I'm 360 meters above the streets of Manhattan
climbing up the side of an actual skyscraper.
The winds up here can get pretty extreme,
which don't just make the climb more exciting,
but also pose a unique set of engineering challenges.
When solved correctly,
they result in some of our most iconic buildings.
When not, well, there's one we'll be visiting later
which almost got blown over.
On of the first things you'll notice when entering New York
is this city's an extremely blustery place,
almost like the city is some how magnifying wind speed.
To find how much that's true,
I've brought with me this anemometer, or wind speed gauge,
in order to get some measurements from around the city.
At the moment, it's about .5 meters per second.
Not particularly high,
but let's see what it's like in other locations.
The regions around waterways and Central Park
had the lowest wind speeds
with the highest around Midtown and the Financial District.
The region between was a bit more sedate.
If we change our perspective, we see that high wind speeds
correlate with a high density of skyscrapers.
That's because of something known
as the Street Canyon Effect
where tall buildings redirect wind onto the street.
This effect is compounded by New York's gridded layout
which acts to channel in and accelerate winds
flowing parallel to the road.
Conversely, meandering routes, smaller buildings, and trees
impede the wind's flow
which is what gave the low-rise regions and Central Park
comparatively sedate speeds.
Another factor to consider,
especially relevant for skyscrapers,
is that wind speed increases the higher up you go.
That's because there is less friction with the ground.
Up at the top of the Rock,
wind speeds are a couple of meters per second.
While down at Rockefeller Plaza it was more like one.
With these effects combined,
a rough calculation shows us that during a storm,
the Empire State Building should be experiencing
some 20 million newtons of horizontal wind loading,
roughly equivalent to hanging nine Statue's of Liberty
off the side.
To find out what sort of affect that what this wind loading
has on structural stability,
and to find some methods to over come it,
I've come to Central Park
with a few miniature skyscrapers models and...
(blower whirling)
an electric leaf blower.
Our first model takes the form
of an aggressively box-like tower
that would look out of place on the engineering campus
of any major university.
This is going to be a control,
a skyscraper with out any form of wind mitigation.
The structure oscillates back-and-forth.
Our residents will not be having a good time.
I first thought that this wobble was caused by variations
in wind speed and direction,
but I have a leaf blower that is fixed in position,
so it's got to be caused by something
a bit more complicated.
To find out what, I ran some simulations.
Here is a model I made in SolidWorks,
which yes, in retrospect,
I realize probably wasn't the right tool for the job.
But it's what I had at hand.
Here we have a horizontal, 2D cross-section
of a skyscraper, with wind flowing from left to right.
These arrows indicate the air's velocity.
But to better highlight structure,
let's switch to vorticity,
which essentially tells us if the air is spinning clockwise,
counterclockwise, or if it's stationary.
The fluid's inertia causes it to detach
from behind the skyscraper.
This creates a low-pressure region
directly behind the obstacle.
Nature hates this imbalance,
and eventually, almost like magic,
you can see these vortices forming
as first one side and then the other
gets sucked in and blown away.
This makes the pressure field oscillate as well,
generating a force causing the building to sway.
Older skyscrapers like the Empire State
and Chrysler Buildings,
overcome these effects by building really strong,
really sturdy structures,
which are less likely to feel the effects of the wind
by nature of them being really heavy.
That's equivalent of replacing a paper skyscraper
with one made out of cardboard.
While our second model still wobbles,
it does so to a much lesser extent.
Unfortunately, building in this way is extremely expensive
and woefully wasteful,
so today, other approaches are used.
Moving down the line, the next option
is just to put a really big hole through your skyscraper.
This reduces the wind loading
and therefor, reduces wobble.
You can also adapt to the certain few architectural needs.
The Shanghai World Financial Center has one such hole,
as do these high-rise apartments in Hong Kong.
While some attempts have been made
to incorporate wind turbines into these holes,
their success is rather limited.
Those in London's Strata SE1 rarely spin.
Back in New York, 432 Park Avenue
uses these wind hole features to exploit a loophole
in New York's zoning laws.
Floors reserved for structural and mechanical equipment
don't count against a building's maximum size.
