This is a hole on top of the Shanghai World
Financial Center.
This is a 660-ton steel ball hanging inside
of Taipei 101.
And these are massive clockwise balconies
on the Burj Khalifa, the tallest building
ever made.
These design choices might seem like gimmicks
to give these skyscrapers their iconic looks.
But behind each feature is a brilliant engineering
trick designed to one thing:
Confuse the wind.
Wind can cause a bunch of issues for buildings:
broken windows, structural damage, and discomfort
for the people inside.
And today’s super-skinny skyscrapers have
to deal with a particular wind-induced phenomenon
called vortex shedding.
This happens when wind flowing past a building
creates vortices, strong swirls of air
that magnify the damaging effects of wind.
In low winds, these vortices cancel each other
out.
But in higher winds, they create alternating
low pressure zones that make the building
rock back and forth.
As the wind speed increases, so does the intensity
of the back and forth movement.
Every object also has its own natural sway
frequency
and when that matches with the frequency
of vortex shedding, it creates a dramatic
spike in the intensity of swaying.
On the top floors of a high rise, that kind
of swaying can be nauseating, plus it can
damage the integrity of the building.
But architects have an arsenal of tricks to
reduce movement.
The first one?
Tapering.
The higher up you build, the stronger the
wind force gets.
So to reduce surface area where the wind is
stronger, designers can simply make a building
skinnier as it gets taller.
They can do that with tapering — like The
Shard in London
or with periodic setbacks, like the
Willis Tower in Chicago.
Then, designers can soften edges.
Hard edges aren’t good on wind, so you’ll
often see skyscrapers with round corners.
But architects can achieve a similar effect
with small cutouts from the edges.
Take Taipei 101, for example.
The building was originally designed with
square corners,
but when a scale model was tested in a wind
tunnel, the designers saw a lot of swaying.
Here are the results after designers added
sawtooth corners.
They reduced movement by 25 percent.
The next option is pretty simple. You can
just open it up with holes.
Skyscrapers like Saudi Arabia’s Kingdom
Centre and Shanghai’s World Financial Center
do this with a single gap up top, allowing
wind to pass right through where it’s blowing
the strongest.
But 432 Park Avenue in New York achieves this
effect with several double-floor cutouts that
allow wind to pass through along the length
of the entire tower.
There’s also twisting.
This wind resistance technique makes for some
of the most stunning skylines today.
Dramatic spirals redirect the wind, guiding
it upward and off of the building.
That’s the same wind resistance trick used
by some industrial chimneys and car antennas.
Corkscrew shapes like this were impossible
to build until fairly recently, thanks to
advancements in software and material science.
And they’re also promising from a sustainability
perspective.
During the design process on the Shanghai
Tower, for example, adding the iconic twist
reduced the wind load by 24 percent, saving
developers $58 million in structural material.
Finally, there’s the technique so good it’s
invisible: damping.
Dampers are mechanisms designed to absorb
the energy from a building’s movement, counteracting
the effect of the wind.
Skyscrapers do this in two major ways.
First are slosh tanks: these are containers
filled with several tons of water.
The water sloshes back and forth, and its
weight displacement helps keep the building
from swaying.
Second are tuned mass dampers: massive weights
suspended in the middle of a building.
These were traditionally hidden away in building
design, placed on empty floors along with
other technical equipment.
But they don't have to be. Taipei 101's
tuned mass damper has been a popular tourist
attraction since it opened in 2004.
They even have a mascot for it: Damper Baby.
It’s a little weird.
These shapes, holes, and counterweights form
a secret design language hidden inside of
our skylines.
And as more people move out of rural areas
and into urban ones,
skyscrapers will keep getting taller and
skinnier.
These technologies are what’s making that
future possible
and letting us keep building into the sky.
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