Pii

Hazard level

Minor

FIGURE 21.2 Defining performance objectives.

Moderate

Severe a function of the intensity of the earthquake load. As suggested earlier, a hazard level denotes the severity of the earthquake. A minor hazard level would represent an event with a higher probability of being exceeded during the life of the structure, while an increased hazard level representing a more severe event would indicate a reduced probability for the same duration. Damage states may be quantified in numerous ways, but two of the most common approaches are physical damage states based on the degree of inelastic response of members and expected losses including economic and indirect losses.

Since a structural system is composed of both primary and secondary elements, the specification of a performance level can be distinguished depending on the structural function. Hence, the structural performance level can be different from the nonstructural performance level for a given performance objective.

21.2.1.1.1 Description of Hazard

Earthquake ground motions, or the characteristics of the expected ground motions, are generally used in conjunction with a performance objective. Given the uncertainty in predicting ground motions, the task of establishing a site-specific design event is a difficult but critical part of any PBD methodology. The description of the hazard is meant to represent the seismic threat given knowledge of the potential earthquake sources at the site. This is typically accomplished through a probabilistic seismic hazard assessment that results in a hazard curve describing mean annual frequency of exceeding a certain spectral acceleration magnitude.

The specification of ground shaking is inevitably linked to two factors: (1) site geology and related site characteristics, which may be classified by soil profile or other soil parameters such as shear wave velocity, undrained shear strength, etc. and (2) site seismicity, which includes information on the seismic source. Ultimately, the design earthquake needs to be quantified in terms of a response spectrum as shown in Figure 21.3. The spectrum is characterized by three critical points: the peak ground acceleration (PGA) S0, the region of maximum spectral demands (Sm between Tm and Tn), and finally a description of the spectral demands beyond Tn.

To allow for a range of performance objectives, it is necessary to define more than a single hazard level. The prevalent thinking in most PBD guidelines is to consider at least three levels:

• Hazard level I (or a basic service level earthquake). Earthquakes representing this level of hazard are expected to occur more frequently, and therefore, the likelihood of such an event during the life of the structure is high. Probabilistically speaking, this event is typically equated with a 50% probability of being exceeded in 50 years or a mean return period of 72 years.

• Hazard level II (or a design level earthquake). This is generally the design level event. Earthquakes at this level of hazard are normally assumed to have a 10% probability of being exceeded in 50 years.

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