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skyscraper. Photo source: Ermell / Wikimedia Commons, CC0 looks out of the window at home, still towering buildings, but when he approaches the window, he notices that there are several "pipes", perhaps lightning rods, standing on most of the buildings.

Before, I might have thought they were standing quietly in the busy city, but now I see a picture of some engineers dragging measuring instruments to the roof and telling the world, "tall buildings cannot remain moving." And they are still making sound all the time.

In the 1929 July issue of Skyscraper, a survey found that there were already 377 20-storey buildings in the United States, 10 of which were more than 152m. But something strange happened: even with a breeze, some staff on more than 40 floors could feel the building shaking and may even have symptoms similar to seasickness. No one studied the behavior of tall buildings in the wind until around 1929, when David Cushman Coyle invented a portable horizontal pendulum seismograph. He picked some windy days and took the equipment to the roofs of tall buildings in New York. The device consists of two levers, each carrying a mirror that reflects incident light onto another roll of photosensitive paper. He leveled the two levers to measure the lateral movement and surface tilt of the tall building, which was eventually recorded on photosensitive paper.

Coyle found that each tall building behaves differently in the face of the wind. The wavy lines recorded on the photosensitive paper indicate that tall buildings may "shiver" 40 times every minute, and less frequently eight times. Perhaps, Coyle thought, it was these small but repeated vibrations that made the indoor hanging lights shake up to a few feet and rippled the water in the bathtub. However, no structural safety problems were found in the tall buildings he tested.

Photo Source: Popular Science Monthly, January, 1931 this seems to be the "secret" of engineering-as buildings get taller and thinner, they are more likely to be shaken by the wind.

If an external force is applied to the pendulum, it can leave the equilibrium position. After that, if an external force is applied all the time, what it does is forced to vibrate. But even if the external force is removed, it can still vibrate freely, that is, it vibrates at a certain frequency under the condition of no resistance, and the corresponding frequency is called natural frequency.

Now, we need to think like engineers: tall buildings are not entirely rigid and can be set up. In that case, an inverted pendulum can be used as a physical model of a tall building.

Therefore, under the action of continuous wind, tall buildings will deviate from the equilibrium position and be forced to vibrate. Moreover, high-rise buildings are also an obstacle to the wind, so they choose to make a detour from both sides of the building. At this time, the wind will form a vortex and fall off at the back of the building, which is called vortex shedding. In addition, the wind bypasses the pressure difference at the same time, which in turn produces suction (suction force), which sucks the building over. This in turn affects the flow field around the building, which in turn wobbles the building back and forth. Resonance occurs when the frequency of vortex shedding is equal to the natural frequency of the building, which is called vortex-induced vibration in engineering.

The vortex-induced vibration formed by bypassing the cylinder. Photo source: A. Placzek / Wikimedia Commons, CC BY-SA 3.0 and then return to the inverted pendulum. As the length of the pendulum increases (the floor increases), the vibration period becomes longer and the vibration frequency decreases. Therefore, it is not only strong winds that can cause damage, but slower winds (lower frequency) are more likely to cause vortex-induced vibration of tall buildings. for example, the obvious sloshing of the Saige Building in Shenzhen in 2021 is mainly caused by vortex-induced vibration.

Engineers keep this "secret" in mind, but when designing, they are not only thinking about how to make tall buildings resist higher wind loads, but also looking for ways to "coexist" with the wind. Under this premise, more importantly, engineers also need to design to "hide" the people in the room so that they can't feel the shaking of the building, even though it happens almost all the time.

Feel the shaking in fact, it is very difficult for humans to sense speed, for example, in a uniform speed car or train, without a reference, we can hardly feel that we are moving forward. However, we are particularly sensitive to acceleration, such as sudden braking and when the plane lands, the change in speed causes the body to react immediately and is actually felt. What engineers want to know is how sensitive people at the top are to acceleration.

Trained fighter pilots sometimes have to withstand four gravitational accelerations, while the vibration acceleration of a 100-story building is only 1% of the gravitational acceleration, which is difficult for most people to perceive. However, compared with testing the vibration acceleration of buildings, it is more difficult to determine the sensitivity and tolerance of different people to acceleration. For example, in a series of experiments called "moving room", the researchers simulated a moving room in a wind tunnel and studied the subjects' sensitivity by changing the moving frequency and acceleration. It was found that one of the five subjects might think the room was still moving when the room stopped moving. This illusion is similar to the fact that the next train started first, but we thought the train we were on had started first.

