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Items that are hanging on supports react to earthquakes act as though supported on a pendulum. If the pendulum is 12" or less, its natural frequency is outside the range generated by earthquakes. Because of this, the piping and ductwork will move back and forth during an earthquake, but the motion will not be amplified with time. As long as there is nothing that won’t prevent 3-4" of lateral motion and the hanger rod is allowed to swing freely (includes some kind of swivel), pipe/duct systems have consistently survived seismic events with minimal problems. If this motion is restricted by local obstacles, damage can occur and the system would require restraint.
Although from a philosophic viewpoint it would seem that the same rules should apply to equipment as apply to pipe and duct, this is not the case. Equipment behaves more erratically than piping or ductwork during a seismic event because it is more local in nature. Large distributed systems like pipe and duct will tend to move laterally or even exhibit relatively minor variations in the vertical loads but will not tend to rock. Localized components like equipment will rock and as such can generate considerably higher tensile loads in the support system. In some cases this may not be an issue, but there are definitely situations where the installation geometry will develop loads that can fail the support system.
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