Reply to Ian’s Post: Shake or Stand?

After reading Ian’s blog post ‘Shake or Stand?’, I suddenly made a connection to my physics class. This is because in his post he had mentioned the words ‘natural frequency’ and coincidentally, we’re learning about waves and natural frequencies in physics. So, what is natural frequency? A frequency that’s natural? No, it is the frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed, or somehow disturbed is known as the natural frequency (the Physics classroom). In simple English, it’s a frequency that causes an object to vibrate back and forth. In his post he also mentions that each building has it’s own unique natural frequency. I did a bit of research and found that the natural frequencies of vibration of a building depend on its mass and how stiff the building is. So, a taller building will have a lower natural frequency because it’s heavier and taller which makes it more flexible (IDEERS). Why is this important? It is important to determine a building’s natural frequency so then you know how the object vibrates, which in turn you can counteract that vibration by using a damper. A damper is a heavy weight that vibrates in the opposite direction of the building thus effectively canceling out any vibrations caused by the building.

The Damper that engineers use in Taipei 101 to counteract any vibrations in the building.
(Figure 1: The Damper that engineers use in Taipei 101 to counteract any vibrations in the building.)

Ian’s post mentions a building reaching it’s natural frequency potentially from vibrations from earthquakes, but a natural frequency of a building doesn’t occur just from earthquakes. It’s also possible for the natural frequency of a building to be reached from wind. As stated in the post, an earthquake causes the ground to vibrate, which in turn sends shockwaves up to the building that causes it to vibrate. The same principle applies to wind, except this time the vibrations start from the top. Wind is weaker at lower elevations because friction (e.g. mountains, hills, buildings, trees) will cause the wind to slow down, and wind is stronger at higher elevations because there is little to no friction. (Air Pressure and Wind). So the higher the building, the more susceptible it is to strong winds. Now, in the present day, engineers have revolutionized their thinking when it comes to buildings, instead of fighting against nature, why not work with nature? The Shanghai World Financial Centre (SWFC) applies this concept. At 492 meters all, the SWFC is the third tallest building in the World. At this height, it means that it is very susceptible to strong winds and it has a huge surface area at the top, which could then lead to the possibility of the building tipping over.

(Figure 2: Picture showing how the wind flows around a building.)
(Figure 2: Picture showing how the wind flows around a building.)

Think of an empty water bottle, it you blow towards the bottom of the bottle it requires a lot of strength to knock it over, however, if you blow towards the top of the water bottle, it tips over very easily. This is the same concept for a building. Which is why engineers have decided to put a big opening at the top of the SWFC, to allow the wind to flow through the building, effectively decreasing the possibility of the building tipping over and vibrating as a result from strong winds.

Figure 2: Picture of the Shanghai World Financial Center
(Figure 3: Picture of the Shanghai World Financial Center)

But wind doesn’t just affect buildings; it also affects other structures such as bridges. A famous example is the Tacoma Narrows Bridge in Washington State. On an early morning of November 7th, 1940, under strong wind conditions, the bridge began to twist and turn in a vertical motion (transverse vibration) (Wikipedia). As the wind became stronger and stronger, it caused the amplitude of the vibrations to increase because the wind was ‘putting in’ more energy then the flexing of the structure can dissipate. The Tacoma Narrows Bridge (aka Galloping Gertie) soon reached its natural frequency and snapped in half. The bridge was rebuilt and this time, the engineers were much more careful and regarded wind with the respect it deserves. A local camera store clerk recorded the collapse of the bridge on film and to this day, it serves to engineering, architecture, and physics students as a warning that nature is not to be underestimated.

Figure 3: Picture showing the collapse of the Tacoma Narrows Bridge in Washington State
(Figure 4: Picture showing the collapse of the Tacoma Narrows Bridge in Washington State)

Click here to see footage of the collapse of the Tacoma Bridge.

Click here to learn more about Resonance and Natural Frequencies.

Click here to learn more about Natural Frequencies and Simple Harmonic Motion

Works Cited:

Resistant Buildings – Vibrating – The Natural Frequency of a Building. IDEERS from Bristol University, 2008. Sun. 27 Feb. 2011.

Resonance and Standing Wave – Natural Frequency. the Physics Classroom, 2011. Sun. 27 Feb. 2011.

Tacoma Narrows Bridge (1940). Wikipedia, 2011. Sun. 27 Feb. 2011.

Wind. Air Pressure and Wind, 2003. Sun. 27 Feb. 2011.

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