"Hard-fighting" against a Category 14 typhoon, how does this "wind-resistant magic weapon" protect skyscrapers?

"Ma Ke", "Bebejia", "Prasang"... Since September, strong typhoons have landed one after another in the eastern and southern coastal areas of my country, bringing strong winds and heavy precipitation. Especially "Bebejia", when it landed, the maximum wind force near the center was 14 (42 meters/second), and the lowest central pressure was 955 hPa. It is known as the strongest typhoon to land in Shanghai since 1949.

Videos circulating on social media showed that when "Bebejia" made landfall, the "wind-resistant artifact" damper on the 125th floor of the Shanghai Tower, a famous landmark building in Shanghai, swung back and forth noticeably with an amplitude of about 1 meter.

▲The damper started working. Source: Shangguan News

During the previous typhoon, some people filmed a video of the building shaking in the wind, which made many netizens shout "scared"! Some even thought that the foundation of the building was problematic and it was a "shoddy construction". Some netizens simply did not believe that the building would shake in the wind and said it was "Photoshopped"!

When a typhoon hits, will the building really shake? Is it really okay? What is the purpose of the damper weighing thousands of tons?

Skyscrapers: Not only during typhoons and earthquakes, but also during normal times

When skyscrapers encounter typhoons and earthquakes, they are subjected to changing wind loads (i.e. the force exerted by wind on the structure) and ground vibrations, which will produce reciprocating dynamic deformation, which is what we feel as "shaking". The vibration of structures is unavoidable and is the inevitable reaction of structures under dynamic action.

In fact, in normal times, buildings also vibrate, but the amplitude of the vibration is very small and it is difficult for people to notice it. Under strong loads such as typhoons and earthquakes, the vibration amplitude of the building increases accordingly. In addition, skyscrapers also need to "shake" to consume the large amount of energy input by typhoons and earthquakes.

It can be seen that there is no scientific basis to judge a building as a "shoddy construction" just because it shakes during a typhoon.

Before the construction of a building begins, engineers will conduct structural design. Based on the building's location, engineers will estimate the earthquake and typhoon effects that the building may be subjected to, and make corresponding designs to minimize the building's structural response under these external loads (i.e., the general term for all external environmental forces on the structure).

In coastal areas with strong winds, wind loads are often the controlling factor in structural design, so engineers often use wind-resistant designs. Wind-resistant designs must meet the design requirements of structural strength, stiffness, comfort, and fatigue resistance to ensure that the structure will not collapse or crack under wind loads, nor will it undergo excessive displacement, thereby ensuring the safety of the structure.

You may wonder why you can’t feel the shaking when you are in a building during a typhoon?

In fact, the main factors affecting human comfort are vibration frequency, vibration acceleration and vibration duration. The vibration duration mainly depends on the wind action time, and the vibration frequency of the structure is difficult to adjust, so the method of limiting the vibration acceleration of the structure is generally used in structural design to make people feel more comfortable.

▲Human body vibration comfort control limit＊1 gal = 1/100 m /s^2

When the vibration acceleration of a building reaches 0.15 m/s^2, people inside it will feel the building shaking.

Damper: If the wind doesn’t move, I won’t move; if the wind moves south, I will move north.

High-rise and super-high-rise buildings are prone to shaking under the influence of high-altitude wind speed, which not only increases structural fatigue and reduces building safety, but can even cause residents on high floors to feel dizzy. Therefore, using physical principles to reduce vibration response and reduce structural fatigue is an important technical issue in the seismic design of super-high-rise buildings, and dampers are one of the targeted solutions.

This device was previously widely used in aerospace, aviation, military, guns, automobiles and other industries, and has been used in construction, bridges, railways and other structural engineering since the 1970s. In particular, for some skyscrapers, when the building sways under strong winds, the damper can offset the swaying of the building by generating reverse motion, using its own huge weight to counteract the impact of wind on the building, thereby slowing down the swaying of the building.

Tian Wei, associate professor of the Department of Physics at the University of Shanghai for Science and Technology and member of the Science Popularization Committee of the China Artificial Intelligence Society, further explained in an interview with the media that the operating principle of the damper is like a person on a shaking "Internet celebrity bridge" (suspension bridge), moving the body in the opposite direction of the shaking of the "Internet celebrity bridge" to achieve balance. He said, "If a strong wind blows from a certain direction, the counterweight is like a huge "pendulum" swinging in the direction of the wind, generating a force opposite to the wind direction, thereby reducing the shaking of the building and offsetting the impact of the strong wind on the building."

