Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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Summary of Present Practices on Dam Safety Standards
Summary of Present Practices on Dam Safety Standards
Pages 15-34
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From page 15... ...
Responses to these inquiries and the pertinent actions of the technical societies are summarized in Appendixes A and B The data from 10 federal organizations, 35 state and local agencies, 9 private firms, and4 professional engineering societies provide a comprehensive overview of current practices in the United States and, to a great extent, in foreign countries.
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From page 16... ...
there is considerable variety in the classification systems that have been adopted, and this variety often makes difficult any precise comparisons between criteria used by different agencies. Most systems for classifying dams specifically utilize dam height, volume of water impounded, and character of the development in the relevant downstream area as parameters in regard to probable effects of dam failure.
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From page 17... ...
SPILLWAY CAPACITY CRITERIA Table 3-2 shows all the spillway capacity criteria as stated in agency standards in terms of either design rainfall or design floods reported to be in current use by the entities responding to the committee's inquiries. Criteria based on estimates of probable maximum precipitation (PMP)
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From page 18... ...
The 10,000-, 1,000-, and 150-year frequency floods are listed in the criteria of the Institution of Civil Engineers, London. Table 3-3 gives an approximate comparison (based on the classifications used for the National Dam Inspection Program)
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22 _` o U Cal Cal 3 es 4_ UD Cal 4~ .
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From page 23... ...
· Apparently as a result of the National Dam Inspection Program for nonfederal dams carried out by the Corps of Engineers in the 1977-1981 period, several state dam safety agencies have adopted the spillway capacity criteria used in those inspections. · Several states have adapted the standards used by the Soil Conservation Service for the design of the tens of thousands of smaller dams constructed under that agency's programs.
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From page 24... ...
seismic zone map, as it appears in the Uniform Building Code, 1979 Edition. The Soil Conservation Service, the Corps of Engineers, and some states, when dynamic response analyses are not required, employ seismic zones for determining the minimum seismic coefficients for pseudostatic analyses.
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25 Cal ·= be ¢ U
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28 Cat ._ o ~5 G)
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From page 29... ...
Earthquake Intensity and Magnitude Scales Both the Modified Mercalli intensity scale (Table 3-5) and the Richter magnitude scale are in use to describe earthquakes, although clam safety criteria usually refer to the Richter scale.
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From page 30... ...
Seismic Design Terminology The following terms in reference to ground motions at the dam site, or to earthquakes causing those motions, are presently used by various government agencies ant! other entities with respect to seismic design criteria: DBE design basis earthquake (Planning Research Corporation)
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From page 31... ...
It is a rational and believable event that is in accord with all known geologic and seismologic facts"; by the Tennessee Valley Authority as, "the earthquake associated with specific seismotectonic structures, source areas, or provinces that would cause the most severe vibratory ground motion or foundation dislocation capable of being produced at the site under the currently known tectonic framework"; and by the Interagency Committee on Dam Safety as, "the hypothetical earthquake from a given source that could produce the severest vibratory ground motion at the dam." The term maximum credible ground motion (MCGM) or equivalent terminology is used by the U.S.
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From page 32... ...
by the U.S. Army Corps of Engineers as, "the maximum level of ground motion that can be expected to occur at the site during the economic life of the project, usually 100 years"; by the Tennessee Valley Authority as, "the earthquake for which the dam is designed to resist and remain operational"; and by the Nuclear Regulatory Commission as, "that earthquake which, considering the regional and local geology and seismology and specific characteristics of local subsurface material, could reasonably be expected to affect the plant site during the operating life of the plant." Regulations proposed by the State of Missouri specify a fraction of probable maximum acceleration (PMA)
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From page 33... ...
Hydrodynamic pressures also were taken into account by similar methods in some cases. Dynamic Response Analyses Analysis of the dynamic response of a dam to specified earthquake ground motion, when it is located in seismic zones 3 or 4 (and under some conditions, in zone 2)
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From page 34... ...
For concrete dams, the dynamic response analyses determine the instantaneous total (dynamic plus static) stresses at both faces of the dam and at designated locations.
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