Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 358

9.6.3.3. Ductility classes and local ductility
In addition to the “L” ductility class (Eurocode 2), two others, “M” and “H”, can
be used (one class per structure). They correspond to two different ductility demand
levels involving plastic hinge rotation capacities, with “H” class materials being
higher.

To ensure such rotation capacities, a ductility coefficient should have a minimum
value in all the sections concerned, determined as shown in section 9.5.3. In
practice, the minimum value is obtained by incorporating sufficient transverse
reinforcements to confine the concrete within the core they encompass. The concrete
inside is able to stand shrinkages higher than those tolerated by non-confined
concrete without any damage.

For critical areas in columns, the following relationship between both quantities
is proposed in Eurocode 8:

b

c

DZ t 30 P H v 0.035

[9.5]

wd I d sy d ,

b

0

Here, Z wd refers to the mechanical percentage of confinement reinforcements, D
expresses confinement effectiveness, Q d is the reduced normal load, H sy,d is the strain
elastic limit of the reinforcements, b c the total width of the element and b 0 is the
width of the confined concrete.

This expression shows that it is not necessary to confine the concrete if the
required strain is lower than or equal to 0.35%.

9.6.3.4. Beam/column nodes
Nodes are special strain localization areas consisting of limited dimension
volumes that demand a specific analysis of stress transfer. Beam-column nodes, for
example, transmit the loads that are concentrated within compressed areas of
concrete sections and within traction reinforcements. Such loads are often redirected
inside nodes to allow moment inversion. The nodes have to be designed to withstand
shear loads, which they are subjected to in three directions. Here again, good design
involves placing transverse reinforcements in the right position in sufficient
quantities. Compression within diagonal bracing struts should be limited and bar
anchoring within nodes plays a prominent part.

Edge nodes and frame intermediate nodes have quite different behaviors, as do
opening and closing nodes.