On Categorization of Seismic Load As Primary or Secondary for Multi-Modal Piping Systems

Categorizing the seismic load requires calculating the input level associated with the ultimate capacity and comparing it to the level associated with the plastic yield. Therefore, an analysis of the seismically induced ductility demand in oscillators of variable frequencies was carried out by running non-linear time response analyses, the seismic input motion being simulated as samples of a stochastic process of central frequency f_c. The response of oscillators with frequencies, f₀, varying from 0.1 f_c to 10 f_c, was systematically analyzed. For every oscillator, 10000 time-responses were performed, corresponding to 1000 input samples multiplied by 10 input levels, covering a wide range of ductility demand up to 20.

Output is that seismic loads should be regarded as secondary for flexible oscillators (f₀ < f_c) while it should be regarded as primary for very stiff oscillators (f₀ > cut-off frequency of the input motion, f_cut), with intermediate situations for f_c < f₀ < f_cut. A practical engineering rule is presented to incorporate this result when calculating the primary part of seismically induced streeses in a multimodal piping system. This rule is currently tested in the framework of the OECD-NEA international benchmark MECOS.