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WHAT'S COMING IN ABATEMENT AND MITIGATION METHODS: ACTIVE AND PASSIVE SYSTEMS FOR EARTHQUAKE PROTECTION Ian G. Buckle National Center for Earthquake Engineering Research Earthquake protective systems for buildings and bridges are innovative solutions to the seismic hazard problem. Also referred to as structural control, these systems have the potential to substantially improve the seismic safety of a wide range of structural types for nominal cost. Passive, active, and hybrid systems together make up this family of protective devices which enable a structure to withstand earthquake loads, not by adding strength but by diminishing the structure forces. Passive systems, which include base isolation, are the most well developed of the three systems mentioned above. Isolation decouples a structure from earthquake ground motion with consequential reductions in earthquake forces. Today there are more than 125 structures, worldwide, which have been built using this technique. In the United States, 15 buildings and bridges have now been completed with at least another 15 under construction or in the design stage. Decoupling is achieved through the use of isolation bearings that support the weight of the building while permitting relative movements to occur between the ground and the building. The technique can be applied to both new construction and the retrofit of existing structures. In the latter case, significant cost savings can be achieved when compared to conventional methods of strengthening. The isolation bearings can either be made of high quality elastomers or use sliding surfaces (e.g., stainless steel and Teflon) to permit the required decoupling to occur. Other passive systems include mechanical energy absorbers which may be installed in structures so as to dampen relative motion during earthquakes. In some bridges, hydraulic shock absorbers have been used, whereas in buildings visco-elastic dampers have been incorporated into bracing elements and wall panels. Active control involves the use of an active energy source to maintain (or control) a structure within predetermined limits during an earthquake. Laboratory and computer models have been studied extensively and their feasibility demonstrated. Both active mass dampers and active bracing systems are now being used in trial installations in prototype buildings. Early results from the field indicate performance is as expected. Hybrid systems involve the active control of base isolated structures and are attractive because many of the advantages of active control are retained while enhancing reliability, reducing power demands and extending the range of applicable structures. Earthquake protective systems are the direct result of aggressive research programs funded in the United States by the National Science Foundation, the National 21

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Center for Earthquake Engineering Research and the Department of Energy. Other countries, who also are active in the design and implementation of these systems include New Zealand, Japan, France, and Italy. By the turn of the century, such systems will be commonplace and the construction of the intelligent building will be one step closer. 22

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rH / no. 120 !• Federal Construction Council. Task Grouo on - Seismic safety technology