MECOS is a Post-Fukushima OECD/NEA initiative, with the following main objectives:

• To quantify the existing margins in seismic analysis of safety class components and assess the existing design practices within a benchmark activity.

• To make proposals for new design/evaluation criteria of pressurized piping systems, accounting for their actual failure mode under strong input motions.

The first part of MECOS consisted of gathering information on i) current design practices and ii) dynamic seismic tests on piping system carried out around the world that could be suitable for benchmarking. Part 2 is a benchmark exercise on piping system tests, and Part 3 are proposals for new criteria. The purpose of the proposed paper is to introduce the work which is undertaken in Part 3 in order to propose design criteria that address the observed fatigue-ratchetting failure modes as well as plastic instability. It includes revisiting the past test results as well as the interpretations that were carried out and conclusions that were drawn at that time, and reanalyzing them in the light of recent developments. Recent experimental programs carried out in Japan and in India will also be addressed.

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