Basic mechanical tools, such as modal analysis, allow the evaluation of the elastic vibration period of a reinforced concrete structure depending on its mass and initial uncracked stiffness. However, during an earthquake, the progressive damage of structural and nonstructural components yields to the degradation of their stiffness, thus resulting in a progressive increase of the vibration period of the structure as a whole. This is theoretically expected and has been experimentally validated by different authors through dynamic identification tests on monitored structures.

Also, the damage amount on structural and nonstructural elements, also known as Damage State (DS), can be determined by visual inspection of real buildings struck by earthquakes (different scales exist for this aim) or predicted by means of numerical analyses by relating the exceedance of a certain Engineering Demand Parameter (EDP) threshold to the attainment of a certain DS. Different studies exist in the literature validating this kind of approach based on real data. Also, upon an earthquake, the observation of a certain DS in a building is the basis for the decision regarding its usability, potentially after enforcing some fast and non-invasive interventions.

In this work, numerical incremental time-history analyses are performed on case-study reinforced concrete residential buildings. The analyses results are used to establish the expected DS attained by structural and nonstructural components and, so, by the considered building, due to the occurrence of an earthquake characterized by a certain intensity measure: this is also related to a certain usability judgment, as already mentioned. In tune, the analyses results are also used to evaluate the period elongation of the building at increasing intensity of the seismic demand.

The objective of the study is establishing a preliminary relationship between a) the DS attained by a certain building due to the occurrence of an earthquake with a certain intensity, and, so, the consequent usability judgment, with b) its period elongation. This is done in order to establish whether the vibration period elongation – which can be practically evaluated by means of ambient vibration tests – can be used for the assessment of structural usability after an earthquake as a tool for decision supporting in the aftermaths of a destructive event.