Seismic vulnerability checks on relevant buildings in case of collapse

On the obligation of seismic vulnerability checks for significant buildings in the event of collapse.

We would like to share some reflections made on the obligation of seismic vulnerability checks on strategic or relevant buildings in the event of collapse.

Obligation to verify the seismic safety of relevant buildings in case of collapse.

The Ordinance of the President of the Council of Ministers 3274 of 20/3/2003 establishes that

It is mandatory to verify, to be carried out by the respective owners, in accordance with the rules referred to in the aforementioned annexes, both of the buildings of strategic interest and of the infrastructural works whose functionality during seismic events assumes fundamental importance for the purposes of civil protection, both of buildings and infrastructural works that can assume relevance in relation to the consequences of a possible collapse.

The Resolution of the Emilia Romagna Region Council 1661/2009 of 2 November 2009, in Annex B, provides a list of the categories of buildings and infrastructural works that may be relevant in relation to the consequences of a possible collapse.

Circular DPC / SISM / 0031471 of 21 April 2010 (Circular on the state of seismic checks provided for by OPCM 3274/03 and future programs.) States that “the verification (of seismic safety ed) is mandatory, while the intervention, unless you have sufficient ordinary resources “.

He also affirms that “the aforementioned checks are the responsibility of the owners or managers of the works, who are in any case responsible for failure to carry out the same“.

Circular DPC / SISM / 0083283 of 04 November 2010 (Clarifications on the management of the results of the seismic checks carried out in compliance with art.2, paragraph 3 of the ordinance of the President of the Council of Ministers n.3274 of 23 March 2003) summarizes for the owners of buildings defined as relevant in the event of collapse, the obligation to verify seismic safety and the obligation to plan interventions.

After several extensions, the deadline for the seismic safety checks has been definitively fixed at 31/03/2013.

Results of the seismic vulnerability verification

The reference seismic action depends on the following parameters defined by the Ministerial Decree 14/1/2008 Technical standards for constructions (NTC08):

  • seismic site hazard;
  • intended use and class of use.

The seismic safety of the building also depends on:

  • from the geometry of the building;
  • from the gravitational loads present;
  • the quality of the materials used.

Usability of buildings subjected to seismic vulnerability verification

Carrying out seismic vulnerability checks and finding safety levels below 100% does not imply the lack of accessibility of the building.

It is important to be aware that the assignment of a “safety assessment” (or “technical verification”) represents only the beginning of a path made up of successive iterations, which see the client protagonist of a process, precisely “iterative” together with the technicians in charge, aimed at assessing the state of health of the building (even beyond the conventionalisms imposed by the rules, often stringent and not entirely suitable) , to evaluate the needs of the Client itself, to evaluate any limitations of use and / or methods for “prudent” use and, finally, to evaluate any interventions that offer the best benefit-cost ratio, taking into account all the technical, economic and organizational needs that emerged during this process iterative.

Par. 8.3 of the NTC08 prescribes that the safety assessment must make it possible to establish whether: the use of the building can continue without interventions; the use must be modified (downgrading, change of destination and / or imposition of limitations and / or precautions in use); it is necessary to increase or restore the bearing capacity. Furthermore, for existing works it is possible to refer to safety levels different from those of new works and it is also possible to consider only the ultimate limit states (Par. 2.1 of NTC08).

The same NTC08, in par. 8.5, emphasize that <<In existing buildings, the situations that can be concretely encountered are the most diverse and it is therefore impossible to provide specific rules for all cases. Consequently, the model for the safety assessment must be defined and justified by the Designer, case by case, in relation to the reliable structural behavior of the building, taking into account the general indications (…)>>.

In admitting that a safety level lower than that required for new buildings is acceptable in existing buildings, the standard introduces the important and innovative concept of “limitations and / or cautions in use“.

For actions controlled by man, the possibility is therefore admitted, within a given intended use, to diversify the “real conditions of use” by adapting them to the capabilities of the structure.

For natural actions (including seismic action), the current regulatory framework (national and regional) does not require, in general, that existing buildings are adequate to withstand the conventional measure envisaged for the design of new buildings, nor does it set specific thresholds, even if less restrictive.

The continuation of the activities in progress is therefore not subject to compliance with predefined safety levels (seismic and structural in general) by the buildings that host them.

This concept is also expressed and justified in the “Ministerial Circular 2 February 2009, n. 617 – Instructions for the application of the “New technical standards for construction” referred to in Ministerial Decree January 14, 2008 “, of which the relative excerpt is reported:

<<The results of the verifications must allow to establish what measures to take so that the use of the structure can comply with the safety criteria of the NTC. The alternatives can be summarized in the continuation of the current use, in the modification of the intended use or in the adoption of appropriate precautions and, finally, in the need to carry out an intervention to increase or restore the bearing capacity, which may fall into the specific case of the improvement. or adjustment. (…) It is clear that the aforementioned measures are necessary and cannot be postponed in the event that the checks relating to the actions controlled by humans are not met, ie mainly to the permanent loads and other service actions; more complex is the situation that arises when the inadequacy of a work is manifested with respect to environmental actions, which cannot be controlled by man and subject to wide variability over time and uncertainty in their determination. For the related problems, it is not possible to think of imposing the mandatory nature of the intervention or the change of intended use or, even, the decommissioning of the work, as soon as it is found to be inadequate. The decisions to be taken must necessarily be calibrated on individual situations (in relation to the severity of the inadequacy, the consequences, the financial resources and the implications in terms of public safety). It will be the owners or managers of the individual works, whether they are public or private bodies or individual citizens, to define the most suitable measure, possibly identifying one or more levels of shares, commensurate with the remaining nominal life and the class of use, with respect to which it is necessary to carry out the intervention to increase safety within a predetermined time.>>

Possible seismic improvement interventions

In all cases where the client intends to undertake a seismic improvement path of a building following an initial seismic safety check, it is appropriate, according to the writer, to act as follows:

  • evaluate some use scenarios with a class of use lower than the one with which the verifications that allow a reduction of the reference seismic action were carried out;
  • evaluate any change in the intended use and the variable loads associated with it;
  • deepen the investigations on materials and elements in order to improve the level of knowledge of the building and increase the resistance values to be considered in the checks;
  • deepen the analysis of permanent loads with further tests (even partially demolition);
  • evaluate some use scenarios that allow the reduction of seismic masses, possibly avoiding the use of certain areas of the building;
  • update the safety checks, also considering a nominal life of the building of less than 50 years which are normally considered for new buildings (for example 10 or 20 years);
  • determine the intervention time with the formula (TINT CU) / TSLV= – ln (1 – 0,1) = 0.105 indicated by the Technical Scientific Committee of the Emilia Romagna Region in Opinion on technical checks, related issues and consequent decisions. (Ref. Prot. Int. N. 6, Meeting of 27 July 2010, Session n. 4)
  • evaluate the possible reinforcement interventions to improve the seismic behavior of the building, given the absolute voluntary nature of the interventions put in place and given the lack of a minimum safety level to be achieved;
  • of the interventions identified evaluate the implementation planning according to the financial availability of the Client.

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