PSI - Issue 78

Alessandro Pisapia et al. / Procedia Structural Integrity 78 (2026) 568–575

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Fig. 7. Comparison between the theoretical and numerical model.

4. Threshold values of non-dimensional diaphragm stiffness In this section, a limit value for the non-dimensional stiffness parameter is proposed, based on the previously presented simplified theoretical models and considering the limits imposed by Eurocode 8. Specifically, by assuming a maximum acceptable deviation of 10% with respect to the solution obtained under the assumption of an infinitely rigid diaphragm in its own plane (i.e., the criterion suggested by Eurocode 8), Figs. 2(b) and 4(b) allow the identification of a threshold value for which, for example, for = 0 , results in 0.88 and 1.54 for the cases with 3 and 4 seismic-resisting frames, respectively. These threshold values increase with increasing values of the dimensionless shear deformability. In particular, by adopting a limit value for the dimensionless displacement equal to 1.10 , in line with the general criterion suggested by Eurocode 8, the following threshold values of non-dimensional diaphragm stiffness are obtained: = 3 3 1 6 + 4 3 (10) in the case of 3 seismic-resisting frames; = 5356 + 4326 (11) in the case of 4 seismic-resisting frames. In order to check the assumption of the infinitely rigid floor diaphragm, it is necessary to verify that: ≥ (12) 5. Conclusions This work presented a preliminary study on the in-plane deformability of floor diaphragms subjected to seismic actions, following a theoretical approach. Specifically, two single-story configurations were investigated, each featuring three and four identical seismic-resisting frames along the direction of the seismic action. To assess the accuracy of the theoretical model, a comparison was carried out with results obtained from a finite element model developed in SAP 2000. Then, for the analysed cases, the threshold values of the non-dimensional diaphragm stiffness were proposed in accordance with the deformability limits prescribed by Eurocode 8. Future developments will include the extension of the study to configurations with a greater number of vertical frames, in order to evaluate potential implications for the assumption of infinite in-plane diaphragm stiffness. Subsequently, the analysis will be expanded to regular multi-storey buildings, with particular attention to the first