BBC iWonder How Do Skyscrapers Stay Up

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http://www.bbc.co.uk/guides/zqyr9j6




1.The rise of the tall building

Presented by

Roma Agrawal

Structural engineer

Our city skylines are dominated by skyscrapers which have become taller with more ambitious, quirky designs. As well providing a practical solution to maximise space in cities, they are a way for a country to reflect its wealth and ambitions for the future.

Chicago and New York fought for the prize of being home to the world’s highest building in the 20th Century with the Sears Tower and the Empire State Building. But in the 21st Century, China and the United Arab Emirates have soared ahead in the race for the sky, with the Burj Khalifa in Dubai now the tallest building in the world.

So as these structures get higher and more eccentric, how do engineers stop skyscrapers from falling over?

2.Foundation is the key


Structural engineers like Roma Agrawal face many challenges when they design buildings, from overcoming gravity to ground that is far from solid. So how do engineers tackle these problems?

3.Soft ground

The Shard weighs 18000 tonnes. All this weight is supported by a concrete slab and concrete piles in the foundations. Clip: Britain Beneath Your Feet (BBC One).

Foundations are vital to anchor buildings but many skyscrapers around the world are not built on solid ground.

This is the case in London where many of the largest structures are built on the city's soft clay. One such building is the 95-storey Shard, the tallest in western Europe. It needed to go deep underground to create its foundations.


The Shard sits on top of a large concrete slab, which is held up by hundreds of concrete piles. The piles are needed to take the rest of the load and go 53 metres down beneath the surface into the clay until they reach a layer of stiff sand. This is further than most skyscrapers in New York – the foundations of the Empire State Building are only 16 metres deep.


In Dubai, the Burj Khalifa had to overcome a challenge posed by salty water running underground in between the soil, sand and rock. This water can often be eight times saltier than seawater and is extremely corrosive.


So engineers needed to use a special concrete that does not allow much salty water to pass through.


They also used a process known as cathodic protection where another metal is added to the concrete base to protect the steel in the foundation. If salt water eats through the concrete then it will be this other metal that corrodes, not the weight-bearing steel.

4.Fighting the wind


The Citigroup Center, formerly known as the Citicorp Center, has a mechanical counterweight inside it. This helps deal with the force of the wind or an earthquake.

Skyscrapers face another opponent – the wind. When it blows, its horizontal force is trying to tip the whole building over to the side.

Base

We might think that the wind is happening high above us, but the effects are also felt beneath our feet. The force of the wind on a building can cause the foundation to shift.

But if the foundation is spread over a wider area – as with the Burj Khalifa – it is less likely to move.


Cores and tubes

One way to resist the wind force is to have a strong middle – or core – made from thick concrete walls to create a stiff backbone.


But this can only do so much, so other engineering solutions need to be applied as well. By using stiff columns and beams on the outside of the building a strong tube can be formed across the whole building. This enables skyscrapers to have outlandish designs, like the Gherkin in London.


The tallest towers tend to have a combination of these two systems, allowing buildings to rise ever higher into the sky.


Technology

Some buildings, like the Citigroup Center building in New York, even use a computer system to move around huge weights within the building, depending on which direction the wind is coming from. This helps reduce the amount the building sways under different wind forces.


Engineers will need to keep employing all the tricks of their trade to keep up with ambitions to make taller and more spectacular buildings.


5.Foundations past, present and future