DEVELOPMENT OF AMPLIFIED SEISMIC RESPONSE SPECTRA FOR STRUCTURAL ANALYSIS OF DECOUPLED SMALL BORE PIPES

Abstract eng:
In various applications, not at least within the nuclear industry, piping systems are required to be structurally verified for seismic loads. Piping systems often have connected small bore pipes which also have requirements for seismic qualification. If the ratio of run-tobranch pipe moment of inertia is large, the small bore pipes can often be decoupled from the main pipes when performing the structural analysis. There are many reasons why decoupling is preferred in comparison to performing a coupled analysis. Firstly, from a project perspective, it is desirable to finish the design and verification of the main pipes before the work on the small bore pipes is started. Secondly, with decoupled pipes the analysis models become smaller, meaning reduced analysis time and reduced risk of encountering convergence difficulties for dynamic loads. Finally, decoupling the small bore pipes makes it possible to distinguish between primary and secondary loads, which will take away unnecessary conservatism in the evaluation of the small bore pipes. Though there are advantages of decoupling, the analyst will be faced with the question of how to consider the impact of the main pipe oscillations in the analysis of the small bore pipe. Seismic loads are generally applied to pipes as response spectrum loads derived in a seismic analysis of the supporting building structure. A clear disadvantage of a response spectrum analysis is that it only calculates bounds for forces, moments, displacements and accelerations and that it does not result in any frequency dependent quantities. There is no established industrial practice how the impact on the small bore pipe from the main pipe should be considered at the decoupling point and in many cases simplifications that give non-conservative results are made. This study presents a method to create an amplified response spectrum at the small bore piping connection point making use of modal analysis results and response spectrum input data. The method is validated by comparison with results from coupled analyses. The benefit of the proposed method is also demonstrated by applying it in evaluation of a small bore pipe.

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