The Mooving Room . Source: Beno t G. Bardy but now, scientists think that the most sensitive people may feel the shaking of the building at 0. 03 to 0. 04 gravitational acceleration and feel uneasy at 0. 1 to 0. 2 gravitational acceleration. Over the past 30 years in the United States, engineering standards have stated that in the face of an once-in-a-decade hurricane, the highest floor of skyscrapers (for housing) should not shake at a frequency of more than 0.15 to 0.18 gravitational acceleration. At this point, engineers may use dampers to counteract the vibration of the building or change the shape of the building, such as the spiraling structure of the Shanghai Central Tower and the specially made hole above the Shanghai World Financial Center.

The building is "making a sound" in physics class, you may have seen a teacher demonstrate vibration with a tuning fork: tapping a tuning fork to make a sound while making the next ball vibrate. Similarly, when the wind knocks on a building, it can also produce sound, but it is usually difficult to be heard because the frequency is too low (less than 20 Hz). But there are exceptions, but the voice is not so beautiful.

In July 2021, news.com.au published a report on the website that some residents in Melbourne complained that a night in a high-rise apartment was like being in a horror movie, with an inexplicable creaking from the roof making it difficult for them to fall asleep. At that time, Melbourne experienced several days of bad weather. Builders said that high-rise apartments would have "made a sound" in strong winds, but they would also conduct further investigation. So, will the weather really make tall buildings "sound"?

For this problem, we have to start with the materials of the building. Buildings are mostly made of steel, concrete and plastic, which expand and compress as the temperature rises and decreases, and these materials may tighten or rub against each other, resulting in banging, clicking, crackling, and so on.

However, BonnieSchnitta said that most of the sound from high-rise buildings comes from pipes and non-bearing walls, without affecting the main bearing structure of the building. Schnitta is the founder and CEO of SoundSense, an acoustics consulting firm in New York. She also said that when the roof and floor are not vibrating synchronously, it may also produce a gunshot-like banging sound due to the tension between the top and bottom. However, these can not be solved, the most commonly used solution is to install flexible hinges at the inflection point of the wall.

Finally, when Hurricane Alicia hit Houston in 1983, two engineers, Robert Halvorson and Michael Fletcher, braved strong winds to the roof of Wells Fargo Square (then known as Allied Bank Plaza). They want to test the performance of the 71-story building here during the hurricane. Although the floor under their feet vibrated violently up and down, they were convinced that the structure of the building was not damaged because it was carefully designed by their engineers.

Although many building collapses have been recorded in history, such as the collapse of Sanfeng Department Store in South Korea in 1995 and the collapse of Terminal 2e at Charles de Gaulle Airport in France, most of these are due to structural load-bearing problems. If the early design and wind tunnel test simulation are rigorous and accurate enough, the occurrence of similar disasters may be reduced.

Reference link:

Https://www.insidescience.org/news/why-skinny-skyscrapers-are-so-loud-and-how-quiet-them

Https://gizmodo.com/how-much-do-skyscrapers-actually-move-1707522178

Https://books.google.co.jp/books?id=BCgDAAAAMBAJ&pg=PA28&lpg=PA28&dq=New+machine+proves+skyscrapers+shiver+in+wind&source=bl&ots=8CPpRZDqiq&sig=ACfU3U0fyPQsWAUVxhOKKWb6JB98y-a9kw&hl=zh-CN&sa=X&ved=2ahUKEwiSpZvAmsf3AhVR-mEKHaa3AQIQ6AF6BAg_EAM#v=onepage&q=New%20machine%20proves%20skyscrapers%20shiver%20in%20wind&f=false

Https://www.theb1m.com/video/how-tall-buildings-tame-the-wind

Https://discovery.ucl.ac.uk/id/eprint/10123414/1/Littler_10123414_thesis.pdf

Https://www.news.com.au/finance/real-estate/like-a-horror-film-melbourne-highrise-residents-haunted-by-creepy-sound/news-story/e7d00d50f979bc5de55083e4e951c256

Https://minnesota.cbslocal.com/2014/01/07/good-question-why-do-our-houses-make-noise-when-its-cold/

Https://asa.scitation.org/doi/10.1121/10.0007620

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Wang Yibo, revised by Wang Yu

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