Having the title of the world's tallest skyscraper is an exemplary achievement for developers across the world. Ever since we started to build vertically, we've been competing to see who can build the tallest structure.
Skyscraper heights have exploded in recent years thanks to advanced engineering and renewed monetary investment. However, most of the world's tallest buildings are hiding a dirty secret: they're not actually that tall.
What makes these buildings so tall?
The tallest points on the world's tallest buildings do actually reach staggering heights, but the actual building part rarely does. Developers have long placed spires or towers on top of skyscrapers to just inch out the competition and win the title of world's tallest building. These pinnacles often add hundreds of feet to the height of buildings, all with relatively low cost. They also reduce the complexity of engineering.
The Council on Tall Buildings and Urban Habitat, CTBUH, studied all of the world's tallest buildings and found some interesting data. They discovered that nearly 60% of the world's tallest buildings, called supertalls, actually wouldn't be supertalls without added wasteful space on top, such as spires. The organization calls this wasteful height "vanity height."
A closer look at vanity heights
Take for example the Burj Khalifa, the current holder of the world's tallest title. It stands at 828 meters tall from base to top of the spire. However, the highest occupied floor only goes as high as 585 meters. This means that there are 244 meters of non-occupiable space on the skyscraper that otherwise makes it so tall. This equates to roughly 29% of the structure.
The Bank of America tower in New York is even worse. This structure stands 366 meters tall, an otherwise respectable height for a tall building. However, useable floors stop at just 235 meters, meaning that 131 of the 366 meters of height are left unusable. That equates to 36% of the entire building. That may seem bad, but it's not even the worst supertall in terms of wasted space in the world. That honor, or rather disappointment, goes to the Burj al Arab.
The Burj al Arab stands in at 321 meters tall and is the world's tallest hotel. You've likely seen photos of the building's striking construction, but you'll notice that the top is mostly architectural fluff. All that fluff equates to 124 meters of non-occupiable space – or 39% of the entire structure's height.
You can see that this problem isn't just limited to these buildings either in the chart below. 95 out of the 360 meters of the Pinnacle building is all vanity or 27%. Emirates Tower One has 113 out of 355 meters as vanity space or 32%. Finally, the 450-meter-tall Zifeng Tower has 133 meters of non-occupiable height or 30% of the entire structure. This is an incredibly persistent problem when it comes to supertall structures.
These findings from CTBUH mean that while these structures are still tall, their height isn't as impressive. It takes much less engineering to construct these un-occupiable spaces and thus, their creation is almost always a ploy to win the title of the world's tallest.
The problem of vanity height isn't necessarily a new one, but it is getting worse. The Chrysler building is 21% vanity height and the Empire State is 2% vanity height. These were some of the first structures to start the trend in the 1930s. It was during this era that the competition for the world's tallest building began heating up and only accelerated into the 21st century. Humanity's drive to have the biggest, tallest thing means that sometimes engineers make buildings taller than they really need to be.
Is there anything inside the vanity height of buildings?
After learning about the free space at the top of most modern skyscrapers, you might wonder if there's anything at all up top. The short answer is yes.
In the top of most modern skyscrapers is something called a tuned mass damper, or TMD. This device helps to stabilize the height of the building as it sways in winds or in earthquakes.
It should be noted though, in reference back to "vanity height", the world's tallest structures would still have a TMD in most cases if they didn't have as much vanity height. That's to say that just because there is something useful in the so-called vanity height, the height itself is rather unnecessary.
Drawing back to tuned mass dampers, they're essentially giant ball-like masses suspended to counteract the dynamic forces of the building.
The damper in each building is tuned to that particular structure. That means that when the structural frequency of the building is excited, the damper will begin resonating. However, the damper resonates perfectly out of phase with the building. If the structure sways to the right, the damper perfectly sways to the left. This action counteracts the sway of the building and dissipates the inertia of the damper into the structure.
The concept for tuned mass dampers was first applied to structure design in the early 1900s. At first, though, the idea was applied to the structure of ships to mitigate hull rock and vibrations. It wasn't until about the middle of the century that tuned mass dampers were first used in structures.
At first, they were used to counteract forces that were unique to very tall buildings. Dynamic loads like thousands of people moving inside at once, or large wind forces pushing on the large surface area of skyscrapers. By late 1950 these mass dampers were implemented to counteract earthquake forces.
The best way to think about how tuned mass dampers work is this, harmonic vibration. By tuning the way that the large physical mass is held in place, you can make it move in a way that is perfectly harmonic with the movements of the building. Doing this doesn't completely cancel out the building's movement, but it does make the peaks and valleys much less intense.
When it comes to structural loading, this is a good thing. Sharp, fast, large forces can quickly take down a building, but if you slow down the impact of these forces over a greater period of time, the building can absorb and move as it needs. This is in many ways like crumple zones in a car.
The tuned mass damper is the crumple zone of skyscrapers, absorbing intense forces and spreading them out to help the overall building not move or react as much.
So, up the top of these super-tall buildings sits giant tuned mass dampers and maybe some mechanical equipment. For the most part, however, their design is that of vanity. After all, having the title of world's tallest building is quite the achievement.