressed. There's a downside, of course: forces don't dissapear. A mean to stand the additionally developed forces has to be provided. To make that particular column slenderer, other columns will get more compressed and, consequently, thicker. The drawings below will help you to understand it.

Our three friends are standing over the columns. The three columns stand exactly the same load, the weight of one person.  In the drawing, one arrow represents the weight of a single person, from now on, 1W.

Suppose that, for some reason, it is decided that the central column should be thinner. We need to make use of the previous trick: instead of the central column we're going to put a string, and we're going to pull from it exactly the weight of two people, 2W (that's two arrows). The lateral columns become compressed 1W, while the central is tensioned 2W -the drawn deformed shape is exaggerated to emphasize that things do deform-.

Our three friends stand over the roof again. Each column is additionally compressed with their weight, 1W. But this time the final loads on each column are quite different from the first drawing: the central column stands 1W (like in the previous situation), but conversely, it is still tensioned (previously it was compressed). Additionally, the lateral columns are twice as compressed! Consequently, they have to be thicker. Result: thinner central column, bigger lateral columns. Two sides of the same coin.

Why have I explained this peculiar design concept? The answer, in the next post.

One, two... buckle!

Do not buckle, it's the law

OK, if you have followed the blog during its not so long history, the answer to this last visual perception test was quite obvious. Of course, the deformed one. One of the balloons weighted more than it should, and consequently, the stick to which it was stuck buckled.
That's the kind of deformation I want to explain you briefly today.
And what's buckling, then? It's a kind of deformation, so to say. Nothing new, just told you that.
Let's go a little further: it happens to things under compression. And still one step further: you'd rather it wouldn't happen.
Why so?
Make a little experiment. Take a cane (a stick, a ruler, anything that's thin will do), and press it lightly against the floor. That way, you're applying compression to it. Now, increase slowly the pressure you apply. While you press, it will remain straight. Until...all of a sudden, you will notice that the stick bends out!
That's buckling. It happens so unexpectedly. All at once, it buckles, and it no longer stands any load.
And that's why it is a very dangerous problem. When it happens, there's no way to keep it under control.
Imagine that one of the columns of your house disappeared. It could be a disaster. If one of the columns of your house buckled, you'd face a quite similar problem. It would be almost the same as it wasn't there.
Therefore, structures have to be designed not to buckle. Conversely to the traffic sign, "do not buckle, it's the law!".

Visual perception test 03