FLOOD09139

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Federal Guidelines for Earthquake Analyses and Design of Dams
• Date: 3/1/1985
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

If analyses are judged to be required, the analyses should begin
with the simplest appropriate methods and conservative assumptions.
If, from this first analysis, the structure is judged able to
resist earthquake forces, then that analysis should be sufficient.
If not, progressively more detailed analyses should be performed
and the structure designed accordingly. Regardless of the methods
of analyses, the final evaluation of the seismic safety of the dam
should be based on engineering judgment and past experience and not
just on the numerical results of the analyses.

2. Concrete Dams

a. Safety Concerns

(1) Types of Instability

Safety concerns for concrete dams subjected to earthquakes
involve two types of instability: sliding on a plane of
weakness and excessive cracking of the concrete. For
gravity dams, sliding instability is possible due to an
earthquake-induced vibratory motion on a plane of weakness
at, above, or below the foundation-dam interface. For an
arch dam, sliding instability can occur through the
fai lure of the abutment support. Sliding instabi li ty for
a buttress dam can occur at or below the foundation-dam
interface. Instability due to excessive cracking of the
concrete can occur in the upper half of a gravity and arch
dam and at any location of a buttresss dam, especially at
points of abrupt changes in the cross-section.

(2) Foundation Importance

Of the two types of possible instability discussed above,
foundation and/or abutment failure is the chief source of
concern for concrete dams. In contrast to the dam itself,
the supporting medium consists of natural materials of
varying composition, irregular joints, and planes of
weakness. The strength of this medium has to be estimated
from exploratory borings and tests on only a small
fraction of the material present. Key zones of weakness
are difficult to detec t.

(3) Field Performance

No major concrete dam is known to have failed due to
earthquake-induced ground motion, although some have
experienced strong ground motion. The field performance of
concrete dams during earthquakes is discussed in section H.

b. Defensive Design Measures

The application of defensive design measures is the most
dependable approach to alleviate safety concerns. Defensive
design measures for concrete dams include:

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