Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 352

9.5.3. Local ductility and behavior coefficients

For frame-type structures, if the elements have a homogenous strength, plastic
analysis reveals that plastic hinges form at the ends of each column or beam
element, providing brittle failure modes have not appeared before their formation.
Capacity dimensioning suggests the measures that have to be taken to obtain
behavior in accordance with the theoretical formation diagram of plastic hinges. The
dissipating capacity of the structure is linked to the hinge's ability to withstand
rotational strain in the plastic field without a reduction in strength. This ability is
achieved using assiduous construction layouts and/or section selection. For higher
behavior coefficients, layouts are restricted. To correctly control plastic strains, it is
better if connections between elements do not plasticize. For this reason, they are
generally over-dimensioned by applying capacity design regulations, so that the first
plastic hinges form near the connections instead of forming within the connections
themselves.

The dissipative capacity of a structure is thus mainly determined by capacity
design, careful choice of sections and construction layouts. Structures are then
classified into ductility classes , each class being characterized by the more or less
strict application of these rules. The value of a structure's behavior coefficient is a
function of its ductility class.

For ductile structures, a relationship exists between the ductility available in the
plastic hinges (expressed by a conventional rotation ductility coefficient P I ,), and the
behavior coefficient. Eurocode 8 proposes the following relationship for reinforced
concrete structures:

P 2 q 1 if T t T

°

I 1 C

[9.4]

§ T ·

®

C

P 12 q ) if T T

¨ ¸

°

I 1 C

T

© ¹

¯

1

which expresses the fact that stiff structures are less ductile. That relationship is
operational, as P I is connected to the layouts taken to confine concrete within the
plastic hinges.

9.5.4. Ductility classes and behavior coefficients

Whereas the PS 92 regulations proposed only one level of ductility to designers
(corresponding to a “medium” ductility), in Eurocode 8, three classes are suggested.
These are: