How do technicians give an objective judgment on the earthquake reserves of an existing building? How can we understand how and when to intervene to improve the seismic performance of a building? In technical jargon, we speak of seismic vulnerability assessment, that is, a parameter that indicates the predisposition of a building to suffer damage or collapse during an earthquake. This value is a number obtained by means of numerical simulations more or less detailed. The estimate of this performance, which is in fact an absolute figure, is then compared to what should be, today, the actual seismic capacity of the building: the figure in question is called risk index and it is a metric that allows technicians to be aware of the severity of any seismic deficiencies, thus ensuring effective conditions for reinforcement intervention. This article delves into the topic of seismic risk and provides an application overview of the practical conditions that occur daily in territories with high seismic risk.
The risk index
THE’risk index is one of the possible quantifications of the seismic capacity of a building. This is the index that relates the maximum (seismic) accelerations that the structure can withstand to those that the building would actually have to withstand if built today, i.e. in compliance with current industry regulationsAs it has been defined, the maximum value that this index can assume is 1: the structure in this case can be said seismically adequate. The more the number tends to 0the more the structural deficiencies grow, the greater the risk! But why if a building is old then there is a possibility that does not have the right seismic capacity? For several reasons, but the most important is that knowledge of seismic risk has evolved over the years and with it has evolved the ability to technically deal with seismic actions, which by their nature remain unknown!
The structural engineer’s sensitivity to understand the problems present, as well as his ability to represent them through a mathematical model appropriate, play a significant role in the results of the resulting technical assessments. In many cases, also due to the lack of adequate investigation times, structural vulnerability assessments are carried out in a haphazard manner simplified, so conservative. However, this entails, to the advantage of a cautious view of the expected seismic performance, a photograph that is not always truly realistic of the actual state.
The intervention scenarios
Even in cases where the calculation of the risk index is done in an approximate manner, the structural vulnerability assessments that characterise it and therefore allow its evaluation allow, to date, to screen the potential structural interventions to mitigate seismic riskas well as outline possible future investment strategies in this sense.
In this perspective, in fact, the current Italian legislation already describes some particular scenarios: the severity of the problem encountered and the possible consequences on public safety, as well as the economic availability of intervention, are the main actors that outline the risk mitigation strategies. Two types of intervention are mainly distinguished:
- Seismic improvementthat is, a structural improvement is made which does not however completely cure the observed seismic criticalities, even if it significantly increases the current resistance of the work;
- Seismic retrofittingthat is, all the structural interventions necessary to obtain a structure with expected seismic performance comparable to that of a new structure, that is, designed according to current seismic standards.
Let’s briefly look at some particular cases.
The schools
For example, school buildings are dealt with in detail in current technical regulations. In particular, they require that any seismic improvement interventions used to mitigate this risk lead to a post-intervention risk index of at least equal to 60%. It represents a different prescription from other types of constructions, where any other percentage of intervention is accepted insteadprovided that it improves by at least 10% compared to the initial condition.
The strategic buildings
Even strategic buildings, classified as high-use class buildingsfollow a similar reasoning to that already explained in the case of schools. Reference is made to buildings with public or important functions, such as for example to preclude any rescue activities in the event of natural disasters, or in which activities are carried out that, in the event of significant damage, could cause serious damage to the environment (think for example of a nuclear power plant).
The infrastructures
Infrastructures have been in the eye of the storm for some years now, especially after the events related to the tragic collapse of the Morandi Bridge. We are talking above all about elevated structures, such as bridges and viaducts, in some cases very susceptible to earthquakes due to some non-performing past construction/design practices. In these cases, for example, the regulations allow a Relaxation of project limitsensuring the achievement of a seismic adaptation condition even reaching values of risk index equal to 80%rather than 100%.
Too much damage even without collapses
The collapse is the most blatant and extreme form of damage of a building under earthquake, but it’s not the only one. The loss of functionality of the building or its extensive damage to the external partitions and system components play a central role, in some cases even dominant, in the design hypotheses. If until some time ago the main objective was to make earthquake-resistant buildings that do not collapsetoday science and technology are turning towards design solutions that limit possible “secondary” damages to a minimum (also implicitly satisfying the previous objective), representing on many occasions non-negligible economic values or trigger further cascading problems in the corporate context, when hit by a natural disaster.
