Sound & Vibration

Sound & Vibration

Earthquake control of resiliently mounted systems is an area of primary concern for acoustical and mechanical engineers. The Northridge, CA earthquake of January 17th, 1994, provided several valuable lessons regarding protection of these systems. The lessons learned from this earthquake are highlighted with actual site photographs that are useful and dramatic. The photographs are combined with several tables and illustrations that provide practical guidelines for future mechanical system installations.

One function of an acoustician or a mechanical engineer is to decide what equipment requires vibration isolation, as well as what type of mountings and what static deflections to use. The picture gets more complicated, however, when this equipment also requires seismic protection. There is a conflict between what the acoustician or mechanical engineer is trying to accomplish and what the seismic engineer must accomplish. Although there is no question that an isolation system without any type of seismic device is superior acoustically, the engineer has to select the seismic systems to be used with his carefully planned vibration isolation system. Techniques that allow both disciplines to be somewhat satisfied have been developed based on information from the most recent earthquakes.

The earthquake of January 17th, 1994, (Northridge, CA earthquake) confirmed the lessons learned in the Loma Prieta earthquake of 1989, when properly designed and applied hardware protected mechanical sys tems from seismic shock. When the rules were not strictly followed, the result was often support failure. Figure 1 shows the overall map of the Los Angeles area, including the epicenter of the Northridge earthquake. Also indicated are some of the areas where the investigation teams studied the mechanical and electrical systems in modern structures. The examined systems cover a full gamut: non-isolated as well as isolated; mechanical and electrical; floor mounted and suspended systems. These systems showed dismal failures in some instances and no damage in others.