Diaphragmatic behaviour at the storey level

In building structures the floors act as horizontal diaphragms that collect and transmit the Clause 4.2.1.5(1) inertia forces to the vertical structural systems and ensure that those systems act together in resisting the horizontal seismic action.

The action of these diaphragms is especially relevant to complex and non-uniform layouts of the vertical structural systems because, in these cases, as indicated above, the inertia forces created in the distributed masses of the building have to be transmitted along more complex and longer paths within these diaphragms.

Diaphragmatic action at the floor levels is also important where systems with different horizontal deformability characteristics are used together (e.g. in dual or mixed systems), because in those situations the interaction between these different structural systems varies along the height of the building, and compatibility between them is ensured by the diaphragmatic action of the floors.

Accordingly, floor systems (and the roof) should be considered as part of the overall Clause 4.2.1.5(2) structural system of the building, and provided with appropriate in-plane stiffness and resistance as well as with effective connection to the vertical structural elements.

Particular care should be taken in cases of non-compact or very elongated in-plan shapes and in cases of large floor openings, especially if the latter are located in the vicinity of the main vertical structural elements, as these elements attract large forces which have to be transmitted effectively by the floor elements connected to those vertical elements.

Clause 4.2.1.5(3) Diaphragms should have appropriate in-plane stiffness for the distribution of horizontal inertia forces to the vertical structural systems, and, in many cases, at the conceptual design phase the choice of a rigid diaphragm approach is appropriate because it distributes the deformation in the vertical elements more uniformly. Furthermore, a building structure with rigid diaphragms allows for simplifying assumptions for its modelling and analysis (see clause 4.3 of EN 1998-1).

The validity of the assumption of a rigid floor diaphragm depends on whether its deformation is or is not negligible in comparison with the deformation of the vertical elements. A note to clause 4.3.1(4) of EN 1998-1 indicates, as a general rule, that this assumption maybe made if the horizontal displacements in the floor plane are not changed by more than 10% by the deformation of the floor itself. If this not the case, the flexibility of the floor diaphragm should be accounted for in the modelling of the structure.

Besides stiffness, resistance of the floor diaphragm and its connections should also be checked, either implicitly or explicitly. This matter is dealt with in general terms in clause 4.4.2.5 of EN 1998-1, and more specific provisions for reinforced concrete and timber diaphragms are presented in clauses 5.10 and 8.5.3, respectively.

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