In Italy, the structural design of buildings is inseparable from guaranteeing an adequate level of performance against seismic activity. These sudden events are neither predictable nor preventable and often arrive with a highly destructive potential.

On that note, the only way to try to limit the damage is by strengthening the structures through targeted measures, which are able to currently improve their response under those types of horizontal activities induced by earthquakes.

To this end, it is possible to carry out a series of works that increase the performance level of the buildings only if carried out by competent and qualified professionals.

All possible interventions can be framed in two macro-classes that operate with two completely different approaches. In particular, we can speak of two different philosophies of intervention:

  • the seismic isolation of the structure by means of special devices to decouple the seismic effects from the structure
  • seismic dissipation concerning the precise controlled damage of the structural elements, not prejudicing, in any case, the overall stability of the building

Projects like this are currently made feasible and desirable thanks to the tax breaks that, this year included, the state has made available to citizens hoping to improve the performance level of new buildings or for interventions on existing buildings located in areas with high seismic risk.

The Sismabonus allows owners to intervene on their own properties saving considerable amounts of money and securing buildings that in most cases had previously been designed in the absence of a specific seismic legislation (Law 2 February 1974 No. 64, Official Gazette No. 76 of 21 March 1974).

What is seismic isolation?

Seismic isolation is a construction technique aimed at limiting the transmission of mechanical energy from telluric waves that, through the ground, can hit buildings.

The term “isolation” is indicative of the objective of such constructions, which is precisely to create a complex that is “isolated” from the effects induced by seismic shocks.

Seismic isolation is a civil engineering intervention that presupposes the interposition between the base of the structures in elevation (superstructures) and the foundation system (substructures) of specific devices called seismic isolators.

These are supports characterized by a much lower rigidity towards horizontal cutting actions than against axial vertical actions, which behave like real springs that filter the actions transmitted from the substructure to the superstructure.

The aim is to decouple the superstructure from the motion of the ground, trying to make the seismic isolators absorb most of the seismic demand for deformation. In addition, the isolated system is more flexible and consequently the structure’s own period shifts towards higher values, which translates into a reduction in seismic demand in terms of response accelerations.

The design of the insulation system can be aimed at the integral absorption of the seismic demand (allowing the structure to remain in the elastic range) or only partial. A similar mechanism is based on the use of viscous energy dissipation systems, associated with the insertion of special devices and capable of reducing the dynamic forces that commonly develop under earthquakes.

The main types of seismic isolators available on the market are as follows:

  • Elastomeric insulators (rubber bearing)
  • Elastomeric insulators with lead core (lead-rubber bearing)
  • Friction-based isolation system
  • Sliding pendulum isolators (friction pendulum system – FPS)

Seismic dissipation: what is it?

Seismic dissipation is the ability of structural elements to absorb the energy released by the earthquake through widespread damage that does not affect the stability of the structure as a whole. The dissipative capacity is closely linked to the ductility of the structural elements that make up the building and therefore to their deformation capacity in the plastic range. To achieve these requirements, the structure must ensure the formation of multiple dissipative zones before reaching collapse. This means tolerating even substantial damage to structures that safeguard the safety of their occupants.

This goal can be achieved both for the design of new buildings, providing a capacity design that improves the overall ductility of the structure through a series of measures that avoids the establishment of fragile breaking mechanisms (hierarchy of resistances), as well as for the design of interventions on existing buildings (which in many cases appear to have been designed in the absence of a specific seismic regulation),  allowing for a series of works that ameliorates the structural response to horizontal actions, such as external steel bracing or precise energy dissipation devices, which function like real seismic energy fuses.

In general, these works are non-invasive or in any case minimally invasive compared to the installation of seismic isolators.

Isolation or dissipation: what approach to take?

As we have seen, the two approaches aim to achieve the same objective, the protection of human lives during a seismic event, but in different ways. On the one hand we have insulation that allows limited or a total absence of damage on the superstructure, while on the other we have an extensive as well as also considerable damage but still with an adequate safety margin against gravitational loads. It turns out to be clean, therefore, that base isolation can be the main solution for new construction structures or for the adaptation of existing ones that need to remain operational even in the post-seismic phase (hospitals, buildings and strategically relevant structures). While pursuing the increase of the seismic performance of a structure by implementing a series of interventions that increases its dissipative capacity is to be considered, to date, the basic technique to be adopted for new and existing buildings of an ordinary nature, which, although substantially damaged after a seismic event, are still capable of guaranteeing a safety margin against non-seismic actions. At this point it is natural to ask: how come we aren’t always relying on the insulation technique at the base of the structures when we talk about anti-seismic design? The answer can be found, in addition to the technical reasons stated above, mainly in the costs of construction and maintenance of the insulation intervention and in the invasiveness of the devices. It turns out that if for new buildings, at least theoretically, an anti-seismic design that includes the technique of insulation at the base is always a possibility, the same cannot be said for interventions on preexisting buildings, which to be isolated at the base have to be substantially modified at the level of its foundation, and this is not always feasible.

Finally, it is fundamental to point out that Italy is an area characterized by a considerable basic seismic hazard, with Sardinia being the only exception, and that the majority of the buildings not only do not comply with the performance requirements imposed by the seismic standards in force but have been designed in the absence of a specific anti-seismic standard. To pursue a generalized improvement, which allows the safety of people to be safeguarded in the event of extreme events, a widespread intervention throughout the country is desirable before being faced again with the damage in economic and human lives terms resulting from the next catastrophe, or as the Japanese would say before Numazu (the huge catfish that lived, according to mythology, in the mud beneath the Japanese archipelago) wakes up yet again.