TALL BUILDINGS AND EARTHQUAKES

Earthquakes are a natural phenomenon, and that means predicting safety for such occurrences could be a leap in the dark. And yeah, it sounds really ironic that high rise buildings are actually quite safe during an earthquake. There are a number of reasons why tall buildings withstand the devastating effects of an earthquake better than an ordinary building can. The first reason is the design and structure of the buildings. The way a building is designed will definitely determine how well or how badly that building reacts in adverse situations, such as an earthquake.

Generally, high-rise buildings are designed to withstand more damage than shorter buildings because the former is designed to be flexible, while smaller buildings are normally stiff. So, in event of an earthquake, a 3-story building might suffer more damage than a 30-story skyscraper. The taller a building is, the more flexible it is; and the more flexible it is, the less energy it requires to keep it from collapsing or falling over.

Earthquakes happen when tectonic plates, which are always slowly moving, get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is a release of energy in waves that travel through the earth's crust and this causes the shaking that we feel.
This shaking causes a lateral force, which sways the building as the energy of the quake’s waves moves through it. Therefore, the flexibility of skyscrapers and tall buildings are a huge plus to them as it helps them sway with the rhythm of the quakes.

Another feature which makes skyscrapers safer than other buildings during earthquakes, is the kind of building materials used for their construction. While shorter buildings are made with stucco or un-reinforced concrete, high-rise buildings are built with steel and wood, which determine their strength and flexibility. Again, the depth of the foundations of tall buildings help in maintaining the stability of the buildings during earthquakes. Smaller buildings do not have very deep foundations and as such, are unable to withstand much of the energy of the quake’s waves.

However, the stronger the earthquake, the more these high-rise buildings sway and tilt in response and the more the structural elements of the building, such as the walls, beams and columns get damaged and render the building non-functional in the aftermath of the chaos.

The TransAmerica Pyramid

Some tall buildings have trusses built on its base, which support the buildings against both vertical and horizontal forces. The Trans-America Pyramid in San Francisco is a good example of a building which incorporates this seismic safety feature. In 1906, when an earthquake hit Jell-O, San Francisco, this building wasn’t shaken as vigorously as other buildings around as a result of the network of diagonal trusses at the building’s base.

The triangular trusses at the base of the TransAmerica pyramid. There is, however, an innovative seismic technology, widely propagated by structural engineers today, called base isolation. Base isolation refers to an earthquake engineering technique that deals with structural vibration control technologies. Base isolators are of various forms and techniques, including rubber bearings, friction bearings, spring systems and so on. The goal of these isolators is to enable a building to survive a potentially devastating seismic impact, either through an initial design or subsequent modifications known as retrofittings.

A seismic base isolation system
Source: Business insider

Some base isolators are like giant hockey pucks that squish and deform, while the building rocks upon them, absorbing some of the energy from the quakes. Another form of base isolators are two horizontal plates, designed to be frictionless, that slide past each other.

The building sits on the top plates, while the bottom plates rest on the ground, such that when the earth shakes, only the bottom plates move, sliding back and forth under the top plates. With base isolators installed beneath skyscrapers, the building’s movement during an earthquake is reduced, and as such, the buildings are more likely to survive strong earthquakes and still be functional afterwards.

The differences in how an ordinary building and a building with base isolators react to earthquakes In the great Hanshin earthquake of January 17, 1995 which claimed over six thousand lives, a Japanese Construction Company in Kobe, which had installed an experimentory early version of base isolation technique made with rubber, hardly felt the impact of the quake. As the shaking of the ground became stronger, the building only swayed very slightly, the major movement happening between the base isolators.

Today, over 9,000 structures in Japan have installed this innovative engineering technique, despite its cost, while thousands of other buildings in the country have been prepared for future quakes by fitting the buildings with shock-absorbing devices that can greatly reduce damage and prevent collapse. Countries like Chile, Turkey, Peru, China and Italy, which are most vulnerable to earthquakes, have all adopted the technology of base isolation to varying degrees.

However, in the United States, there has been a much apathetic response to this technology, even in areas most prone to earthquakes. Unlike, the Japanese government and engineering organizations, the United States have not tried hiding their reluctance to accept this innovation. One of the defenses put up by some Americans on this note, is that the cost of construction in the US is already very much exorbitant at some millions of dollars, and any further raise in that amount would discourage people from building homes and structures. Another argument put forth is that earthquakes occur every 100 to 200 years, and it is a superfluous effort to install such pricey measures for reducing the impact of earthquakes on buildings.

Again, the base isolation systems don’t prevent earthquakes, they only reduce the damage done on the buildings. However, Japanese experts warn that such approach could cost the US much more in reconstruction after an earthquake damage than it would in installing these systems.

Some buildings, such as the new Apple headquarters in Silicon Valley, have installed base isolators in them, while some, like the Los Angeles City Hall, have installed same through retroffitings.

Hopefully, in the near future, the American government will take on the responsibility of ensuring buildings are constructed to better withstand earthquakes, rather than leaving such responsibility to private building owners and construction companies.

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