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Get an engineeer's-eye-view of the tallest buildings in the world, to learn what challenges they face as they reach for the sky and wonder, how tall can we build?

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For nearly 40 years, New York City's Empire State Building was the tallest building in the world.  Topping out at 381 meters, it now feels like something of a relic.

The tallest building currently under construction, Saudi Arabia's Kingdom Tower, will, if all goes as planned, be two and a half times taller than the Empire State.  Yes, we are talking about a 1km tall building.  How is that possible?!  And how high can we go?

I'm sure the answer to the second question is, "No one really knows," but a good look at the Kingdom Tower and other super skyscrapers will give you a sense of the challenges that come with building something that's as tall as, like, from here to the nearest Starbucks.

Kingdom Tower was originally conceived as a mile high building, or 1.6km, but the project was scaled back after soil testing revealed layers of weak, porous limestone and sandstone underneath.

To provide a sturdy foundation for this ginormous structure, 270 long concrete cylinders called piles have been installed, as much as 1.8 meters in diameter and at depths up to 110 meters.

Working up from there you have building materials to consider.  Three decades ago the tallest buildings on Earth were mostly made of steel but steel is crazy heavy so today super structures are built mainly with concrete or combinations of steel and concrete.

Now, transporting concrete 800 meters above the ground requires some creative engineering and it may be one of the trickiest limitations to get past if we're gonna go beyond the 1km mark.

For example, to construct the 820 meter Burj Khalifa in Dubai, currently the world's tallest building, engineers have to invent a 15cm wide pressurized tube to transport wet concrete to the top and they had to do it all at night so that in the heat of the day it wouldn't dry en route to the 163rd floor.

Some engineers say the future of building materials may be in carbon-fiber composites. the light-weight and super strong materials that are already used to build fancy bicycles and modern airplanes.  The downside is the cost.  Carbon fiber is very expensive.  And even if a builder could afford enough of it to get to 1km in height, there's still the question of how you keep the building standing because the taller it gets the harder it is to stabilize.

Wind in particular is a constant challenge for building designers and it's more complicated than just making sure the foundation is strong.

The tallest skyscrapers today have to regularly withstand wind speeds of over 150km/h.  To do that, most are built with joints at the corners where steel beams can expand and contract, allowing the building to sway.

That's a good start, but you also have to contend with wind vortices.  As the vortices keep moving, they quickly separate from the building and when they do, each one essentially pulls the structure toward it.  These eddies of air end up yanking the building from side to side, perpendicular to the flow of the wind.

This is called vortex shedding and you might've seen it's weird-looking effect on a flag pole or a light post on a windy day.

Wind tunnel tests have helped engineers to find some solutions to the phenomenon, like rounded edges and notches on a building's face that can act like winged shock absorbers.

But then, more practically, just about every skyscraper architect will tell you about the elevator problem.  I mean you could build the world's largest building, but if you can't get people up there to buy T-shirts, what's the point?

So the elevator problem is two-fold.  First, elevator shafts can't exceed 600m in length.  At that point that used to move the cars just become too heavy to work.

In the Burj Khalifa, engineers solved this problem by creating what they call sky lobbies where passengers heading to the top get off and take a second elevator ride.

But modern super skyscrapers, they're also limited by elevator speeds.  No one wants to take 15 minutes to get to work on the 120th floor but if you go too fast, then you're talkin' about air pressure and ear popping, not to mention queasy passengers.

But most elevators don't exceed speeds of 10m/s and those have air pressure controls and roller guides to dampen vibrations.  

To move people more quickly, one option might be electromagnetically driven elevator cars that don't need any cables.  One company has already created a prototype using technology similar to Maglev Motors in new train systems.

So it's gonna be 2019 at the earliest when the world gets its first look at a kilometer tall building and we could learn how engineers defeated some, or, hopefully, all of these problems.

Will we go higher?  Probably.  But until somebody comes up with super cheap carbon fiber and perfects the electromagnetic elevator, the steps are gonna be incremental and few and far between.

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