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

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
the overall displacement and acceleration demands of earthquake ground motion
for structures having different period and damping characteristics, without the need
to perform detailed numerical analyses. A new concept was proposed in 1960 by
Newmark and Hall, by constructing spectra based on accelerations, velocities and
displacements, in short, medium and long period ranges, respectively. This
concept remained a proposal until after the Northridge and Kobe earthquakes,
when the importance of the velocity and displacement spectra was recognized.
More recently, another methodology based on the drift spectrum of a continuous
medium in opposition to the concepts of discrete medium has been elaborated for
structures situated in near-field region of an earthquake (Iwan, 1997). This concept
is based on the observation that the ground motions in near-field regions are
quantitatively different from that commonly used for far-field earthquake regions.
For near-field earthquakes, the use of the equivalence of MDoF systems with only
SDoF gives inaccurate results, because the effects of the higher vibration modes
are ignored. Therefore, a new direction of research works for ductility of structures
in near-field regions began to be explored.
Moreover, the recent technological computer advances permit static and dynamic
analyses in elastic and elasto-platic ranges and allow to obtain more refined
results, using the design spectra in current design, with a more correct calibration
of the design values. At the same time, a time-history methodology can be applied
for important structures using a recorded accelerograms and the behaviour of the
structures under seismic actions can be evaluated in a more precise way,
according to the spectrum methodology.
Recently, this concept has been criticized because large deformations, such as
those necessary for the building components to provide the required ductility, are
associated to strong earthquakes with local buckling, cracking and other damage in
structural and non-structural elements, with a very high cost of repair after each
event. In order to minimize this damage, a new approach in seismic design has
been developed, mainly based on the idea of controlling the response of the
structure, by reducing the dynamic interaction between the ground motion and the
structure itself. This concept is very different from the conventional one, according
to which the structure is unable to behave successfully when subjected to load
conditions different fro the ones it has been designed for. The control of the
structural response produced by earthquakes can be obtained by various means,
such as modifying rigidities, masses, damping and providing passive or
counteractive forces (Housner et al, 1997). This control is based on two different
approaches, either the modification of the dynamic characteristic or the
modification of the energy absorption capacity of the structure. In the first case, the
structural period is shifted away from the predominant periods of the seismic input,
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