Example application of reliability assessment of coastal structures

Example application of reliability assessment of coastal structures

Lamberti, A.

TU Delft

ICCE 1992

1992-10-01

During the last few decades the believe that absolute security could be provided has been recognized to be mistaken in many branches of engineering. This happens as a consequence of the imperfect predictability of load intensity, structural characteristics and structural behavior, as well as a consequence of human gross errors during the design and construction phases or during the using (exploiting) period. In the field of coastal engineering this became particularly evident when breakwaters were constructed in deep-water condition, i.e. where water-depth ceases the beneficial effect in limiting the design wave height. Beyond this special case, it is rat her common in coastal engineering practice to dwell with uncertain environmental conditions and sometimes even with uncertain structural behavior. A method which can account for the multiform information available and which can produce a rational risk evaluation is therefore highly advisable. A rationalization of the risk assessment produce some advantages: 1 - The various uncertainties are rationally incorporated and balanced in the assessment of the reliability of the structure. 2 - A more balanced design of components of the overall system can he made, avoiding the coexistence of overdesigned components blowing up the costs and of underdesigned components causing the risk. 3 - Cost of the structural solution to risk (building a stronger structure) can be assessed and compared to the non structural solution (reducing the uncertainty). 4 - A better insight into how the failure probability is built up from the various uncertainties is obtained: priorities can he established for further research aiming to improve the description of the system and reduce the uncertainty margins. In the example presented failure and risk are looked at from the client point of view. CONCLUSIONS A complete and objective risk assessment is not always possible due to the lack of reliable models for some of the relevant failure modes. A model is suitable for this kind of use, if its reliability has been tested and quantified (the statistics of the model related variabie Z should be known). The statistics of all the relevant parameters should be known at least in a simplified form (a central value and one or two fractiles characterizing the distribution on the cautious side). A good pile of scientific and technical work must still he done in order to translate the actual knowledge of the basic phenomena into a rationally based level 1 risk assessment procedure, suitable for the everyday design praxis. Nevertheless even a not completely objective risk assessment can be useful to answer in a conclusive manner such questions as: - How deep the preliminary environment characterization should be brought? Does risk come only from the climate vagary or also from our imperfect knowledge of the past and of the present? Does it come from our poor knowledge of some relevant phenomenon? - What are the relevant failure modes? Could we substantially reduce the risk by controlling carefully the construct ion? - Is a physical model or any other more reliable and expensive evaluation tool useful or necessary? Should an overdesign he preferred to a strictly controlled construction or a more deep analysis?

reliability
assessment
coastal
risk analysis
safety

TPN1100

http://resolver.tudelft.nl/uuid:02485a19-1d62-4ba0-a9ff-2bda3b22d6de

ICCE 1992 local organising committee

Design and Reliability of Coastal Structures, short course during the 23rd ICCE in Venice

Hydraulic Engineering Reports

report

(c) 1992 A. Lamberti