The seismic risk present within the Italian territory is well known and can be identified mainly in two respects: the first concerns the high seismicity of our country which in the previous centuries led to important earthquakes that mainly affected some areas of central Italy. - southern. The second aspect is inherent, however, to the high vulnerability of the Italian building heritage which is characterized by being very heterogeneous and in some cases decidedly unsuitable to be able to cope with actions of a certain intensity such as seismic ones.
If with regard to the first point it is not possible to do anything, as it is the cause of natural tectonic movements, with regard to the seismic vulnerability it is necessary and necessary to intervene in an adequate way.
Therefore, intervention strategies become fundamental to mitigate the seismic risk on structures present in the Italian territory with the aim of making them less vulnerable to seismic actions.
In the last twenty years, the history of seismic protection of structures has placed the emphasis on achieving a performance in terms of ductility. By ductility we mean the ability of the structure to embed deformations in the plastic field and therefore to be damaged as a result of seismic actions. Since a building is damaged, it dissipates energy in the form of heat (hysteretic dissipation), reducing the effects of the earthquake in the form of smaller accelerations and smaller forces.
Today, operating according to the current regulatory framework, the ductility of the structures is achieved through the design philosophy of "Capacity Design", with which controlled damage areas are identified in which to concentrate the dissipation, in such a way as to safeguard the main structural elements. In practice, the criterion of the resistance hierarchy has the purpose of guaranteeing an adequate over-resistance of the fragile (non-dissipative) structural parts, which must remain in the elastic range, so that only the elements or dissipative zones can plasticize.
An alternative way to dissipate the energy deriving from an earthquake is to insert within the structure of the structural control devices, ie elements that provide additional dissipation of energy. This technology can be envisaged within an existing building, with the aim of protecting it, but it can also be used as a seismic-resistant element of a new structure in addition to the design philosophy of the hierarchy regarding the other traditional elements with the aim of giving the structural complex a high dissipative capacity, without resorting to damaging the members.
By exploiting only the dissipation linked to the damage to the structure, an effective reduction of the overall seismic forces is produced: think of the concept of the design response spectrum obtained by dividing the elastic spectral ordinates by a behavior factor q greater than 1,5. The higher the value of the q coefficient, the smaller the actions that the structure will be called to resist. Unfortunately, the same thing cannot be said of displacements: in fact, the damage "granted" to the structure will lead to a decrease in the stiffness of the vertical elements and consequently displacements at most equal to those that the same structure would have had, designed to remain in the elastic range. .
These displacements in turn lead to increasing deformations in the sections of the earthquake-resistant elements which will prove to be the major cause of damage to the non-structural elements.
It is now known that, for an increasing number of structures, the value given by the cost of the structural elements is much lower than the cost of the non-structural elements and the value of the content. Making the choice to design the structure admitting that it is damaged would by no means exclude protection against non-structural elements and the contents, since the displacements would remain the same as in a structure with elastic behavior.
Similarly, a design with low behavior factors (non-dissipative structures) is counterintuitive as this would expose the structure to accelerations (and therefore forces) that are not compatible with the secondary elements.
A protection obtained by additional energy dissipation reduces the seismic forces acting on the structure and, at the same time, decreases the elastic deformations of the system, also protecting the non-structural elements and the contents of the building.
In other words, the use of devices suitable for dissipating additional energy allows the entire value of the structure to be protected.
The structural control technology involves the use of devices capable of modifying the overall dynamic response of the structure, reducing its oscillations and responding to the safety and usability requirements of the product, without the need to resort to plastic deformations. The dissipation is due to non-linear behaviors inherent in the devices, mostly distributed within the structural structure.
Structural control devices are generally cataloged in the literature in two categories:
- passive control systems;
- active control systems.
Below we will go to an overview regarding active and passive devices.
Passive control systems
Passive control systems are devices that work without requiring external energy, using only the motion of the structure to produce relative displacements between the different parts of the system so as to develop the control forces necessary to increase the energy dissipation capacity. (Figure 1).
Figure 1 - Operation diagram of a passive control system
In the case of use of passive systems, the characteristics of the overall structure in terms of stiffness / capacity are modified, obtaining a more favorable dynamic response to seismic actions.
The most used passive control systems are friction dampers, base isolation devices (elastomeric and sliding pendulum bearings), viscous dampers, dissipative braces and tuned mass dampers mass damper).
Active control systems
Active control systems, on the other hand, are organs that operate require a certain amount of energy coming from the outside, such as to allow the operation of electro-hydraulic, electro-mechanical or electrical actuators, which provide the control forces on the structure.
In these cases it is necessary to develop a control algorithm capable of controlling the actuators which in turn will be able to generate the control forces necessary to dissipate the seismic energy. The operation of this algorithm will be subordinated to the processing of the data measured by some sensors placed in strategic points inside the building, able to measure the response of the structure hit by the earthquake (Figure 2).
Figure 2 - Diagram of operation of an active control system
The control forces are generated by the actuators, controlled by a control algorithm, whose decisions are based on the feedback, coming from sensors, which measure the response of the structure and on the feedforward information, coming from the excitation measurement.
The best known active devices are active mass dampers, also called Active Mass Damper (AMD).
Active Mass Damper
The Active Mass Damper (AMD), are the active and most advanced version of the passive Tuned Mass Damper device.
They consist of an auxiliary oscillating mass, driven by an actuator, which exerts a force on the mass, causing it to vibrate. The control force exerted on the structure is given by the inertia force of the mass thanks to the principle of action and reaction. The AMD system has the advantage of being able to reduce the response of the structure in a wider range of frequencies than the passive device from which it derives, using a completely negligible moving mass when compared with the total weight of the structure.
The first implementation of an active control device on a building was with the adoption, in 1989, of an AMD system in Tokyo within the Kyobashi Seiwa (Figure 3).
This 11-storey structure mounts inside two active masses of 5 tons in total (the weight of the building is approximately 400 tons) to counteract both the transverse motion of the building and the torsional motion from the actions of the wind.
Figure 3 - Kyobashi Seiwa from Tokyo
ISAAC Antisismica has patented the first AMD-type active control system in Europe capable of significantly improving the overall behavior of a structure subject to a seismic event.
The system provides for the installation of the actuators on the roofing surface of the building to be protected (Figure 4) and of accelerometric sensors positioned in strategic points of the structure (Figure 5). A central computer coordinates and controls the entire system. The purpose of the latter is to counteract the motion of the building during the earthquake, reducing the oscillation amplitudes of the building and consequently making the forces acting on the resistant structural elements less intense. This process occurs through the generation of forces by the machines, determined by the control algorithm implemented within the central computer. The latter measures the accelerations of the building at the points where the sensors are positioned and, consequently, calculates the forces that the machines must generate.
Figure 4 - AMD proposed by ISAAC
Figure 5 - Example of accelerometric sensor installed on the structure
The adoption of active control systems within civil structures constitutes the future of seismic protection, as it allows to obtain improvements in the dynamic behavior of the building in the event of an earthquake comparable to those obtainable with the adoption of passive systems, but taking advantage of the less invasiveness during the installation phase. A system of this type, in fact, allows to intervene on buildings without disturbing the inhabitants and allows to reduce to a minimum the ancillary works preparatory to the installation of the system.
Figure 3 was taken from the following work: "Semiactive control of civil structures for natural hazard mitigation: analytical and experimental studies" - Richard E. Christenson - Department of Civil Engineering and Geological Sciences, Notre Dame, Indiana, December 2001
Author: Fabio Menardo