Sway With Me

Scientists estimate that by 2030, the number of people living in cities will rise 30% to five billion. We can’t spread out (unless we go off planet), so we must go up.

– Ricia Sturgeon-Hendrick, ELEVATOR WORLD July 2015

You can expect the number of tall, super-tall and mega-tall buildings to increase in the near and far future. This is fine with me. These structures are amazing, beautiful and are incredible feats of engineering.
One of the many challenges of building a high-rise is that it needs to be rigid enough to withstand strong winds, yet flexible enough to survive an earthquake. One fact that many people often forget is that these structures actually sway in winds. That thought is pretty frightening, but it is a necessary function for these buildings.
Let’s take a look at one example that helped redefine what was once thought impossible.

Enter: Taipei 101

taiwan-465784_1280


Located in Taipei, Taiwan, construction began in 1999 and was complete in 2004. Along with its completion came the highly sought-after title of world’s tallest building – – a title it held until more recently when it was surpassed by the Burj Khalifa in 2010. Taipei 101 has 101 floors and tops out at 1,667 feet (508 meters). For those keeping score, the Burj Khalifa is 2,722 feet (830 meters).
At first glance, you may think the 101 is a strange looking building, with its saw-tooth or step-like corners (also called reentrant corners). This design, however, reduces the buildings response to wind by 30 to 40 percent – – a critical feature considering Taiwan is subject to heavy winds from typhoons. The goal is to reduce “vortex shedding,” which is a wind phenomenon resulting from skyscrapers. The typical cost-effective shape when designing a skyscraper is a square or rectangle. It doesn’t take an engineer to realize that shape is not particularly aerodynamic. One method is to round off, or streamline the corners like the examples below: (click to enlarge)


The designs above make the building more aerodynamic, allowing winds to pass more smoothly.
The designers and engineers of Taipei 101 claim the structure can withstand winds up to 134 miles per hour (216 km/h).
The chopped corners of the 101 actually delay the formation of the dangerous vortices.
So problem solved, right? Well yes, if only Taiwan was not also subject to earthquakes.

Enter: The Damper

I recently started reading about and watching videos about dampers in buildings. I honestly had no idea such things existed!
In many tall buildings these dampers, usually located toward the top of the building, act as pendulum to counteract the sway caused by the winds. In other words, they push the building in the opposite direction. Check out Taipei’s record-setting damper:

That is one big ball. Photo credit: https://www.flickr.com/photos/rinux/2885419140

That is one big ball. Photo credit: https://www.flickr.com/photos/rinux/2885419140

Here is a nice illustration showing where it is located.

Photo credit: https://commons.wikimedia.org/wiki/File:Taipei_101_Tuned_Mass_Damper.png

I told you it was big! Photo credit: https://commons.wikimedia.org/wiki/File:Taipei_101_Tuned_Mass_Damper.png

 

 

This particular damper is unique for several reasons. One is that, unlike most dampers, Taipei 101’s is visible to its visitors; as it spans across 5 floors.
Another is that it is the largest (and most expensive) damper in the world. It weighs an impressive 728 tons (660 metric-tons) and cost US$4million. Now you can see why they’d want to display it!
The ‘golden orb’ as it is often referred to, consists of 41 circular steel plates of various sizes. Each plate is 4.92 inches (125 mm) thick. All are welded together to create the 18 foot (5.5 m) diameter sphere.

To prevent the damper from moving too much or continue to move due to its own weight, are several hydraulic arms bolted to the floor and attached to the orb. This makes it a “tuned mass damper.”

The general rule is that a building should not sway more than 1/500th of its height. If my math is correct, Taipei 101 could sway up to 3.3 feet (1 meter). Certainly, the residents and visitor would appreciate it if that distance were much less.
Thanks to this massive damper, it is… or at least they do not feel it as much. However, on August 8, 2015, during Typhoon Soudelor, the damper swayed a record 39 inches or 3.25 feet (0.99 meters)! And guess what? We have the video! Check it out:

Just remember: that is a more than 700 ton, 18 foot steel ball swaying more than 3 feet continuously over 1,600 feet in the air.
All buildings sway. If they’re tall enough, they probably have a damper inside too… just something to think about the next time you are enjoying the view from a high-rise.

Thanks for reading,

– Caleb

References:

The Heights: Anatomy of a Skyscraper by Kate Ascher; purchase at elevatorbooks.com
Wikipedia: https://en.wikipedia.org/wiki/Taipei_101#Structural_facade

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