Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 361

Plastic hinges must be organized at the base of ductile walls or cores, which
implies that they are connected to the foundations without the soil being able to lift,
otherwise, the hinges would not be able to form. As a consequence, the foundations,
whether deep or shallow, should be designed to oppose any lift below the critical
area planned at the foot of the wall, and to transmit the moments of tilt, considering
a possible transfer of vertical loads through longitudinal girders. In the case of pile
foundations, this implies traction within piers.
To ensure correct operation with regard to the assumptions chosen, it is better to
use walls that are regular. Sudden section reductions (thickness as well as length) on
the upper floors can cause the formation of additional plastic hinges, and an overall
less controlled behavior.
As walls or ductile cores mostly work in bending, critical areas are subject to
rotation ductility pulls related to the required behavior coefficient, in a predominant
bending situation and with restricted normal force. Thus, the behavior coefficient
can reach quite high values. Should the normal force influence the behavior of the
plastic hinge more markedly, the available ductility would be more restricted and the
behavior coefficient should logically be limited. Besides, in that kind of wall, the
reduced normal force is restricted to 0.4. Furthermore, the failure mode depends on
the influence of the transverse force and, in practice, of the wall slenderness. This is
the reason why the behavior coefficient is bearing a k w conversion factor ranging
from 0.5 to 1 depending on the slenderness.
For this type of walls, Eurocode 8 has chosen two ductile levels, as for the
frames. In order to fulfill the ductile demand, construction layouts allowing the
confinement of the critical area concrete are recommended, and capacity design
applies. Consequently, the main column construction layouts referred to above are
found in the walls. Moreover, flanges can be placed at both ends to improve the
behavior of the compressed area.
Capacity design is needed to assess safety issues relating to failure because of
shear and the potential additional acceleration caused by the hinges being designed
with over-resistance. As a consequence, it is recommended that the bending moment
diagram of a building should be displaced upwards to take into account the effect of
the shear force on the failure mode at the base. This shifting also allows an
acceleration rise above the plastic hinge to be taken into account. This compensates
for uncertainties in the distribution of bending moments and shear forces , and
ensures that areas located above the critical zone stay within the elastic region,
which should guarantee restoration after insult. It has been proved that shear load in
a wall is not reduced (with regard to its elastic behavior); therefore, designing so that
the shear load is increased by 30% within “M” ductile walls and by a higher
multiplying coefficient in “H” ductile walls assures compliance with Eurocode 2