The Taipei 101, at the time of its completion in 2004, was on the cutting edge of engineering and technology, breaking the records for tallest building in the world and fastest elevators in the world. Another impressive engineering feature resides on the 87th floor: a 728 ton weight, called a tuned mass damper, that keeps the structure from swaying.
On the surface, this concept doesn’t make sense. Putting the weight at the top of the tower would make it less stable, not more, right?
Take the difference between standing up and sitting down, for example. A person who is standing is more likely to be blown over by a gust of wind, than is a person sitting down. This is because the person’s center of mass is higher when standing than when sitting.
In physics, the center of mass is the point inside or outside an object at which an object’s mass is considered to balance. For example, the center of gravity of a solid ball, like the damper of the Taipei 101, would be in roughly the center of the ball, like this:
For a skyscraper with no weight on top, the center of gravity would probably look something like this:
For a skyscraper with a weight at the top, though, the combined center of mass of the tower and the weight would probably be somewhere higher:
As long as nothing or nobody tries to tip the building over, this isn’t a problem. If for any reason the center of mass moves outside of the building’s footprint, a collapse is possible. This seems to put skyscrapers with dampers at a disadvantage, because they need to move fewer degrees to tip over:
Therefore, by moving the tower’s center of mass upward, a skyscraper’s damper reduces the amount the building has to move in order to fall over. How can this stop the building from tilting? This comes down to mass, weight and inertia.
Mass is a tricky term to define. Merriam Webster’s collegiate dictionary, 10th ed defines mass as “the property of a body that is a measure of its inertia and that is commonly taken as a measure of the amount of material it contains and causes it to have weight in a gravitational field.” The two critical terms in this definition are inertia and weight.
Inertia is an object’s “indisposition to motion.” For example, moving a little matchbox car is very easy, because it has little mass. Pushing an actual car, however, generally can’t be done by one person alone, because the car has too much mass. When describing this scenario, it is important to distinguish between mass and weight. Weight refers to the presence of a gravitational field. For example, an object floating around in space still has mass, because it has inertia, but it is weightless, because it is not under the influence of gravity.
Therefore, the damper’s inertia at the top of the tower makes that portion of the tower more resistant to movement when a gust of wind strikes. Given a strong enough gust of wind, however, the tower will still move. How does the damper help with that?
The answer to that question comes down to weight. The damper is balanced so that it always wants to return to the center position where the tower is completely upright. When the tower moves, the damper is moved out of its stable position, and its weight begins to pull against the tower’s sway through the suspending cables. This brings the tower back to its upright position.
Obviously, this is only the basic concept of a skyscraper damper. The reality that most people miss is much more complicated, since engineers have to account for the building’s weight, structural features, and location. This damper is only one part of a much greater work: the Taipei 101.
Gröndahl, Mika. “Reducing Skyscraper Sway.” The New York Times, 6 Aug. 2015, www.nytimes.com/interactive/2015/08/06/realestate/Reducing-Skyscraper-Sway.html?mtrref=www.nytimes.com&assetType=REGIWALL. Accessed 30 June 2020.
Lewis, Robert. “Taipei 101 | Building, Taipei, Taiwan.” Encyclopædia Britannica, 2019, www.britannica.com/place/Taipei-101. Accessed 30 June, 2020.
Willett, Megan. “Asian Skyscrapers Dominate A New List Of The World’s Fastest Elevators.” Business Insider, 23 Jan. 2013, www.businessinsider.com/the-8-fastest-elevators-in-the-world-2013-1. Accessed 30 June 2020.
Higgins, Michelle. “Keeping Skyscrapers From Blowing in the Wind.” The New York Times, 7 Aug. 2015, www.nytimes.com/2015/08/09/realestate/keeping-skyscrapers-from-blowing-in-the-wind.html. Accessed 30 June 2020.
Featured Image Credit:
Pixabay User Peggy_Marco. https://pixabay.com/photos/taiwan-taipei-101-china-taipei-101-1954652/