Earthquakes - Geotechnical Engineering Investigation - page 969

11.4.5 Seismic Hazard Analysis

Method Selection
General
The method selected for dynamic analysis varies from the relatively simple conventional
approach to comprehensive analytical procedures. The selection depends upon the degree
of risk and hazard.

Structure Purpose and Type
Pertains to failure consequences (the risk).

Conventional structures , moderate risk, include industrial plants, moderate-height
buildings, fossil-fuel power plants, moderate-height dams, etc.

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Lifeline structures , moderate to high risk, include roadways, railways, tunnels,
canals, pipelines (gas and liquid fuel, water, and sewage), electric power, etc.

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Critical public structures , high risk, include schools and hospitals.

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Unconventional structures , high to very high risk, include nuclear power plants,
50-story buildings, long suspension bridges, large dams, offshore drilling plat-
forms, etc.

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Earthquake Occurrence and Magnitude
Pertains to the hazard .

High hazard : Frequent occurrence of high magnitude events (I VI) or occasional

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occurrence of high-magnitude events (I VIII). Zones 3 and 4 in Figure 11.13.

Moderate hazard: Frequent occurrence of I VI and occasional occurrence of

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I VII. Zones 2A and 2B in Figure 11.13.

Low hazard : Generally areas of no activity, or events seldom exceed I V, and

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activity is normally low. Zones 0 and 1 in Figure 11.13.

Approach
Conventional or standard design is applied for conventional structures in low-hazard areas,
and considers only g forces.
Comprehensive or site-specific design is applied to unconventional structures in all areas
and to conventional structures, critical public structures, and lifelines in high-hazard
areas. The approach considers all site response factors in the development of the design
earthquake (see Section 11.4.6) , which are employed in more comprehensive analysis.

Standard Approach
Generally standard studies use preliminary values of ground motion (seismic coefficient)
obtained from published seismic zone maps (Figures 11.13- 11.44) , a preliminary struc-
tural analysis, and a simplified assessment of soil liquefaction and deformation. Standard
studies may be satisfactory for final design and evaluation in seismic zones 1 or 2A
(Figure 11.13), and are used to set the scope of site-specific studies.

Comprehensive Analytical Methods
General
The combined influence of ground accelerations, their frequency contents, and, to some
extent, ground shaking duration, in relation to the period and damping of the structure,