Building for Earthquakes


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Building for Earthquakes

  • Chances are 2 out of 3 that you’ll be home when the next earthquake strikes, and 1out of 3 that you’ll be in bed. So your home’s ability to withstand an earthquake affects not only your pocketbook but also your life and the lives of those who live with you.


Imagine for a moment that your house is anchored to a flatcar on a moving train. Suddenly the train collides with another train and the flatcar stops abruptly. What happens to your house? If it’s a wood-framed house, as most houses in California are, it probably would not collapse, although your brick chimney might topple over.

  • Imagine for a moment that your house is anchored to a flatcar on a moving train. Suddenly the train collides with another train and the flatcar stops abruptly. What happens to your house? If it’s a wood-framed house, as most houses in California are, it probably would not collapse, although your brick chimney might topple over.



This analogy introduces an important concept. The jolt to your house during the train wreck is analogous to the shocks the house would receive during a large earthquake. The response of the house and its contents (including you) to these jolts follows the principle of inertia. The principle of inertia says that a stationary object will remain stationary. A home not bolted to its’ foundation will slip of the foundation because of its’ inertia.

  • This analogy introduces an important concept. The jolt to your house during the train wreck is analogous to the shocks the house would receive during a large earthquake. The response of the house and its contents (including you) to these jolts follows the principle of inertia. The principle of inertia says that a stationary object will remain stationary. A home not bolted to its’ foundation will slip of the foundation because of its’ inertia.



Ductile buildings such as wood and steel-frame structures tend to bend and sway during an earthquake. In contrast, brittle structures made of brick or concrete block joined together with mortar, or adobe buildings from California’s pioneer days are unable to deform during an earthquake with out collapsing.

  • Ductile buildings such as wood and steel-frame structures tend to bend and sway during an earthquake. In contrast, brittle structures made of brick or concrete block joined together with mortar, or adobe buildings from California’s pioneer days are unable to deform during an earthquake with out collapsing.



The Armenian SSR Earthquake

  • On December 7, 1988, at 11:41 a.m. local time a magnitude 6.9 earthquake shook northwestern Armenia and was followed four minutes later by a magnitude 5.8 aftershock. Swarms of aftershocks, some as large as magnitude 5.0, continued for months in the area around Spitak. The vast majority of the damage occurred in unreinforced masonry structures.



In this earthquake both design deficiencies and flawed construction practices were blamed for the large number of building collapses and resulting deaths. Many of the modern multi-storied buildings did not survive. Twenty-five thousand were killed and 15,000 were injured by the earthquake. In addition 517,000 people were made homeless.

  • In this earthquake both design deficiencies and flawed construction practices were blamed for the large number of building collapses and resulting deaths. Many of the modern multi-storied buildings did not survive. Twenty-five thousand were killed and 15,000 were injured by the earthquake. In addition 517,000 people were made homeless.



During the 1994 Northridge Earthquake, similar damage patterns to those in Armenia were observed throughout the San Fernando Valley.

  • During the 1994 Northridge Earthquake, similar damage patterns to those in Armenia were observed throughout the San Fernando Valley.



A common failure in California’s recent earthquakes was the two- or three-garage with living space overhead. Many condominiums have most of the first floor devoted to parking, with apartment space in the upper floors. The large amount of empty space at the garage door means less bracing against earthquake forces than in standard walls, so these open areas are the first to fail in an earthquake.

  • A common failure in California’s recent earthquakes was the two- or three-garage with living space overhead. Many condominiums have most of the first floor devoted to parking, with apartment space in the upper floors. The large amount of empty space at the garage door means less bracing against earthquake forces than in standard walls, so these open areas are the first to fail in an earthquake.





Similar problems arise, although on a smaller scale, with large picture windows, sliding-glass patio doors, double doors or patio covers.

  • Similar problems arise, although on a smaller scale, with large picture windows, sliding-glass patio doors, double doors or patio covers.



Soil Types and Shaking Amplification

  • Because of certain conditions, seismic waves may cause certain areas to shake up to 10x harder during an earthquake, this is called site amplification. The chief contributor to the site amplification is the velocity at which the rock or soil transmits shear waves (S-waves). Shaking is stronger where the shear wave velocity is lower.





Fire!

  • There is also a formidable threat of fire, such as resulted from the 1906 San Francisco, the 1923 Tokyo, and the 1995 Kobe earthquakes.










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