PSI - Issue 78

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

286

1. Foreword In the cognitive process of new and existing reinforced concrete (r.c.) constructions and in the choice of interventions to be envisaged in the design or strengthening phase, it is very important to evaluate the behaviour of the decks, with particular reference to their capacity to distribute seismic actions, operating in their own plane, between the various pillars and the consequent redistribution of stresses on the structural elements. Both § 4.3.1(4) of Eurocode 8 part 3 and the instructions for the application of the Italian technical regulations (§ C7.2.6 of C.S.LL.PP. Circular no. 7/2019) specify that the deck can be assumed to be rigid in its own plane if, when modelled by attributing to it the real stiffness, its horizontal displacements do not exceed, at any point and by more than 10%, the corresponding absolute horizontal displacements in the seismic design situation, as resulting from the assumption of a rigid deck. These indications make it necessary to adopt a calculation model that takes into account the real stiffness of the decks, to be compared with an infinitely rigid deck model. As an alternative, the Italian regulations allow decks to be considered as rigid if they are made " in reinforced concrete, or in latero-concrete with a reinforced concrete slab at least 40 mm thick, or in a mixed structure with a reinforced concrete slab at least 50 mm thick connected to the steel or timber structural elements by suitably sized shear connectors. " Since the vast majority of the decks are in hollow-core concrete, the hypothesis of a rigid floor is only used in the presence of a reinforced slab (40 or 50 mm thick, depending on the type of horizon). In the following paragraphs, with particular reference to existing buildings in which such a slab is very often unreinforced, it is shown when, on the basis of simulations carried out using a simplified calculation model, the The simplified calculation model used for the simulations is intended to estimate the tensional state and deformability of the decks, due to the actions induced by the earthquake on the intermediate floor and roof slabs, by means of a simplified procedure developed on the basis of the assumptions illustrated below. The building considered in the simulations has a rectangular plan, with a reinforced concrete frame structure and pillars with a square cross-section and regular spacing, constant and equal to 5.50 m in both X and Y directions, but with a different number of pillars in the two directions. Fig.1 shows the schematic plan of the building, indicating beams and columns, the latter at the intersections of beams; the parameters m and n set the number of columns in the X and Y directions, respectively, and in the example used for a first set of simulations are m = 8 and n = 4. assumption of an infinitely rigid deck can also be adopted for unreinforced slabs. 2. Illustration of the simplified calculation model used for the simulations

Fig.1 . Plan representation of the arrangement of beams and columns. The mass of the building is assumed to be present, totally and solely, on the floors and uniformly distributed (load p str equal to 10 KN/m 2 ); the stiffness of the columns is evaluated as K= α ∙ E ∙ ℎ 3 (1)