Wind is one of the most important factors
that architects and engineers must consider
when designing tall buildings.
While skyscrapers might appear to be highly-strengthened, immovable structures,
all tall buildings are in fact designed with a degree of flexibility in mind.
This is principally due to the impact that
wind forces (known as “wind loads”) have
on a building as it becomes taller. Whilst
you might be experiencing a pleasant breeze
at street-level, the force of the wind generally
grows much stronger the higher up you travel.
While the steel and concrete used in a skyscraper’s
superstructure is designed to bend and flex
to absorb the impact that these wind loads
have,
the degree to which a structure is able to move
can have a significant impact on the comfort of those inside the building.
When buildings first began to grow tall in
the 1890s and 1900s, height limits were imposed
- such as those introduced in Chicago - to
prevent their masses from blocking sunlight.
In New York City, ordinances were passed that
allowed tall buildings to develop on the basis
that they were set-back after reaching a certain
height.
This allowed sunlight to reach street-level
whilst breaking-up the facade and reducing
the impact that high winds had on these early
towers.
By the 1960’s however, larger, box-like
skyscrapers began to come to prevalence,
bringing with them a whole host of wind load engineering challenges.
The first problem that began to arise was
increased wind velocity at street level.
This was principally caused by the “street
canyon” phenomenon; an effect that sees
large buildings redirect wind down their facades
- which effectively act like canyon walls
- and funnel it along streets at much higher
velocities than in low-rise, suburban areas.
The street canyon effect was particularly
notable in Manhattan where the heavily formalised
grid structure of the city blocks offered
little to break up and deflect winds once
they began to blow.
Additionally, as wind moved around the top
of these tall structures, vortices were being
created in a process known as vortex shedding.
This process - much like water flowing down
a stream - acts differently on obstacles depending
on how streamlined they are.
In the case of these buildings, their sheer
block walls created a “bluff” obstacle
that wind had to flow around.
As strong winds moved around these structures,
areas of low pressure emerged on the opposite
side of them, creating suction forces that
pulled at the buildings, causing them to sway
back and forth.
While any such movement may initially be minimal,
high winds can create vortices that can match
the frequency of the building they are moving
around - causing noticeable swaying and shaking
motions for those inside.
This phenomenon led engineers to begin testing
models of tall buildings in wind tunnels at
design stage, assessing the potential impact of high winds on structures before they were constructed.
By doing this, project teams were able to
develop innovative approaches to managing
wind loads, reducing their impact on tall
buildings and enabling them to rise even higher.
The first and by far simplest way to reduce
the impact of high winds on a tall building
is with an approach called “corner softening”.
Corner softening sees sharp edges smoothed-off
of a structure to make it more aerodynamic,
or small cutouts created on the edges of an
structure to “scramble” prevailing winds
and reduce the strength of the vortices they
create.
A prominent example is the ornamental design
of Taiwan’s Taipei 101, where relatively
minor cutouts on the building’s corners
reduced movement by as much as 25%.
Tapering a building as it rises also breaks
up the uniformity that causes vortex shedding.
Kuala Lumpur’s Petronas Towers and The Shard
in London, both use this technique to reduce
the effect that high winds have on their structures.
Taking things a step further, alternating
a building’s profile as it rises and including
setbacks can also reduce the strength of vortices
as they move around buildings.
Some of New York City's early skyscrapers achieved this in response to the setback ordinances of 1916.
But perhaps the most notable example today
is the 828 metre Burj Khalifa in the United
Arab Emirates (UAE) - the world’s tallest
building at the time of filming.
This remarkable structure uses a range of
techniques to tame the wind and achieve its height -
including an extreme taper, multiple
setbacks and high degree of corner softening.
We’ve demonstrated how the Burj Khalifa’s
carefully crafted design manages the wind
in this simulation - developed using SimScale’s
software.
With more than 150,000 users worldwide,
SimScale is an easy-to-use could-based engineering simulation platform
that enables anyone to create powerful high-fidelity simulations in a web browser.
The platform can be tried for free through the Community account, which gives access to thousands of
public simulations to promote knowledge sharing and to crowdsource advice.
Creating a twist in a building’s form can
also reduce the impact of vortex shedding.
With every floor offset to the last, the number
of “bluff” areas across these structures
is considerably reduced, minimising - or in
some cases completely eliminating - the locations
where vortices can form.
Perhaps the most breath-taking example of
this technique can be seen in China’s megatall
Shanghai Tower; which tames the wind and rises
to becomes the world’s second tallest building
by elegantly twisting throughout its 632 metre
height.
Another way to reduce the impact of high winds
on tall buildings is to increase their porosity
- “cutting out” parts out of the structure
and allowing air to flow through, as well
as around the building mass.
This technique has been used in a number of
high profile skyscrapers around the world
- including Saudi Arabia’s Kingdom Centre
and the World Finance Center in Shanghai, China.
But the most impressive example can be in
seen New York City’s 432 Park Avenue.
With an incredible width-to-height ratio of
1:15, the tower is one of the world’s most
slender skyscrapers and the most prominent
manifestation of New York’s emerging super-skinny
residential tower trend to date.
The 426 metre tower features double floor
cut-outs at 12 storey intervals throughout
its height, allowing wind to pass through,
as well as around its extremely thin structure.
We have again demonstrated the impact of this
approach in a SimScale simulation.
Some buildings using this approach have tried
incorporating wind turbines into their voids
in an effort to harness wind energy and convert
it to electricity.
Despite the obvious benefits of this, wind
turbines on skyscrapers never really seem
to have caught on.
In addition to the techniques that tall buildings
use to tame the wind, some are also fitted
with dampers that counteract motion where
it occurs.
These incredibly heavy instruments are suspended
- often on the upper levels of skyscrapers
- and sway as buildings move, counteracting
motion and creating a more stable environment
for those inside.
Several tall buildings already use these dampers
- including Taipei 101 in Taiwan and the Trump
World Tower, 432 Park Avenue and 53W53 in
New York
The advancements in wind load engineering
made over the last 100 years have enabled
the incredible skyscrapers we see today to
become a reality.
With further research underway and an incredible
range of new technology innovations currently
entering the construction industry, we could
see tall buildings rising even higher in the
years ahead.
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