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Earlier this month I made a trip from Ireland to Dubai and as the plane climbed to cruising
altitude we were treated to the most amazing sunset, from 12 kilometres in the sky.
I had this moment of awe and gratitude that we live in a time where experiences like this
are possible and the feeling just got stronger when I arrived in Dubai and saw the Burj Khalifa.
The building defies belief, I lived in Malaysia for 3 years and used to love watching the
Petronas twin-towers glimmering in the distance as I drove through the hills of Bangsar.
Those towers held the title of the tallest building until 2004, but the Burj Khalifa
is almost twice the height.
I found myself looking up the tower and where I expected it to stop it just kept going,
soaring further than I thought possible.
It really got me thinking about how it’s engineers managed to overcome the challenges
that faced them.
The battle to build higher captured the world’s attention in the 1929 as the Chylser building
raised it’s secret spire like a proverbial middle finger to the Bank of Manhattan Trust
building, after months of intense competition to become the world’s tallest building.
It cemented Chrysler as a powerhouse in the American automotive industry, becoming a symbol
of the company's power and technological prowess.
The soaring skyscrapers of Manhattan were not only a symbol of the power of the companies
that built them, but were seen as an expression of America’s optimism and wealth.
The explosion in growth in America was largely fueled by the invention of the elevator and
the availability of cheap structural steel.
Frank Lylod Wright even proposed a The Illinois a 1.7 kilometre tall building in 1956 and
while the building was theoretically possible, it was completely unpractical.
Elevator technology had not advanced far enough and building sway would have been a huge issue
for comfort.
Tall slender structures like this are susceptible to wind induced vibrations.
Anyone that has seen lamp posts shaking in the wind will have seen this in action.
So what’s happening here?
Let’s place a cylinder in a wind tunnel and examine what happens as we increase the
air velocity.
In a steady flow of air, you would assume that the net force on the cylinder would be
in the same direction, like this.
And you would be right, at lower speeds this is the case.
Here the light pole would just bend in that direction and while the wind speed and direction
may fluctuate, you wouldn’t see the consistent back and forth vibrations.
As we increase the speed, the air begins to separate from the surface of the cylinder
creating two symmetrical eddies behind the cylinder.
Eddies are regions of slow moving swirling fluid.
You will see these a lot in rivers where branches or bridge pillars block the flow.
Here is one in my hometown of Galway, Kayakers use these eddies when they need to rest from
the fast moving mainstream.
If we keep increasing the fluid velocity these eddies will grow and the force on the cylinder
will also grow, but as long as these eddies are symmetrical the force will remain in the
direction of fluid flow, but there is a critical moment where the system loses it’s stability.
The energy gradient from the main stream and the slow moving eddies becomes too high and
the eddies begin to oscillate, at this point a phenomenon called vortex shedding occurs
and the resultant force is no longer directly downstream.
It teeters between the alternating low pressure zones as the vortices are shed on either side
of the cylinder.
This can become a massive issue if the frequency of the shedding matches the resonant frequency
of the structure.
That means that the direction of sway and the direction of the force become synchronised
and the amplitude of the swaying is allowed to grow as energy is being stored between
each cycle.
Every building dissipates some of that oscillation energy through natural damping through it’s
materials and through friction at the joints, but this is not always enough.
In these cases it is essential that the engineers add mechanical dampers.
These are usually hidden away in the guts of the building, but the world’s former
tallest building, The Taipei 101, decided to open their 730 metric ton tuned mass dampener
to the public.
On August 8th 2015 a category 5 typhoon slammed into Taiwan and set the Taipei 101s mass dampener
into motion and it was all recorded on a web camera.
So what’s happening here, how does this help stabilise the building.
When the tower is displaced the mass dampener does not move with it immediately, it is left
behind and then begins to sway independently of the building.
Now this is where the tuned part comes in.
The engineers will have tuned the damper to the same frequency as the building, so when
the building sways to the right the damper sways to the left and vica versa.
This creates an opposing force to the sway which is transferred to the building through
these piston dampers and thus the kinetic energy is dissipated and the magnitude of
the resonant motion is reduced.
Now what amazes me is that the Burj Khalifa has no mass damper.
It simply relies on clever aerodynamics from stopping those vortices from ever getting
organised enough to cause harmonic motion.
The reason light poles sway so easily is that they have a consistent cross-section, allowing
those vortices to slough off uniformly along the poles height.
So the same force is being applied at the same time along the entire length.
One way engineers combat that is by placing these helical spirals along the length of
cylindrical structures.
You occasionally see this with chimney stacks, but also in offshore platforms as vortex shedding
can also happen in liquids.
The helical fins disrupts the fluid flow along the length of the hull, preventing the vortices
from forming coherently.
The Burj Khalifa works in a similar manner albeit in a much more elegant fashion.
The building’s footprint was inspired by the desert hymenocallis flower and while this
is a beautiful design.
It provides an optimal amount of window space while also allowing the steel reinforced concrete
frame to take this shape.
This central core provides excellent torsional resistance while these y-shaped buttresses
provides fantastic lateral bending resistance, similar to how an I-beam works.
(on screen)I’ll explain that in more detail in a future video.
As the tower grows the building steps back consecutively like this, This spiralling pattern
works exactly like the helical fin on the platform earlier.
It prevents the vortices from sloughing off the building coherently along it’s length
and so stop them from exciting the buildings resonant frequency.
This is the genius of the building and why is doesn’t need a mass damper.
The architects put meticulous care into the buildings aerodynamic design using modern
computational analysis and wind tunnel tests to ensure the structural integrity of the
building.
It is clear that with the continuous improvement of technology, building these supertall buildings
is becoming less difficult and we are going to continue seeing the title of tallest building
in the world swap hands in the coming years, especially as the pressure to build higher
grows.
In 2007, the total urban population of the world surpassed the 50% mark, 20 years ago
that figure was just 33% and that statistic is expected to approach 80% by 2050.
Creating a functional city with adequate water and energy supply and everything else that
comes with a densely packed population will become an enormous challenge in the coming
years.
It is likely that these supertall buildings will become less of a decadent symbol of power
and wealth and become a necessary and fundamental part of the modern city.
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