IEEE-344 CHECK: Determination of multi axes table independence and stationary
Shake table testing according to IEEE-344 (seismic qualification of safety related equipment in nuclear power plants) must demonstrate that the motion in the different axes of the table are Stationary and Independent. IEEE-344 CHECK performs calculations to verify this.
A time history motion is Stationary if its frequency energy distribution is relatively constant over the typically 30 second duration of the earthquake. This is verified using the method of Kana by breaking the earthquake up into several sections and evaluating the PSD of each section and comparing it to the average PSD of all sections. If the deviation from average does not exceed the Kana/IEEE-344 criteria, then the trace is considered Stationary and acceptable per IEEE-344. The purpose for this check is to assure that all modes of vibration of the test object are close to being simultaneously excited during the entire earthquake and hence can interact and provide a conservative test.
Two time histories are said to be Independent, for the purposes of IEEE-344 if their Coherence is 0.5 or less over the entire frequency range of interest, or if their maximum absolute value correlation coefficient is 0.3 or less. (The actual criteria 0.5 and 0.3 are sometimes reduced per the particulars of the testing organization.) The purpose of these criteria is to assure that all modes of vibration are excited regardless of their direction of maximum coupling to ground motion and hence produce a more conservative test.