ISAAC technology: how does it protect the structure on which it is installed?
To understand how ISAAC technology protects the structures during an earthquake, first of all it is necessary to briefly describe the behavior of a building subjected to a seismic event.
If we exclude the actions of the wind, or the moving masses that may be inside the structure, during normal operation a construction is usually subjected to static and vertical loads. The earthquake, by imposing displacements on the foundations, moves the masses involved in the structure which unload their inertia on the structural elements in the form of horizontal actions. The greater the displacements of the masses, the greater the accelerations and, consequently, the stresses for the structure. If we want to see the system from an energy point of view, we can say that the earthquake introduces energy into the building in the form of kinetic energy due to its displacements. This energy is dissipated by hysteresis and damage to the structural and non-structural elements.
From what we have just seen, we can understand the fundamental principle on which the
operation of an Active Mass Damper: by reducing the oscillations of the building, the horizontal forces discharged on the structural and non-structural elements are reduced, preserving their integrity and bearing capacity. To reduce travel the A.ctive Mass Damper uses a mobile mass that is moved according to a control logic in contrast to the earthquake. Just as the masses of the building set in motion by the earthquake are transformed into horizontal stresses for the structure, the moving mass generates forces that are opposed to those of the earthquake. In this way, the structure controlled by ISAAC technology will undergo less displacement than the same structure without a control system.
Protection of structural elements
Structural elements such as beams and columns dissipate the energy coming from the displacements imposed by the earthquake through the hysteresis process. If you want to describe the behavior of
The element on a graph with the displacements in the abscissa and the force in the ordinate, we will obtain a graph similar to the one shown in Figure 1. It can be seen how the red curve, initially elastic and linear, with increasing stresses begins to widen and enclose an area gradually larger. The area under the curve represents the energy dissipated by the specific element and the various cycles it performs are called hysteresis cycles.
Figure 1 - Hysteresis and damage to the beam-column joint
Energy dissipation occurs through damage to the structural element with consequent loss of bearing capacity. With the installation of the Active Mass Damper we have seen that the movement of the structure can be significantly reduced. Going therefore to see the hysteresis graph of the single structural nodes we would obtain a much tighter curve with less energy dissipation and less damage to the elements. In this way, the load-bearing capacity of the elements and the structural integrity of the building are preserved.
Protection of non-structural elements
Non-structural elements such as cladding or window frames do not play a supporting role inside the building. Even if their integrity provides additional stiffness to the structure and consequently improves its dynamic behavior, their collapse does not constitute a loss of bearing capacity. However, damage to non-structural elements should not be overlooked, as it can be an important source of danger for the occupants of the building and cause significant economic damage. For their behavior in the event of a seismic event, it is possible to make a speech similar to that made for the structural nodes. Even the infill panels have a hysteretic behavior, characterized by greater fragility than beams and columns, which allows them to dissipate energy. The movement of the nodes and the inter-floor drift create a compressed strut inside the masonry panel that can lead to the collapse of the masonry (Figure 2). The use of the Active Mass Damper, reducing structural displacements, reduces the level of compression reached in the infill and allows to preserve its integrity.
Figure 2 - Behavior of the infill walls during the earthquake
In summary, the benefits of Isaac technology can be seen first of all by comparing the displacements, especially on the roof where there will generally be the largest fluctuations, between the current state and the controlled structure. Another very important aspect concerns the reduction of the hysteresis area in the beam-column nodes which implies less damage to them.
Even in energy terms, the benefits are very evident. Going to describe the energy balance of the structure during the earthquake in a graph with the time on the abscissa and the percentage value of the dissipated energy on the ordinate (Figure 3), we will have that for the structure in its actual state all dissipation is entrusted to damping structural with all the damage and hysteresis phenomena that we have seen previously; for the structure controlled by Isaac technology there will be, in addition to the structural damping, a further contribution provided by the machines which involves a considerable reduction in the dissipation of energy entrusted to the structure.
Figure 3 - Energy balances
Author: Guido Ducoli