Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 351

The equivalent linear analysis is mostly based on considerations of ductility and,
as a rule, does not apply in the case of marked geometrical non-linearities, like cases
of uplift between foundation and the soil.
9.5.2. Structure behavior and behavior coefficients
A structure is dissipative if it dissipates energy during hysteresis cycles via the
ductility of its constituent materials.
A structure can be designed as non-dissipative if its behavior remains quite close
to linear elasticity: in such cases, a behavior coefficient equal to 1 would be
assigned to it, and the elastic spectrum could be used. However, when the structures
are designed according to the 2 to 6 structure Eurocode regulations, they retain a
little ductility (limited so-called “L” ductility) and over-strength linked to
hyperstaticity . Therefore, for all structural materials, a behavior coefficient ranging
from 1 to 1.5 (2 for metallic structures and mixed structures) is used, with a few
additional considerations as far as materials are concerned, but without any specific
construction layouts.
When the structure has been designed as dissipative, a behavior coefficient
higher than 1.5 (or 2) can be used, as long as construction layouts allowing the
structure to reach the expected ductility are used as well.
To determine the behavior coefficient of a given structure, the constituting
material of the structure, the bracing type, the ductility level considered and its
regularity must be taken into account. In Eurocode 8, typical data for a building that
is irregular in height is given in Table 9.1, from which it can be seen that the basic
value of the behavior coefficient is equal to 0.8.
As far as reinforced concrete frameworks are concerned, energy dissipation is
directly linked to the number of plastic hinges that have to be formed. Therefore,
hyperstaticity is taken into account by multiplying the reference behavior coefficient
by the D u / D e ratio. Using static calculations, we assume the structure is subject to a
seismic action system obtained by the analysis described in section 9.4.3. The
resulting actions are proportionally increased by an D coefficient. The first plastic
hinge will appear at the D e value (elastic limit), and the structure will become a
mechanism for the D u . Thus, for a reference value of 4.5 and a reinforced concrete
frame, the behavior coefficient can reach a maximum close to 7, because the D u / D e
ratio will be restricted to a value of 1.5. It is possible to avoid calculating the D u / D e
ratio by accepting a fixed value equal to 1.2, in the case of one-bay frames for
example. A similar approach is acceptable for high-ductility walls, for which the
reference value will be equal to 4.