1. Introduction#
To perform a calculation on a reinforced concrete structure composed of walls, slabs, foundations, we can adopt, depending on the challenge and the targeted calculation performance, a:
modeling with volume finite elements in the field occupied by concrete (formwork geometry), with steels also modelled in volume finite elements: this choice should be reserved for local analyses;
modeling with finite volume elements in the field occupied by concrete (geometry of the formworks), with the steels of the reinforcing beds modelled by membrane grids, placed on the middle plane of the reinforcing beds. This situation is for example one of high floors or thick floors;
modeling with finite elements of so-called « generalized » plates, which will support a law of behavior expressed directly in generalized forces (flexure and membrane), representing the behavior of homogenized reinforced concrete in the section. This situation is for example that of structural assemblies based on walls and floors. This modeling uses the geometry of the mean planes of walls, slabs, floors…
It is also necessary to model the reinforced concrete junctions connecting a wall and a floor or a raft. These junctions have dimensions identical to or close to that of the thickness of the connected structural elements: they are therefore essentially three-dimensional in the geometric part of the connection (the « heart »). We can distinguish junctions between 2, 3 or 4 structural elements, which are cross-shaped, T-shaped, or L-shaped.
Various codified construction provisions govern the reinforcement of these junctions. Depending on the design choices, it is possible to consider a good continuity of transmission of the bending moments between the connected structural elements when there is continuity of the reinforcements at the crossing. On the contrary, if they are discontinuous, the possible cracking of the concrete changes the behavior of the connection: a perfect fit is not always representative.
The choices made for concreting also influence: we can have the case of a concrete floor at the same time as the wall, or on the contrary the case of a floor with prefabricated prefabricated slab and receiving an additional compression slab by concreting. The construction phase with the presence of shoring under the floor slabs also contributes to the stress on the junction (dead weight of the concrete and additional loads during the pouring of the floor).
Often the veil extending above the floor the lower lift veil is not sunk and the reinforcements are waiting.
Figure 1-a : Cross-sectional view of a reinforced concrete junction connecting a wall and a floor and its reinforcement: left: solution with brackets; right: solution with brackets; right: U-solution
With the reinforcement solution on the left, the bracket ensures a good fit even in a situation of concrete cracking; with the reinforcement solution on the right, the U allows many more differential rotations of the floor compared to the wall after cracking the junction.
The kinematics imposed on the boundaries of the junction core by the connected structural elements is nevertheless (in the first order) controlled by the kinematics adapted to the current zones of these structural elements of walls, slabs, foundations… The kinematics of the core of the junction itself can be represented with sufficient precision towards simple deformations in the transverse plane.
Figure 1-b : Sectional view of a reinforced concrete junction connecting a veil and a floor: kinematic the heart of the junction.
In this, sail-floor junctions behave differently from beam-column joints, which often show cross-shaped cracks within the core.