Keywords: passive control systems, seismic protection, flexible supports, spring elements, damping, viscodampers.

Passive seismic control strategies are based on the reduction of energy which may affect a structure in case of earthquake events. Some well known approaches make use of frictional, plastic, or other energy dissipating behaviour of special devices. The following presentation reflects some basic ideas for the increase of elasticity and viscous damping for different types of structures. Spring elements provide local elasticity and they attract a great extend of the seismic energy. In many cases they represent the most flexible part of the structure. In comparison to common structural members they are designed for high operational and seismic demands.

Usually Viscodampers are installed beside the spring elements. They have the task to absorb the kinetic energy by the increase of structural damping. Viscodampers serve as a displacement limitation of the structure and of the devices. The safety against collapse and defined states of serviceability of the structures can be assured. Induced accelerations and internal stresses of important members can be reduced drastically when comparing with the behaviour of the unprotected system. In some special cases merely Viscodampers are installed. They possess the ability to damp the relative motion between two structures or between the structure and the "rigid"' vicinity. The decrease of the induced structural responses by viscous damping can be taken from different national and international standards, e.g. Eurocode 8 and provisions of the Architectural Institute of Japan.

The efficiency of the presented strategies is outlined. Results of numerical analyses are presented utilising elastic springs and viscous dampers arranged below or within the discretised models. Several examples of executed projects in seismically active regions are discussed.

Protection of Machinery: Many types of machines can be regarded as more or less rigid from the seismic point of view. Hence, the improvement of the earthquake resistance can easily be achieved by changing the support conditions. The seismic protection of a highly flexible steam turbine / generator set is described. Here a special support system was developed and the foundation was integrated in the machine building.

Protection of Equipment: Equipment, e.g. for electrical purposes, frequently consists of sensitive material or requires a vulnerable assembly. Two examples are shown in this category - a capacitor bank with ceramic footings and an air core reactor with a big single mass high above the ground. The improvement of the seismic behaviour for both structures is based on modified vibration modes and damping. The effects of controlled rocking motions are discussed.

Protection of Buildings: Buildings are usually very complex in regard to the dynamic behaviour, and they require a very high standard in safety questions. The earthquake protection strategy of a RC building in Argentina are discussed. Here, a comparison study was undertaken with a base-isolation system (BIS with rubber bearings) and the chosen base-control system (BCS - helical steel springs and Viscodampers). Important responses like internal stresses and subsoil reactions are computed and compared for the different systems under 11 recorded seismic events.

After a brief outline of the fundamentals of seismic control strategies, some examples for the earthquake protection of machines, equipment and buildings are discussed in the paper. The proposed integration of the T/G deck in the machine building implies economic advantages regarding the total construction costs. It becomes very important for the reduction of seismically induced turbine accelerations and relative displacements. Additionally the earthquake demands of the entire machine building can be controlled by the proposed visco-elastic support of the T/G deck.

Strategies for the protection of equipment are shown using changes in the support conditions. By controlled rocking effects the structural safety can be increased significantly. In comparison to other strategies the horizontal differential displacements in the control surface are in the range of only a few millimetres even under severe earthquake actions. Even sensitive material like ceramic footings in electrical equipment can be protected effectively.

Finally, internal stresses, subsoil reactions, and horizontal motion in the control surface of an apartment building are calculated and the protection level of rubber bearings (BIS) and helical steel springs and dampers (BCS) are discussed. Depending on the frequency content of earthquake both systems BCS and BIS are suitable for the reduction of internal stresses. Vertical subsoil reactions under BIS become relatively high when compared with the system without any precautions. On the other hand the seismic gap for structures using BCS can be chosen with 100 mm only. There are maximum figures between 38 and 95mm and the same figures using rubber bearings can be found in a range between 57 and 420mm. The execution of the chosen Base-Control System is briefly described.

In the discussed examples helical steel springs and Viscodampers are used as seismic control devices. It is well known that springs are acting in the axial direction as they are usually also carrying the dead load of the structure. For the purpose of earthquake protection the vertical demands usually become higher and the horizontal component becomes more important in regard of the target frequency and mode shape of the seismically controlled structure.

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