Equipment must be designed and tested to withstand shock and vibration. Ideally, all equipment would be tested on a shaker table with six-degree-of-freedom control (three translations and three rotations). Such tables and control systems exist but are very expensive. Furthermore, any multi-axis testing requires careful consideration of phase angles between the six degrees.
Another option is to test equipment on a triaxial table where the three translations are controlled, and the three rotational degrees are constrained to zero motion. Testing on a biaxial table is yet another choice.
The most common test method, however, remains testing in each of three orthogonal axes, one axis at a time, on a single-axis shaker. This is simplest and least expensive method.
The question arises “Should the acceleration level be increased for the case of single-axis testing?”
There is a tacit understanding that aerospace and military equipment test levels already have a sufficient uncertainty margin or safety factor so that the levels can be used without further increase. In other words, the specifications are already intended for single-axis testing. In many cases, a uniform level is used in each axis which is the maximum envelope of the maximum expected levels in the three axes plus some margin.
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The standards which address testing equipment for earthquakes take a different approach. The following descriptions are taken from five common standards.
Only KTA 2201.4 gives a scaling formula. This is also the only standard from the five samples which may be freely downloaded.
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IEEE 344-2013 Standard for Seismic Qualification of Equipment for Nuclear Power Generating Stations
8.6.6 Multiaxis tests
Seismic ground motion occurs simultaneously in all directions in a random fashion. However, for test purposes, single-axis, biaxial, and triaxial tests are allowed. If single-axis or biaxial tests are used to simulate the 3D environment, they should be applied in a conservative manner to account for the absence of input motion in the other orthogonal direction(s). One factor to be considered is the 3D characteristics of the input motion. Other factors are the dynamic characteristics of the equipment, flexible or rigid, and the
degree of spatial cross-coupling response. Single and biaxial tests should be applied to produce adequate levels of excitation to equipment where cross coupling is significant and yet minimize the level of overtesting where the cross coupling is not significant.
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KTA 2201.4 Design of Nuclear Power Plants against Seismic Events, Part 4: Components
This document may be freely downloaded: link
5.3.3 Excitation Axes
220.127.116.11 Simultaneity of excitation directions
Simultaneous three-axis testing is preferred. But single-axis testing can be substituted by testing in each of three axes sequentially.
The standard shows, for example, that the uniform single-axis level should be the “square root of the sum of the squares” of the three orthogonal installation site levels.
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IEC 980 Recommended practices for seismic qualification of electrical equipment of the safety system for nuclear generating stations
6.2.9 Qualification test method
As is well known, seismic excitation occurs simultaneously in all directions in a random way. According to this point of view, the test input motion should consist of three mutually independent waveforms applied simultaneously along the three orthogonal axes of the equipment.
However, taking into account that three axial testing installations are rare and that triaxial testing is desirable when significant coupling exists simultaneously between the two preferred horizontal axis of the specimens, biaxial testing with multifrequency independent input motion in the horizontal and vertical direction is an acceptable test.
Tests shall be performed according to 6.3.2 and, in terms of total duration and fatigue induced, are intended to become conservative.
In some cases, single axis tests with multiple, or single frequency excitation are also acceptable methods of test if properly justified considering the effect of coupling between axes.
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Assumes single-axis testing. The base input time history is specified in the standard.
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IEEE 693-2005 – IEEE Recommended Practice for Seismic Design of Substations
The shaker table shall be biaxial with triaxial preferred.
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– Tom Irvine