ETL 1110-2-550
30 May 97
F-7. Engineering and Design Reliability
combination of expert elicitation and deterministic
Assessment of Navigation Structures
and probabilistic calculations. The above
(ETL 1110-2-532) and Stability of Existing
information was combined to produce "economic
Gravity Structures (ETL 1110-2-321)
consequence curves." These curves provide an
economic cost versus time in an event. The
a. Engineer Technical Letter (ETL) 1110-2-
economic consequence curves were then combined
with the frequency profiles from Phase 2 to produce
321 supplements ETL 1110-2-532 and provides
an economic risk. This is a single value for each
guidance for assessing the reliability of existing
field condition that reflects the dollars at risk for
gravity structures founded on rock and establishing
that field condition. Comparisons between the
an engineering basis for rehabilitation investment
different field conditions were provided, as were
decisions. ETL 1110-2-532 provides guidance for
importance values for the components from
assessing the reliability of navigation structures and
Phase 1. This latter information is useful for
establishing an engineering basis for rehabilitation
identifying components important to risk. In
investment decisions. The guidance provided by
addition, the importance values were evaluated to
these ETLs is intended to provide (1) an
predict changes in risk to specific components. The
engineering method for assessing the reliability of
risk values and associated importances would have
structural features based on their current condition;
to be re-quantified for any model and/or data
(2) a consistent uniform method for prioritizing the
changes as discussed in Phases 1 and 2, but the
investments needed to restore or modernize projects
general process should be applicable to
which are approaching or have exceeded their
rehabilitation projects. A detailed uncertainty
design life; and (3) an initial step in defining the
analysis was included in the Phase 3 analysis using
detailed engineering studies needed to estimate the
a Latin-Hypercube process and a Monte Carlo
remaining service life of structural features.
simulation. This process discerned the overall
b. The methodology in these ETL guidance
uncertainty associated with each of the intermediate
steps as well as for the final result (economic risk).
documents uses reliability indices as a relative
The uncertainty values would have to be
measure of the current condition and provides a
requantified for any model and/or data changes as
qualitative estimate of the structural performance.
discussed in Phases 1, 2, and 3, but the general
Structures with relatively high reliability indices will
process should be applicable to the rehabilitation
be expected to perform their function well.
projects.
Structures with low reliability indices will be
expected to perform poorly and present major
Phase 4 - The results of Phase 3 will be used as
rehabilitation problems. If the reliability indices are
input to the decision analysis in Phase 4. This
very low, the structure may be classified as a
analysis will be based on various economic analyses
hazard. Working from a sufficiently large
such as cost-benefit ratios. The cost-benefit ratios
experience base, it should be practical to make some
and economic analysis of Phase 4 are composed
estimates of expected structural performance with
mainly of comparisons between designs and
some engineering judgment. The reliability indices
proposed changes to designs. They are not
will be calculated using the performance function,
expected to concentrate on changes in individual
capacity, divided by demand. The results of the
component reliability improvements. However, the
reliability analyses may be used to identify deficient
process could lend itself to modification for
structures in need of stabilization and to prioritize
rehabilitation studies.
investment decisions. Target reliability indices that
F-7
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