Why do a security assessment?
So long as «the safety assessment, supported by a specific report, must allow you to determine whether:
- the use of the construction can continue without intervention;
- the use must be modified (downgrading, change of destination and/or imposition of limitations and/or precautions in use);
- is necessary increase structural safetythrough interventions” the importance it represents in the decision-making phase of the project is evident “aimed at determining the extent of the actions that the structure is able to support with the minimum level of safety required by this regulation.” (par. 8.3).
In particular, attention must be paid because the boundary between the obligation of only a specific or global evaluation it may not be so clear-cut in relation to a series of critical situations which, although involving a sum of specific elements, can have a notable influence on the structural response of the entire building.
It will be up to the designer evaluate whether the verification «safety can also be carried out only on the elements involved and on those interacting with them, keeping in mind their function in the structural complex, provided that the changed local conditions do not substantially affect the global behavior of the structure.»
When to do the security assessment?
The par. 8.3 of the NTC2018 requires that «the safety assessment must be carried out when necessary only one of the following situations:
- evident reduction in resistance and/or deformation capacity of the structure or some of its parts due to: significant degradation and decay of the mechanical characteristics of the materials (Photo 1)significant deformations also resulting from foundation problems (Photo 2); damage produced by environmental actions (earthquake, wind, snow and temperature) (Photo 3), from exceptional actions (impacts, fires, explosions) (Photo 4) or from anomalous operating and use situations. Faced with widespread structural criticality that could compromise the use for which the building is intended, a general assessment is necessary. To give a simple example, a crack pattern affecting half the structure will certainly have a negative influence, not only on the static safety of the part affected by the failure, but also on the response of the building in terms of dynamic resistance under seismic stress;
- try it serious errors of design or construction;
- change of intended use of the construction or parts of it, with significant variation in variable loads and/or transition to a higher use class;
- execution of interventions that are not overtly structuralif they interact, even only partially, with elements having a structural function and, consistently, reduce their capacity and/or modify their stiffness;
- every time the structural interventions referred to in § 8.4; (regards the seismic improvement interventions referred to in par. 8.4.2, or seismic adaptation referred to in par. 8.4.3)
- works carried out in the absence or non-compliance with the housing titlewhere necessary at the time of construction, or in non-compliance with the technical standards for construction in force at the time of construction.»
There verification of foundations is mandatory only if one of the following conditions exists:
- «in construction major disruptions are present attributable to foundation failures or disasters of the same nature have occurred in the past; (Photo 2)
- they are possible phenomena of tipping and/or sliding of the construction as a result of: unfavorable morphological conditions, modifications made to the profile of the ground near the foundations, seismic actions of the project;
- they are possible liquefaction phenomena of the foundation soil due to the seismic actions of the project.»
Seismic and static actions
The safety rating will be expressed as relationship between capacity and demand.
For them seismic actionswill be expressed by the ratio between the expected maximum ground acceleration that can be tolerated by the structure and that which would be used in the design of a new construction, without prejudice to the fact that the level of seismic safety may not coincide with that of a new construction according to the level of improvement required for the type of intervention, and that any limitations on the use of the building pending the interventions must be established on a number of factors, such as the severity of the inadequacy and the consequences on public safety.
For them static actionsthe checks will be based on the ratio between the maximum value of the variable vertical overload that can be tolerated by that part of the construction and the value of the variable vertical overload that would be used in the design of a new construction, requiring if necessary the prohibition or reduction of use (or overloads), even partial, in the event that the checks relating to human-controlled actions are not satisfied, i.e. mainly permanent loads and other service actions.
