How San Francisco Skyscrapers Increase Earthquake Risk - ForumDaily
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How San Francisco skyscrapers increase the risk of a tragic earthquake outcome

Sailors arriving in San Francisco in the 19th century used two giant trees as landmarks to land in the city's bay. Those redwood trees were cut down during the Gold Rush, but San Francisco has a new beacon: the Salesforce Tower, the tallest office building on the West Coast.

Фото: Depositphotos

Other high-rise buildings built on soft soil and sand at the edge of the bay crowded around this skyscraper, 326 meters in height. They are a symbol of modern San Francisco, but also pose a potential danger to a city located in an area where earthquakes occur frequently, writes The New York Times.

San Francisco lives with the confidence that a large earthquake will happen sooner or later in the city, but more and more skyscrapers appear there due to the serious housing shortage in the country. Although there are strict building codes in the United States, they do not guarantee the protection of buildings against earthquakes, even though many residents believe in this. And experts are sounding the alarm regarding San Francisco’s high-rise buildings.

More than a century has passed since the last devastating earthquake in San Francisco. Eyewitnesses to this phenomenon on April 18, 1906 said that the streets of the city "rose up and down like a ribbon in the wind."

The earthquake then caused a fire that lasted for three days, destroying 500 city blocks and 28 000 buildings.

Half of the city’s population, which at that time was about 400 000 people, remained homeless, many were forced to leave San Francisco.

After decades of hostile attitudes towards the skyscrapers, San Francisco authorities have supported the construction of a more dense and high-rise city center. In San Francisco, there are now 160 buildings whose height exceeds the 73 meter, and a dozen more are planned or are under construction.

California has strict requirements for the construction of schools and hospitals to protect them from large earthquakes. But not for skyscrapers. The five-story building has the same requirements for its sustainability as the 50-storey building.

But skyscrapers expose the city to much greater risk, and their fall or destruction can cause very serious consequences.

Even engineers who design San Francisco skyscrapers cannot give an accurate answer as to how safe and sustainable they are. Earthquakes are too unpredictable, and too few large cities with skyscrapers were heavily pushed to collect data on the response of skyscrapers to earthquakes.

Previous earthquakes revealed the flaws in the construction of some skyscrapers. It was discovered that the widely used welding method was not strong enough, and the building was destroyed during an earthquake in Los Angeles in 1994. After that, many buildings in San Francisco and Los Angeles, built in this way, have not been improved.

California has made significant progress in earthquake preparedness over the past century. Road crossings, bridges and some municipal buildings have been strengthened. Many Californians live in small wooden frame houses, which have been found to withstand earthquakes quite well.

But until recently, high-rise buildings were not in the center of attention of experts on ensuring the seismic safety of San Francisco.

High-rises in California are typically designed using computer simulations. But seismologists say that the effectiveness of this method is very doubtful, because this method works only on paper and has not been tested in practice.

In September 2017, the former mayor of San Francisco, Edwin Lee, commenting on the skyscraper scandal that subsided and bent, ordered city officials to tighten building standards for high-rise buildings and asked for an independent study of the safety of skyscrapers.

This study will for the first time create a detailed database of more than 160 city skyscrapers and assess their condition and safety. But work on the project is still in progress.

For years, San Francisco limited the height of 152 buildings to meters. But this was caused not by considerations of seismological security, but by the cultural and aesthetic views of the citizens.

In 2004, the city launched a program to build new homes, which today are the center of San Francisco, and skyscrapers were built there that exceeded the previous restrictions.

According to Dean Makris, who headed the council on redevelopment of the city at that time, the problem of the seismic safety of skyscrapers "was never a factor" in developing new plans for building the city.

This discussion was triggered by the 58-story skyscraper Millennium Tower, which began to sag and bend shortly after its construction.

When the building was completed in 2009, it received numerous awards for ingenuity and engineering innovation.

The builder and city officials knew about the flaws of the building for many years, but kept them secret until the 2016 year, when news of the sagging of the building became public. The latest measurements taken in December 2017 of the year show that the building has sunk by 46 cm and is tilted by 36 cm.

The threat of falling of this building caused an understanding in the city of what danger a structurally damaged skyscraper could carry. Experts fear that after a strong earthquake there may be several such buildings.

The area around the Millennium Tower is considered one of the most dangerous in the event of earthquakes due to the large amount of sand and clay, which are easily shifted and deformed by fluctuations.

The problem with this building led to calls to tighten construction requirements for skyscrapers.

At the moment, they require the structure to be designed in such a way as to have 90% chance of avoiding complete destruction from an earthquake. But many experts believe that this is not enough, since the destruction of even 10% of buildings can be a disaster for the city.

The construction rules also do not require the building to be livable after an earthquake. That is, many buildings may not collapse completely, but they can be damaged and not be repairable.

Read also on ForumDaily:

How to protect yourself during an earthquake: updated instructions

10 items required in case of emergency evacuation

How to survive in case of emergency in a high-rise building

Hurricanes, tornadoes and earthquakes in the USA: how to stay alive

earthquake San Francisco Educational program
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