The in-plane deformability of diaphragms plays a key role in the seismic response of structures, particularly affecting the distribution of horizontal forces and the overall behaviour of the lateral load-resisting system. This study proposes a new theoretical approach to evaluate diaphragm deformability, based on a simplified yet rigorous mechanical model. The proposed method allows for a direct estimation of the diaphragm stiffness, accounting for the geometric configuration and mechanical properties of its structural components. In particular, two simple cases have been analysed: one regular single-storey building with three seismic resisting frames and another one with four seismic resisting frames. In order to evaluate the accuracy of the model, theoretical predictions have been compared with results obtained from a numerical model developed in SAP 2000 and by means of a simple linear analysis, showing good agreement and confirming the reliability of the approach for preliminary design assessments or rapid evaluation purposes. Finally, for the examined cases, the threshold values of the non-dimensional diaphragm stiffness have been derived according to the deformability limits prescribed by Eurocode 8. The proposed methodology offers valuable insights, especially into the seismic design and assessment of both new and existing buildings.
Pisapia et al. (Thu,) studied this question.