Elastic period of sub-standard reinforced concrete moment resisting frame buildings

The fundamental period has a primary role in seismic design and assessment as it is the main feature of the structure that allows one to determine the elastic demand and, indirectly, 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 both experimental and numerical/analytical studies, some of which have been acknowledged by codes and guidelines worldwide. Moreover, this kind of information is useful for territorial-scale seismic loss assessment methodologies. In the majority of cases, the assessment of the period is considered as function of the structural system classification and number of storeys or height. Reinforced concrete structures, comprising most of the building stock in Italy and in seismic prone areas in Europe and in the Mediterranean region, were built after the Second World War and are designed with obsolete seismic codes, if not for gravity loads only. Therefore, a class of buildings featuring the same height and/or number of storeys may show a significant variability of the structural system. This, along with 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 acknowledging the practice at the time of supposed construction (e.g., simulated design) and with reference to the relative enforced code. The 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.