Prediction of the fundamental period of infilled RC framed structures considering the maximum inter-story drift at different design limit states

The fundamental period of vibration is a key parameter for the spectral approach to seismic design of structures, as it significantly affects the evaluation of the response spectra values. Even though infill elements can significantly increase a building's lateral stiffness, they are not always considered in the structural design process. This is particularly significant at the damage limitation states considered by the European (Eurocode 8) and Italian (NTC2018) seismic codes, where mostly elastic behaviour is expected. Non-structural elements, in fact, can influence the natural elastic period of buildings, determining spectral accelerations different from those evaluated by the simplified formulas provided by the seismic standards and related guidelines. This research aims to develop novel simplified period-height relationships to estimate the fundamental period of vibration of infilled Reinforced Concrete (RC) framed buildings more accurately at various limit states, as a function of inter-story drift value. To collect information about the fundamental period of structures in different damage conditions, several experimental campaigns involving more than 330 RC framed structures have been considered. Nonlinear numerical analyses have also been performed to integrate the database, especially for the Ultimate Limit State (ULS), considering different number of floors, infill distributions, and geometries. Collected data have been parametrized as a function of the damage level suffered by the considered structures, and different empirical period-height formulations have been defined for each limit state. The proposed relationships have been compared with those found in international, European, and national seismic codes, as well as with simplified formulations in previous literature studies. In particular, the results strongly suggested that the fundamental periods estimated according to the EC8 formulation, overestimates the experimental data, resulting in a potential underestimation of the seismic forces and therefore of damage to non-structural elements. To quantify the differences in spectral acceleration and spectral displacement at all design limit states, some examples have been proposed, considering different Italian seismic zones.