ELASTIC PERIOD OF EXISTING RC-MRF BUILDINGS

The fundamental period structures has a primary role in the seismic design and assessment and design as it is the main feature of the structure allowing to determine, at least, the elastic demand and is the basis to assess the required inelastic performance in static procedures. In fact, the definition of easy to manage relationships for the assessment of the elastic period has been the subject of a significant deal of research of both experimental and numerical/analytical studies, some of which has been acknowledged by codes and guidelines worldwide. Moreover, this kind of information is useful for territorial-scale seismic loss assessment methodologies. In the most of the cases the assessment of the period is considered as function of the structural system classification and number of storeys. Reinforced concrete (RC) buildings constituting the most of the building stock in Italy and in seismic prone areas in Europe, were built after the second world war and are designed with obsolete seismic codes if not for gravity loads only. Therefore, a significant variability of the structural system may affect a class of buildings featuring the same height and/or number of storeys. This, along the contribution of the stair module, may affect the elastic periods in the two main directions of a three-dimensional building. In the study presented these issues are investigated with reference to a population of existing RC structures designed via the practice at the time of supposed construction (e.g., simulated design) and with reference to relative enforced code. Elastic period is evaluated for both main directions of the buildings of the considered sample, and regression analysis is employed to capture the dependency of the elastic dynamic properties of the structures as a function of mass and stiffness.