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How does Earthquake Frequency Influence Seismic design?

To design a seismic resistant structure, it is required to predict the earthquake loading. Two approaches are utilized to determine earthquake loading. They are:

a. Equivalent lateral force procedure

b. Modal analysis procedure

Both approaches estimate the earthquake loading considering structural properties and record of past earthquakes in region for where structures to be designed. It is fortunate to earthquake engineer as well as mankind that severe seismic activities are rare, whereas minor seismic activities are frequent. In some regions like Japan, California, Alaska, Chile etc. hundreds of minor earthquake are observed in a month. Of these Japan suffers magnitude>6.0 earthquake Several times in a month. So frequency and intensity is very important in seismic design. These relation are found inverse i.e. frequency of occurrence inversely related to intensity. The recent advancement in earthquake loading prediction as well as seismic design, it is possible to design a structure to survive a severe earthquake not undergoing significant damage. But as frequency of occurrence of such severe earthquake is less i.e. it may not occur in life time of a structure, it is not justified to provide such strength to the structure thus to provide additional cost. Seismic resistant design follows principles:
Structural collapse due to Magnitude 8.9 Japan earthquake March 11, 2011

a. Structure can withstand minor earthquake not undergoing damage.


b. Structure can withstand moderate earthquake, no structural damage are happened but may have some non-structural damage.
The total destruction of Magnitude 8.9 Japan earthquake March 11, 2011
c. Structure will not collapse under average earthquake but they may have structural and non-structural damage.

But some structure like nuclear power plant must not suffer any structural and non-structural damage as severe environmental and radioactive pollution followed by destruction of a region, that were observed in recent Fukushima Crisis. These vital structures that must be operational during and after earthquake, some adjustment are made upon above described principles. 

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