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. Thanks for watching. I really enjoyed making this video and I hope
you enjoyed watching it. I have heard back from loads of you on how
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Another building that uses aerodynamics instead of a tuned mass damper is this highrise in Moscow.
I'm still wondering how they clean the windows.
Thanks for posting Escobeezy! This is probably the most fun I have had making a video. If ye have any questions, feel free to ask me here or on my twitter https://twitter.com/Fiosracht
There are three videos on the front page of this sub that are all sponsored by The Great Courses. . . Thats suss.
When Real Engineering and Wendover Productions post right after one another :D
I can't remember the last time I learned so many things in such a short amount of time. Exceptionally well explained.
Huh, I thought those helical fins were for cooling, I've only seen them on chimneys.