'1

basic elements of engineering reliability analysis

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applicable to geotechnical structures for various modes

of performance. These reliability measures are

intended to be sufficiently consistent and suitable for

(2) In the engineering reliability literature, the

application to economic analysis of geotechnical

term *failure *is used to refer to any occurrence of an

structures of water resource projects. References are

adverse event under consideration, including simple

provided which should be consulted for detailed

events such as maintenance items. To distinguish

discussion of the principles of reliability analyses.

adverse but noncatastrophic events (which may require

repairs and associated expenditures) from events of

catastrophic failure (as used in the dam safety context),

adequacy are expressed by safety factors. A safety

the term *probability of unsatisfactory performance*

factor can be expressed as the ratio of capacity to

demand. The safety concept, however, has short-

stability where the safety factor is below the required

comings as a measure of the relative reliability of

minimum safety factor but above 1.0. Thus, for this

geotechnical structures for different performance

case, reliability is defined as:

modes. A primary deficiency is that parameters

(material properties, strengths, loads, etc.) must be

assigned single, precise values when the appropriate

values may in fact be uncertain. The use of precisely

defined single values in an analysis is known as the

approach reflects the condition of the feature, the

(1) Engineering reliability analysis can be used in

engineer's judgment, and the degree of conservatism

several general contexts:

incorporated into the parameter values.

The estimation of the reliability of a new

structure or system upon its construction and

extends the safety factor concept to explicitly incor-

first loading.

porate uncertainty in the parameters. This uncertainty

can be quantified through statistical analysis of existing

The estimation of the reliability of an existing

data or judgmentally assigned. Even if judgmentally

structure or system upon a new loading.

assigned, the probabilistic results will be more

meaningful than a deterministic analysis because the

The estimation of the probability of a part or

engineer provides a measure of the uncertainty of his

system surviving for a given lifetime.

or her judgment in each parameter.

Note that the third context has an associated time

interval, where as the first two involve measures of the

overall adequacy of the system in response to a load

event.

(2) Reliability for the first two contexts can be

(1) Engineering reliability analysis is concerned

calculated using the *capacity-demand model *and

with finding the *reliability R *or the *probability of*

quantified by the *reliability index *$. In the capacity-

demand model, uncertainty in the performance of the

system is considered reliable unless it fails, the

structure or system is taken to be a function of the

reliability and probability of failure sum to unity:

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