Why do I not have to restrain piping or ductwork that is hung 12″ or less from the ceiling?
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 with 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.