Dampers can generally be divided into two types, one is an energy-absorbing damper and the other is a tuned damper. From the energy point of view, the energy input into a building by a typhoon or earthquake is certain. Therefore, the more energy the energy-absorbing device dissipates, the less energy the structure itself needs to dissipate. Energy-absorbing dampers use this principle to reduce the vibration of buildings. The Petronas Twin Towers in Kuala Lumpur, Malaysia, use energy-absorbing dampers. They use the air bridge between the two buildings to balance the shaking between the buildings. The energy-absorbing dampers are set at the connection point between the air bridge and the building. Under the action of an earthquake or typhoon, it first enters an inelastic state, consumes a large amount of seismic energy, and thus reduces the vibration of the structure.

Tuned dampers transfer the energy of a building to a substructure by adding a substructure. These substructures often weigh hundreds of tons, and when a typhoon or earthquake shakes the building to one side, the damper will also move, and its inertial motion direction is exactly opposite to the direction of the building's motion.

According to the different substructures, tuned dampers are divided into tuned mass dampers (TMD) and tuned liquid dampers (TLD). Tuned mass dampers have been widely used in actual high-rise building structure wind vibration control projects because of their good vibration control effects on high-rise building structures, less impact on building functions, low cost, small footprint, and easy installation, maintenance and replacement.

The Shanghai Tower is 632 meters high, making it the tallest building in China and the only building in my country that exceeds 600 meters in height. Its upper floors usually have to withstand strong winds of tens of meters per second. When a typhoon hits, the Shanghai Tower will shake, with the maximum amplitude at the top reaching 1.4 meters. Its damper, the "Shanghai Eye", is an eddy current tuned mass damper, an innovative technology in my country with the advantages of high response sensitivity, strong durability, variable damping and low maintenance cost. This damper is also the heaviest in the world, weighing 1,000 tons, accounting for about 0.118% of the weight of the building. It is suspended inside the building by 12 25-meter-long steel cables, with a single-side swing limit of 2 meters.

The damper is installed on the third to fourth floors of the building, 583 meters above the ground. When strong winds hit, the giant pendulum will swing in the direction of the wind, thereby reducing the building's swaying amplitude through its own swaying. According to data provided by Shanghai Construction Engineering, the construction company of the Shanghai Tower, this damper can reduce the peak acceleration caused by wind by more than 43%, which can make 90% of the people in the building feel more comfortable.

▲China's tallest building - Shanghai Tower. Image source: Shangguan News

Since dampers can reduce building sway, do all super high-rise buildings need to be equipped with them? Some experts said that the wind and earthquake resistance of super high-rise buildings mainly depends on the building structure and appearance design, which requires "strength in itself". The role of dampers is more like "icing on the cake". On the one hand, it reduces sway, optimizes the experience, and improves the quality of the building; on the other hand, it increases the durability of the structure. According to relevant Chinese regulations, it is not mandatory to install dampers on buildings.

In addition, the range of action of the damper also has an upper limit. For example, the limit value set for the damper of the Shanghai Tower is a single-sided swing of two meters. When the swing of the damper exceeds a certain amplitude, the limit protection device will lock it to protect the building structure and surrounding decorative parts from being hit.

Slight shaking will not affect the building structure

So for high-rise buildings that are not equipped with dampers, do we need to worry when facing typhoons?

A chief engineer surnamed Zhang from Fangchenggang Municipal Housing and Urban-Rural Development Committee said in an interview with the media that when a typhoon comes, people living in some high-rise buildings feel a slight sway, which is a normal phenomenon. The typhoon will not have an adverse effect on the structure of the building itself. According to relevant regulations on building design, when strong winds hit, high-rise buildings are allowed to sway slightly, and buildings over 30 floors often have a deviation of several centimeters. The degree of floor sway is related to the ratio of the area. When the plane building area is larger, the sway will be smaller; while some floors with a narrower plane building area will sway more severely.

According to architectural experts, meteorological equations can be used to calculate that the wind speed in the city is indeed higher than that near the ground. Therefore, when strong winds blow, high-rise buildings are more affected, which is called the "wind vibration effect" in architecture.

Another reason for the increase in high-altitude wind force is the "narrow channel effect" of tall buildings. Due to the obstruction of tall buildings, if the wind's passage surface is reduced by half, its speed will increase by about one-fold. When encountering narrow passages in tall buildings, the wind is even more violent. Therefore, the tops of super-high-rise buildings generally have displacement performance, but the amplitude is different. The higher the floor, the greater the amplitude. Therefore, it is normal for residents living on the top floor to feel "shaking", but this will not affect the safety of the building structure itself.

When constructing a high-rise building, wind is one of the first factors to be considered. For wind resistance requirements, general buildings are required to be able to withstand strong winds that occur once every 50 years; high-rise buildings have higher requirements, generally being able to withstand strong winds that occur once every 100 years. Therefore, as long as the building is built strictly in accordance with the wind resistance design, there will be no danger. If the high-rise building is shaking and there are problems such as wall falling off, the public should pay attention to it and report the situation to the local housing safety department in a timely manner.

Source: Science Popularization China, Global Times, Red Star News, Shangguan News