Learn how structures with large open spaces on the ground floor—called soft-story buildings—are vulnerable to earthquakes but can be retrofitted to be safer in this video excerpted from NOVA: “Making Stuff Safer.” Host and technology columnist David Pogue examines how soft-story buildings have collapsed in past earthquakes and visits a testing site to see how retrofits improve their safety. In order to study methods to improve the seismic safety of soft-story buildings, researchers from five universities collaborated to build a typical soft-story building on a shake table that simulates earthquakes.
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The motion of the ground during an earthquake causes structures such as buildings and bridges to sway, which can be destructive. There are several key factors that affect how much structural damage may result from ground shaking: the intensity of the ground movement, the length of the shaking, the engineering of the structure, and the type of soil. In general, the stronger the shaking and the longer it lasts, the greater the damage. However, proper design and engineering can improve a structure's ability to withstand the strong lateral forces—forces that push side to side—that an earthquake can generate. Buildings may also suffer damage when ground shaking causes soil and water to mix together in a process called liquefaction. The soil becomes liquid-like, and structures may sink, lean, or topple over.
Human understanding of how structures should be designed to limit seismic risk has improved over time. Modern buildings must adhere to seismic codes—regulations that are intended to ensure the safety of structures during earthquakes. However, many older buildings were constructed without adequate support. If a structure cannot support the forces produced by an earthquake, it can collapse. Earthquake, or seismic, retrofitting is the modification of existing structures to make them more resistant to earthquakes.
Soft-story buildings—buildings that have large openings on the first floor—are particularly susceptible to earthquakes. This common type of building typically has a garage or commercial space on the first floor and residential housing on the upper floors. Because the first story of this type of building is much less stiff than the upper stories, it is vulnerable to collapse when subjected to the forces of an earthquake. There have been disastrous collapses of soft-story buildings in a number of earthquakes in recent history. In 2013, the city of San Francisco, which is densely populated and earthquake-prone, passed a law requiring the evaluation and retrofit of multiunit soft-story buildings. Thousands of soft-story buildings will be retrofitted, which will greatly reduce the risks, and tens of thousands of people will be much less likely to be injured by a major earthquake.
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