Which design parameters should you choose to define the seismic action?


Emma Potter

Nominal life

The NTC2018 technical standards for construction introduce a first performance parameter: the nominal life VNo«defined as the number of years in which the work is expected to maintain specific performance levels, provided it is subject to the necessary maintenance”.

It is thetime framechosen by the designer, during which the structures require only ordinary maintenance, at the end of which it is however necessary to carry out physiological extraordinary maintenance and a new analytical check of structural safety. This parameter is related to time-dependent phenomena, such as degradation, fatigue, damage, and related durability. Therefore, the time expiration of the nominal life must represent the opportunity for a new verification of the structureaccompanied by tests on materials, to evaluate the overall safety level of the building after the trials it has undergone over time

The NTC2018 identifies in tab. 2.4.I Three minimum reference values ​​of nominal life, which can be increased by the designer with suitable justification. For residential construction or with low crowding, traditionally the reference term of VNo I am at least 50 years. For infrastructures or in any case in cases where the client and the designer deem it necessary to extend the durability times, the value equal to 100 yearsproviding the construction with greater resistance to seismic actions and related damage.

The use class

A second important factor is the use class defined as follows in relation to the consequences of interruption of rescue operations and post-seismic emergency management:

  • class I: buildings with only occasional presence of people, agricultural buildings;
  • class II: buildings whose use involves normal crowding, without contents dangerous for the environment and without essential public and social functions. Industries with activities that are not dangerous for the environment. Bridges, infrastructural works, road networks not falling into Use Class III or Use Class IV, railway networks whose interruption does not cause emergency situations. Dams whose collapse does not cause significant consequences;
  • class III: buildings whose use involves significant crowding. Industries with activities dangerous for the environment. Extra-urban road networks not falling into Use Class IV. Bridges and railway networks whose interruption causes emergency situations. Dams relevant to the consequences of their possible collapse;
  • class IV: buildings with important public or strategic functions, also with reference to the management of civil protection in the event of disasters. Industries with particularly dangerous activities for the environment. Road networks, bridges and railway networks of critical importance for the maintenance of communication routes, particularly after a seismic event. Dams connected to the operation of aqueducts and electricity production plants.

The correct classification of the most suitable class of use relating to each case study can be deduced inList A of theAttachment 1 of the Decree of the Presidency of the Council of Ministers 21/10/2003, and more specifically in the decrees that each Region has legislated regarding more detailed lists for class III (relevant) and IV (strategic) buildings. Each use class is associated with a relative coefficient CU (tab. 2.4.I).

Which design parameters should you choose to define the seismic action?  Image 2 1

The reference period

The product of the nominal life and the use class determines the reference period for seismic action VR:

VR =VNo · CU

Since this is a semi-probabilistic assessment, seismic safety is considered within a time frame, i.e. the reference period VRin which a certain falls chance (10% in the case of a life-saving limit state) that occurs at least one seismic event of intensity equal to or greater than the expected ground acceleration for the site on which the structure falls.

Represents the closest time limit to planning to guarantee a sufficient level of seismic safety for the intervention to improve or reduce seismic risk, after which the safety of the structure must necessarily be re-evaluated after the effects of time and any damage (including seismic, if earthquakes have occurred). For a residential building in use class II and nominal life of 50 years, the reference period coincides with the nominal life. For a school building with the same nominal life of 50 years and use class III, the reference period will be 75 years.

The greater the value of the nominal life or use class chosen by the designer, the greater the reference period and the related seismic accelerations that the designed structure will be able to withstand.