As of recording,
it's the world's third tallest residential building,
in no small part, due to these five windbreaks,
which artificially, elongate the structure.
Similar wind benefits can be derived
by giving the upper floors a tapered design,
as seen in One World Trade Center, 50 West 53,
and 30 Hudson Yards.
If we change the shape of our building,
we can prevent vortex shedding before it really starts.
This significantly cuts back on vibration.
One way of doing this is including cut-backs from the design
as we have here.
Although there are various other retrovations,
including spirals and different setbacks,
which have a very similar effect.
Running the simulation again,
you can see what while some vortex structures still form,
they are much less defined.
The pressure field confirms the oscillating forces
are significantly reduced.
Combining and playing off of these techniques
is the job of engineers and architects.
But when things go too far,
we risk the lives of the very people
that these buildings were designed to hold.
This is 601 Lexington Avenue, or at least part of it.
As it turns out, filming a skyscraper from ground level
turns out to be rather challenging.
Fortunately though, most of the interesting stuff
is down here at ground level.
Back when the land was purchased in 1968,
it was done so under condition
that they wouldn't be interfering with St. Peter's Church,
which still resides in a corner of the property.
These constraints gave Lead Engineer, William LeMessurier,
a unique engineering challenge.
After days spent professoring at MIT,
he was having dinner at a Greek restaurant just off campus.
Doodling on a napkin, he surveyed his options.
LeMessurier's first idea,
was to design the skyscraper like a table
with forces transferred horizontally and then down
through four corner columns.
Unfortunately, this just wasn't going to work
with a church in the way.
So he moved the columns toward the center.
Unfortunately, this meant that the floors
were no longer supported and would just collapse.
This problem hadn't been faced before
with skyscraper design,
but trees have been doing something very similar
for millions of years, as they support their canopy.
His idea was to uses some branch-like diagonals
to transfer forces towards a central trunk-like column.
Unfortunately, this design was now quite unstable,
so to reduce building wobble,
he added in a Tuned Mass Damper,
recently invented by one of his colleges.
It was under repairs while I was in New York,
but here's a video of back in 2015 during a storm.
Located near the apex of the tower,
a large concrete mass is supported
on some electronically controlled springs.
This helps to absorb up the building wobble,
keeping the entire thing stable.
Upgrading his design from a napkin doodle
into something slightly more professional,
he submitted his proposal.
And in April of 1974, work began on the skyscraper.
Upon it's completion, 601 Lexington Avenue
was heralded for its unique design.
However, soon concerns were raised internally
about the building's structural integrity.
During fabrication, the conventional wisdom
had been that diagonal wind forces on a skyscraper
were negligible.
Appropriately, 601 had been designed
with only the sideways winds taken into account.
However, What the original engineers hadn't realized
is that 601's novel branch structure
would result in diagonal wind loads being carried unevenly;
doubling the stress in some of the beams
while dropping it to zero for the others.
Talking with a student about his interesting design,
LeMessurier was suddenly hit with a realization.
He hadn't taken into account diagonal wind loading
and that was going to be a problem.
Updating his calculations, LeMessurier realized
that if power to the Tuned Mass Damper
were lost during a storm,
and if those storm winds were particularly significant,
then the resulting sway
would cause the doubly-stressed branches to fail
and the entire structure would come crashing down.
Such an event was expected to occur
roughly once every 16 years.
With hurricane season fast approaching,
something had to be done.
LeMessurier put together a team of welders,
working late at night so as to avoid panicking
the civilian population.
After two nail biting months,
the weaker bolted sections were retrofitted
with welded supports
and the structure was deemed safe.
In a final twist, it appears that these frantic retrofits
may not have been entirely required.
Applying modern wind simulations tools,
it appears that the loads would not have actually been
significant enough to break the beams.
However, since there was no way of knowing this at the time,
I think LeMessurier made the right choice.
New York is a city that hasn't just defined skyscraper
as a really-tall building,
but also as an art form shaped by the wind.
Next time you're walking along the blustery streets
of your local city, make sure to keep looking up.
This has been James Dingley from the "Atomic Frontier."
(gentle music)