Though there may not be many, if any skyscraper in Nigeria, but we do have high rise buildings; and the scope of their design is similar to that of a skyscraper, only that in the case of the latter, more has to be taken into consideration – because the higher and more complex a structure, the more meticulous is the attention to detail.
Structural engineers know this, and when designing buildings – especially skyscrapers – consider factors like: ensuring that the structure can overcome gravity, fierce windy and storm conditions, earthquakes and weak soil strata. So how do engineers tackle these problems?
Foundations are vital for anchoring buildings to the earth, but many tall buildings around the world are not built on solid ground. An example is the 95-storey building in London, The Shard (picture above); because much of London sits atop soft clay soil, in building The Shard, it was necessary to go deep underground to create its foundations.
The Shard, upon completion, sits on top of a large concrete slab, which is held up by hundreds of concrete piles (square, rectangular or circular concrete or steel tubes driven into the earth).
The piles are needed to take the rest of the load and go 53 metres down beneath the surface into the clay and beyond it until they reach a layer of stiff sand – which will provide the necessary support needed by The Shard to continue to stand without the risk of deformation or collapse.
Because soils are not uniform, that is, they are not the same everywhere even within the same environment; it is not unsurprising to learn that most skyscrapers in New York do not have foundations that deep – the foundations of the Empire State Building are only 16 metres deep due to the fact that Manhattan sits atop a stratum of mostly rock and rocky soils.
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 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 steel that shares with the concrete the task of supporting the weight of the building.
2. Frames and Shear Walls
Skyscrapers face another opponent – the wind. When it blows, its horizontal force is trying to tip the whole building over to the side.
While it may be easy to think that the wind is happening high above us, 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.
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 also known as shear walls. 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 or frame can be formed across the whole building and this enables skyscrapers to have those outlandish designs that you see.
The tallest towers tend to have a combination of these two systems – shear walls and frame construction method – allowing for more robust buildings that rise ever higher into the sky.
3. Mechanical Counterweight
Some buildings, like the Citigroup Centre in New York use a computer system to move around huge weights within the building, depending on which direction the wind is coming from. This manner of counterbalancing forces reduce the amount the building sways under different wind forces.
And there’s also consideration for seismic activity, especially in earthquake-prone areas of the world like China, Japan and South East Asia, and increasingly more so in areas previously thought to be not-so-prone to earthquake since the Haiti incidence.
So, for those of you who would like a career in structural engineering, take this as your introduction into a truly exciting field!