Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 396

396 Earthquake Engineering for Structural Design

reduced to the minimum. So, for this level, the structure must remain within the
elastic range or it may undergo just unimportant plastic deformations.
- Damageability limit state (DLS), for occasional earthquakes. This limit state
considers an earthquake intensity which produces damage in non-structural
elements and moderate damage in the structure, which can be repaired without
great technical difficulties.
- Ultimate limit state (ULS), for earthquakes which may rarely occur,
representing the strongest possible ground shaking. For these earthquakes, both
structural and non-structural damage is expected, but the safety of inhabitants has
to be guaranteed. In many cases damage is so substantial that the structures are not
susceptible to be repaired and their demolition is recommended.
One can see that the four design levels, developed by Vision 2000 and presented in
Figure 9.2, practically correspond to the three design levels, with the exception of the
serviceability limit state, which is divided in two (fully operational and operational
limit states).

9.3.2DefinitionofEarthquakeDesignLevels

There are two approaches to define the main characteristics of earthquake design for
different limit states.
- Probability approach. Design earthquakes are determined for a given probability
of exceeding in 50 years (Fig. 9.4a).
- Recurrence period approach. The level of acceleration or magnitude is determined
in function of the recurrence period (Fig. 9.4b).
The annual probability of exceeding the peak ground accelerations is considered for
low, moderate and high seismicity regions.
The relationship between the two approaches is presented in the following:

Earthquakedesign Probabilityof Recurrence
level exceedance interval
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Frequent 50% in 50 years 43 years

Occasional 20% in 50 years 72 years

Rare 10% in 50 years 475 years

Veryrare 2% in 50 years 970 years