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

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
As a consequence of the above-mentioned differences in ground motions, there
are some very important modifications in the design concepts:
• in near-field regions, due to the very short periods of ground motions and
to pulse characteristic of loads, the importance of higher vibration mode
increases, in comparison with the case of far-field regions, where the first
fundamental mode is dominant. For structures subjected to pulse actions,
the impact propagates through the structure like a wave, causing large
localized deformation and/or important interstorey drifts. In this situation
the classic design methodology based on the response of a single-degree
of freedom system characterized by the design spectrum is not sufficient to
describe the actual behaviour of structures. Aiming at solving this problem,
a continuous shear-beam model is proposed and the design spectrum is
the result of the shear strain, produced by the inter-storey drift. (Iwan,
1997).
• due to the concordance of the frequencies of vertical ground motions with
the vertical frequencies of structures, an important amplification of vertical
effects may occur. At the same time, taking into account the reduced
possibilities of plastic deformation and damping under vertical
displacements, the vertical behaviour can be of first importance for
structures in near-field regions. The combination of vertical and horizontal
components produces an increase in axial forces in columns and, as a
consequence, increases in second order effects.
• due to the pulse characteristic of the actions, developed with great velocity
due to the lack of important restoring forces, the ductility demand may be
very high. So, the use of inelastic properties of structures for seismic
energy dissipation must be very carefully examined. At the same time, the
short duration of the ground motions in near-field regions is a favourable
factor. A balance between the severity of ductility demand, due to pulse
action, and the effect of short duration must be analyzed.
• due to the great velocity of the seismic actions, an increase in yield
strength occurs, which means a significant decrease of the available
ductility. The demand-response balance can be broken due to this
increasing demand, as an effect of the impulse characteristic of loads,
together with the decreasing of the response, owing to the effect of high
velocity. The need to determine ductility as a function of the velocity of
actions is a pressing challenge for research works.
• if it is impossible to take advantage of the plastic behaviour of structures, at
this high velocity, it is necessary to consider the variation of energy
dissipation through ductile fracture. The fact that many steel structures
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