PSI - Issue 78

Franco Braga et al. / Procedia Structural Integrity 78 (2026) 285–292

292

Since the values of ∆ d are, for all the configurations investigated, always less than 10%, it is possible to use the hypothesis of a rigid floor (remember that the slabs are 4 cm and are not reinforced), as permitted by § 4.3.1(4) of Eurocode 8 part 3 and § C7.2.6 of the C.S.LL.PP. Circular No. 7/2019; in the case of models developed on the basis of linear elastic bonds, it is therefore possible to make use of the hypothesis of rigid floors in all the configurations investigated, regardless of the value of the PGA. In addition, compared to a limit of 10% set as a rule for ∆ d, in the case of a 4 cm slab, the value of 5% was never exceeded for ∆ d; this observation, combined with the fact that the model envisages a stiffness of the deck inversely proportional to the thickness of the slab, prompted the situation to be examined also for less rigid slabs, i.e. with a thickness of less than 4 cm. The last two columns of Tab.1 (columns highlighted in yellow) show the values of ∆ d in the case of a slab thickness of 3 cm; also in this case the values of ∆ d are all ≤ 10%. Finally, it should be noted that buildings with smaller floor plans (such as buildings with a 4 x 3 mesh) and a lower number of storeys have low tensile/shear stresses in the floor slab and higher values of ∆ d, a condition that is reversed for buildings with more generous dimensions such as, for example, 8-storey buildings with a 20 x 5 mesh. 4. Conclusions In the preceding paragraphs, a simplified numerical model has been illustrated which has made it possible to relate the stresses and deformations of the decks to the magnitude of the seismic action by considering different geometric configurations in terms of plan dimensions and number of storeys; furthermore, from a broad spectrum of situations recurring in professional practice, two limit configurations have been enucleated, one with and the other without a stiffening core. The problem addressed is very important in the evaluation of the vulnerability of existing buildings, since most of the existing reinforced concrete buildings have decks with slabs without reinforcement; therefore it is not possible to use the hypothesis of an infinitely rigid deck in its own plane without first making specific evaluations, but the evaluation of the deformability of the slabs in its own plane using specific models presents modelling and calculation burdens and complexities similar to those that must be faced in order to elaborate the complete numerical model of the entire building. The proposed calculation model makes use of strongly cautious calculation assumptions, such as the maximum off centre position of the stiffening core and the reacting section of the floor slab biased at 50%; Nevertheless, the difference in percentage (the ∆ d) between the displacements calculated considering the real stiffness of the decks in their own plane and the displacements resulting from the hypothesis of an infinitely rigid slab in its own plane are less than 10% of the absolute displacements, a condition that allows the calculation hypothesis of rigid members in their own plane to be used even in the case of unreinforced slabs, provided that there is a suitably reinforced perimeter beam. In the light of the above, the simplified procedure foreseen by the Italian regulations, which imposes a minimum thickness of the slab and the presence of reinforcement, therefore appears to be too restrictive. Similarly, the conditions imposed by the European regulations appear to be difficult to apply in professional practice, which, regardless of the complexity of the construction, foresees a single (analytical) procedure and does not contemplate any simplified procedure for the easy taking into account of the stiffness of the decks. References Ministry of Infrastructure and Transport, 2018. Technical standards for construction approved by Ministerial Decree 17 January 2018, Official Gazette of the Italian Republic. Ministry of Infrastructure and Transport, 2019. Application Circular of the new technical standards for construction approved by Ministerial Decree 17 January 2018, Official Gazette of the Italian Republic. European Committee for Standardisation (CEN), 2004. Eurocode 8: Design of structures for earthquake resistance. British Standard.