TOEFL Reading Practice #019 — The Ball That Steadies a Skyscraper

The Ball That Steadies a Skyscraper

¶1 Every building taller than a few hundred meters moves. Wind presses against its face continuously, and for most of the twentieth century engineers treated that movement as a flaw to be built out. The usual fix was to add mass and stiffness until a tower barely swayed at all. Past a certain height, that approach is both costly and self-defeating. A frame rigid enough to resist the wind can crack from the very stress it was built to resist. The real fix looks backward at first. Instead of holding a tower still, engineers let something inside it swing freely. Only one exact condition determines whether that swinging calms a building or shakes it further.

¶2 Taipei 101 is a 508-meter tower completed in Taipei, Taiwan, in 2004, and it shows why. The region gets both typhoons and earthquakes, so the building's frame has to flex. A perfectly rigid structure that tall would concentrate stress at its base and eventually fatigue the steel. The tower is therefore designed to sway, a few centimeters at the top in ordinary wind and considerably more in a storm. The trouble is that this motion can build on itself. Wind gusts arriving at close to the tower's natural period, the rate at which it would swing back and forth on its own, push it further in the same direction each time, a process engineers call resonance. Left alone, that buildup can turn a mild sway into something strong enough to make people on the top floors feel ill, even though the structure itself stays safe.

¶3 Adding more steel does little to solve this problem. A heavier tower still has its own natural period and can still be pushed into resonance. The actual fix seems backward. Engineers do not try to make the tower more resistant to motion at all. Instead, they add a tuned mass damper, a second mass suspended separately so that it can swing on its own. The damper is adjusted so its swing matches the tower's natural period but moves opposite to it, and its momentum cancels much of the building's motion at each swing. The condition is exact. A damper tuned even slightly off the tower's period barely helps, and one tuned to the wrong period can do nothing at all.

¶4 Taipei 101's version of that second mass is a steel sphere. It weighs about 660 metric tons and hangs from four cables between the 87th and 92nd floors. [A] Built from more than forty layered steel plates, it swings like a huge pendulum, and eight hydraulic pistons connect it to the surrounding structure to absorb energy as it moves. [B] The clearest demonstration came on August 8, 2015, when Typhoon Soudelor struck Taiwan with winds strong enough to rattle windows across the city. [C] Cameras inside the viewing gallery recorded the sphere swinging nearly a meter from its resting position, the largest movement measured since the tower opened. [D] Engineers later confirmed that the sphere's swing had been almost exactly out of phase with the building's own motion. That timing is precisely what kept the tower's sway from growing larger as the storm continued.

¶5 The same idea now protects structures far from Taiwan. Tuned mass dampers, or smaller versions of the same idea, have been installed in towers from Shanghai to New York and in long bridges prone to wind-driven oscillation. None of them work automatically. Each mass has to be calculated and adjusted for the specific structure it protects, since a damper tuned for one building's period does nothing useful on another. The lesson engineers drew from Taipei 101 was not that heavier construction is safer. It was that motion, matched precisely against motion, can be the more reliable defense.