Analysis and modelling of the seismic behaviour of high ... - Ingegneria

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
A very important value in seismic design is the ductility limit. This limit is not
necessarily the largest possible energy dissipation, but a significant changing of
structural behaviour must be expected at ductilities larger than this limit. Two
ductility limit types can be defined:
• available ductility, resulting from the behaviour of structures and taking into
account its information, material properties, cross-section type,
gravitational loads, degradation in stiffness and strength due to plastic
excursions, etc;
• required ductility, resulting from earthquake actions, in which all factors
influencing these action are considered: magnitude, ground motion type,
soil influence, natural period of the structure versus ground motion period,
number of important cycles, etc.
Ductility of a structure is provided by satisfying the limit state criterion:
D
γ
a
m
≥ γ D
( 2.1 )
F r
where Da is the available ductility determined from the local plastic deformation and
Dr, is the required ductility obtained from the global plastic behaviour of a structure.
The partial safety factors γm for available ductility and γF for required ductility must
be determined considering the scatter of data with a mean plus one standard
variation and the uncertainties in available and required capacities. Values of
γm=1.3 and γF=1.2 are proposed (Gioncu, 2000) for this check, if the ductility is
obtained by deformation ductility (see Figure 2.2). If the available ductility results
from fracture, a greater value for γF must be used (i.e. γF =1.5). This check must be
included in a performance based seismic design.
Figure 2.2. Deformation and fracture ductilities
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