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

Stairway to Viamala

In Switzerland, there's an old path, dating from roman times, which crosses the Viamala gorge. In 2005, after a previous wooden footbridge was destroyed by a landslide in 1999, a new bridge was built. It was placed in a new location, with  less posibility of falling rocks.

The bridge spans 56 meters, about 70 meter above the valley, and quite a peculiar feature: it connects places with a height difference of 22 meter. It's not only a bridge, but a staircase.

Photo from lixelle

Photo from Marco Zanoli

It's a cable stayed bridge. The wood plattform (the bridge deck) hangs from steel cables which, as you already know, deform accordingly. Its funicular (the shape the cable adopts when it deforms), together with the steep stair, give rise to a powerful structure.

General forces: the weight of the bridge is supported by the upper cables in tension.

The two main steel cables, of just 36 mm of diameter, carry the load by means of tension to the foundations located in both sides of the river. Hanging from them, smaller cables (10 mm of diameter) hold the glue-laminated larch beams. Well, not really. That wouldn't be a good idea: instead, the secondary cables (those hanging from the main cables) are attached to transversal steel beams (HEA 120), and the wood beams are layed above them. This is in order to prevent tension perpendicular to the grain to appear and... believe me, you'd rather not do that. Have a look at the technical drawing here, to see how it is built.

The bridge deck is inclined, and it pushes on its lower support (that's ok, business as usual), but it pulls from the upper support. That's a different story: the bridge's own weight is trying to drag the foundation into the river. How can you avoid that?

The forces you have to stand if you don't want everything to fall down cliff!

Moreover: the cables pull inward (that is, deep into the gorge as well!) thus, something has to be done to prevent the fall: an outward force is more than required. The foundation is responsible for that and, apart from its design (have a look at it here), it must be really heavy. Its own weight is its only tool. Actually (have a look below, at the details section), the foundation of this apparently light bridge weighs seven times more than the bridge itself. It's not what you see, but also what you don't see.

Details

• More information about the bridge, and pictures of the building process.
• Location:
• Traversiner Tobel, Graubünden, Switzerland
• Concept: Conzett Bronzini Gartmann AG
• Year: 2005 (built from May to August 2005)
• Span:
• 56 m (footbridge)
• 95 m (main cable)
• Materials:
• wood
• pine (heartwood) from Nesselboden,
• larch, glue-lam and roundwood (heartwood) from Rongellen
• steel cables and profiles
• concrete foundation
• Weight:
• 26 ton (bridge)
• 192 ton (concrete foundation)