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Old September 3rd, 2004, 06:36 PM   #1
TallBox
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what percentage of a building's height is the depth of it's foundation?

hi

i've always wondered about this... what percentage of a skyscraper's height is the depth of it's foundations?

i realise this relies on stuff like the soil integrity below and what type of site it is...

but can anyone give me a rough percentage for the varying soil types skyscrapers are usually built on? london clay etc etc
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Old September 5th, 2004, 01:50 AM   #2
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OK, the building I work in has 16 stories, and two underground floors (which I guess makes up the foundation). That makes 32%.

Hope that helped.
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Old September 5th, 2004, 02:29 AM   #3
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You correctly stated that the depth of a skyscraper's foundations depends on the type of soil it sits on. A lot of skyscrapers have foundations that reach as far down into the ground so that it reaches solid rock. In some cities it is almost right up near the surface (eg. Hong Kong) while in other cities it can be hundreds of metres below ground. This isn't the case for all skyscrapers though.

Eureka Tower is a 91 level building u/c on Southbank in Melbourne, close to the edge of the Yarra River. It sits on reclaimed land that once was part of a swampland, only a couple of metres above the water table of the Yarra. IT has a special piled foundation with concrete pilings that reach over 30m below ground level. It only has one underground floor.
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Old September 5th, 2004, 03:38 AM   #4
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its more dependent than just the soil type. for example the foundations of swiss re will not be as deep as the foundations of tower 42, one has a concrete core which is much heavier per square inch than a lightwight steel structure. infact foundation depth is dependent on the type of structure you build with.
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Old September 5th, 2004, 04:38 AM   #5
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It varies. Our tallest is 16 storeys, made of reinforced concrete, it has 4 basement levels, 2 of which are exposed(parking) the other 2 are mech stuff. Or second tallest has 2 basement levels, it's 14 storeys.
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Old September 5th, 2004, 05:21 AM   #6
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basement levels arent foundations.
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Old September 5th, 2004, 03:00 PM   #7
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I'm not sure but these pics might give you an idea.

Its the construction site for a 36-storey apartment tower in Singapore. You can see how deep theyve dug below street level.

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Old September 5th, 2004, 04:19 PM   #8
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i see, thanks guys.

gothic,

i understand about the weight per sq inch... but wouldnt it make sense that a lighter structure like swiss re should have deeper foundations than a heavier one (t42)? i.e. have deeper foundations to act as a dampner to the sway of the building at high wind speeds.

or is it cos of swiss re's tapering shape that it doesnt require deep foundations (the top of swiss re is aerodynamic to allow winds to follow the curvature of it) and thus sway is reduced anyway, eliminating the need for deep foundations?
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Old September 7th, 2004, 02:23 PM   #9
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ah-hem!
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Old September 7th, 2004, 02:36 PM   #10
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'Natural' depth of a foundation in Rotterdam is around 25 meters. That's where the first layer of sand can be found, which is a good soil to rest a structure on. Either 5 or 50 floors, most buildings here a founded on that layer. In rare cases, such as a complicated underground infrastructure, foundation goes as deep as 60 meters, where a much thicker sandlayer can be found. Old buildings are founded on wooden piles which is no problem as long as the underground water level stays heigh enough to keep the wood wet.

The city of The Hage has a natural distinction between sandy top layers (dunes) and clay top layers. In the old days, working class residential area's were founded on these spouncy clay layers, while good neighbourhoods are build on dunes. They actually had a lot of troubles with a tram tunnel that crossed the sand/clay boundary.
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Old September 7th, 2004, 04:32 PM   #11
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The depth of Baiyoke 2's foundation is equivalent to 20 storeys!
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Old September 7th, 2004, 04:40 PM   #12
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I wonder if thats because the building is very slim that greater pressure is exerted on its foundations hence slimmer buildings have extra-deep foundations....
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Old September 7th, 2004, 04:49 PM   #13
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That pressure per area unit offcourse is of influence as well, but one could also spread the load if there is an opportunity to create some large underground areas or wide foundations. Some buildings are known to be founded on a thick concrete plate, simply because the surrounding area provided room for it.
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Old September 10th, 2004, 01:08 AM   #14
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There is no simple answer to that question Shaun. There are several major variables. The soil, sub-soil, and the depth to bedrock (if it isn't real deep like in Tokyo) are factors. The total weight of the building is a factor. Earthquake zones have to take that variable into account.

It all comes down to the location's unique situation. It also depends how many sub floors were planned, etc.

It's a good question and I hope somebody will research for us a really detailed answer.
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Old September 10th, 2004, 01:11 AM   #15
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Oh also, for really tall buildings, if they have a dampening system in the roof like Taipei 101, that can also affect how the foundation is designed. A building without one might need a very stiff and deep foundation to counteract lateral forces like wind and earthquakes.
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Old September 10th, 2004, 04:40 AM   #16
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Yeah, here is the Bayoke
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Old September 10th, 2004, 04:43 AM   #17
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Yeah, here is the Bayoke 2 Tower, tall and slim, with huge fundation as you mentionned

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Old September 10th, 2004, 07:21 AM   #18
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Singapore's Golden Mile Complex is a mixed-use complex only 56m tall, yet its foundations are deeper than it is tall.

That is because it sits on a thick layer of marine clay, the pillars are on bedrock.
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Old September 10th, 2004, 08:39 AM   #19
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must be costly, no? Why not build it taller then? do you have any picture Redstone?
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Old September 10th, 2004, 09:08 AM   #20
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An ugly building built in the 1970s.

It sits on what was once very near to the sea.Now the sea had been reclamed, but the marine clay is of course still there.
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