Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 359

In order to analyze such nodes, two models have been proposed:
- a model of concrete confined by transverse reinforcements;
- a diagonal bracing strut model.
Eurocode 2 verifications adopt the same propositions as those put forward by the
International Federation of Concrete ([COL 91] and [COL 99]). These combine
Eurocode 1 and 2 approaches according to the application field.
The shear loads used are those that derive from the capacity design method,
which takes the actual beam strengths into account.
9.6.4. Reinforced concrete bracing walls
9.6.4.1 . Two opposing approaches
Two concepts are used to cover the design of reinforced concrete walls. They are
based on two different methods for counterbalancing the energy injected into the
structure by an earthquake. When a structure moves, the energies playing a part
include: the kinetic energy (at its minimum when the strain of the structure is
maximum), the strain energy (elastic energy and energy dissipated as heat) and,
whenever possible, the potential energy of a structure's dead weight.
With the first design concept, the walls are organized so as to dissipate some
energy. They are considered as isolated vertical beams liable to develop ductility
similar to that of a beam or a column inside a frame. As the maximum moment is
generally at the base of the wall, a plastic hinge is placed in the area to facilitate
energy dissipation. The confinement conditions of concrete needed to ensure
rotation of the plastic hinge are thus set up, similar to a hinge at the base of a
column. Above the critical area, as the bending moment decreases very quickly, the
rest of the wall will remain in a quasi-elastic condition. Obviously, this design
method can only apply if a plastic hinge can be formed at the base of the wall, which
is not the case if the support becomes detached from its foundation, for instance.
Other factors apart from ductility influence the dynamic behavior of a reinforced
concrete wall. These include:
- geometrical non-linearities (such as the sole of the wall lifting above the soil
due to the overall moment of tilt, or a crack opening within reinforced concrete
sections that causes mass lifting) bring into play the potential energy of the weight
involved. As this energy is maximum when the strain of the wall is maximum, part
of the energy injected into the structure by the earthquake is stored. This potential