Passive Seismic Control Systems are called passive because they do not require any additional energy source to operate and are activated only by the earthquake input motion. They are divided into three different types:
- Energy Dissipation Devices
- Base Isolation Devices
- Dynamic Oscillators
Passive control of structure can be obtained with the introducing of viscoelastic material into the structure, by the adoption of dampers, or with the so called tuned mass dampers.
Through internal stressing, friction, cracking, plastic deformations, all vibrating structures are able to dissipate energy and it’s known that the larger the energy dissipation capacity, the smaller the amplitudes of vibration.
However sometimes structures, because of their very low damping (of the order of 1 % of critical viscous damping), experience large drift even for moderately small earthquakes. In these cases Passive Control Systems are really useful because they are all characterized by the capacity to reduce the dynamic response of the structures by absorbing and dissipating vibration energy, converting it from kinetic energy to heat, or by transferring energy among vibrating modes. Frictional sliding, metals phase transformation, deformation of viscoelastic solids or fluids, yielding of metals and fluids passing through confined orifices are some of the ways used for the energy conversion to heat.
Another form of passive control systems is represented by seismic isolation systems in which a flexible isolation system is introduced between the foundation and the superstructure so as to uncouple the motion of the ground and the motion of the structure and to increase the natural period of the system. The increase in flexibility typically results in the deflection of a major portion of the earthquake energy; reducing accelerations in the superstructure while increasing the displacement across the isolation level.
Depending on their construction, Passive Control Systems may also increase the stiffness and the strength of the structure to which they are attached. Among the advantages of the Passive Control, the main ones are :
- It doesn’t require external power source in order to work.
- It is usually relatively cheap, the cost of installing and maintaining them is relatively inexpensive.
- It is inherently stable.
- It works equally during a severe earthquake as well as in a minor one.
Even for all these reasons, the passive protection of structures against seismic vibrations is widely accepted as a very effective technique, both for new constructions and for the retrofitting of existing ones. Thus in the decision process for the choice of a system to reduce vibrations in a structure, passive control should be considered as one of the first options, and just in case of not enough effectiveness, it’s suggested to move to other more sophisticated technique.
Nevertheless as main disadvantage, compared to active systems, in situations that require some adaptability of the control force depending on the structural response, the Passive Control devices turn out to be less advantageous.
Furthermore the increasingly demanding performance requirements push towards the development of new devices, exploiting the peculiar characteristics of new advanced materials. Indeed the present technologies present some limitations, such as problems related to ageing and durability (e.g. for rubber components), to maintenance (e.g. for those based on fluid viscosity), to installation complexity or replacement and geometry restoration after strong events (e.g. those based on steel yielding or lead extrusion), to variable performances depending on temperature (e.g. polymer based devices).
