Seismic vulnerability analysis: what it consists of and how it is calculated
The Italian building stock has a high degree of vulnerability to seismic phenomena. This fact is there for all to see and, as often happens with destructive natural disasters, it is thought that it is impossible to deal with such events.
If it is true that earthquakes cannot be predicted, it is equally true that there is the possibility of intervening on the construction of our country to make it suitable for resisting the seismic actions.
The first step in assessing the criticality of an existing structure is the vulnerability study carried out by a professional in the sector, which allows, through detailed numerical simulations, to understand the actual state of the construction.
Figure 1 – L'Aquila, Government Palace following the 2009 earthquake.
But what does this vulnerability study consist of?
The vulnerability study makes it possible to calculate the safety index of the building. If this index is close to 0, then the structure will be very fragile. As the performance of the building increases, this value will increase, increasing the safety of the structure.
The Technical Standards for Construction of 2018 in § 8.3 deal with the issue of assessing the safety of existing buildings against seismic actions.
To evaluate the level of safety it is necessary to determine the parameter ζE which represents the ratio between the maximum seismic action that can be tolerated by the structure and the maximum seismic action that would be used in the case of design from scratch.
In formulas:
where with PGAc if it indicates the Peak Ground Acceleration of capacity and with PGAD if it indicates the Peak Ground Acceleration of question.
The demand PGA is evaluated as:
where S indicates the coefficient that takes into account the subsoil category and the topographic conditions, while ag is the maximum horizontal acceleration at the site.
The capacity PGA, on the other hand, is evaluated when the failure of the first significant element of the construction occurs, which determines the maximum acceleration at the base to which the structure can be subjected.
In § 8.4 the NTC 2018 go into detail on the interventions that can be carried out on existing structures. The interventions are divided into:
repair or local interventions;
improvement interventions;
adjustment interventions.
The calculation of the parameter ζE indirectly falls within the classification of the type of intervention to be carried out. In fact, in §8.4.2, with regard to improvement interventions, the Regulations state that "[...], for class III buildings for school use and class IV buildings, the value of ζE in any case it must always be not less than 0,6, while for the remaining class III constructions and for those of class II the value of ζE must be increased by a value not less than 0,1".
In the case, however, of adjustment interventions, the value of ζE must always be ≥ 0,80.
The objective is therefore to increase the capacity of the structure or to reduce the demand entering it to increase ζE and therefore to raise the seismic performance of the building.
A suitable technology for intervening in this sense is the Electro-Pro 20x active seismic protection system proposed by Isaac which, through active dissipation, reduces the inertia forces entering the structure. Through the generation of forces carried out by the machine units located on the upper floors of the building, the oscillations of the structure are reduced, the deformation energy of the elements decreases and the cutting edges of the floor are reduced.
Author: Fabio Menardo
