This paper presents practical implementation of a theoretical approach that was introduced at PVP 2020 conference. Analysis of the seismically induced ductility demand in elastic-plastic oscillators of variable frequencies and hardening slope is carried out by running time response analyses. The input motion consists of 1000 stochastic process samples of central frequency fc. Oscillators of natural frequencies, f0, (0.1 fc ≤ f0 ≤ 10 fc) are submitted to a wide range of ductility demand, up to 20. It turns out that seismic loads should be regarded as secondary for flexible oscillators (f0 < fc) and primary for very stiff oscillators (f > fcut, cut-off frequency of the input motion), with intermediate situations. On this basis, we derive a strongly f0/fc dependent evaluation of the seismically induced inertial stresses primary part. In the frame of the conventional linear modal analysis method, practical implementation consists of reducing the input response spectrum: spectral ordinates are minimized in the low frequency domain by a factor that depends on the ductile capacity and hardening slope, unchanged at the ZPA frequency and vary linearly in the medium frequency domain. This approach is tested against nonlinear time-response analysis of a multimodal piping system.

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