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