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

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
are that panel zones are designed to be the weakest element of the node and
inelastic deformations are expected to occur in panel zones.
The ductility of members is another reason for dispute between the code provisions
and researchers, concerning the use of ductility determined at the level of cross-
section (as in Eurocode 3, 2000) or the necessity to use the ductility of members
as proposed by Gioncu and Mazzolani (2002). Code provisions are particularly
qualitative and therefore this procedure is inadequate for a methodology in which
the available ductility is compared with the required one. The basic requirement for
plastic analysis is that large rotations (theoretical infinite) be possible without
significant changes in the resistant moment. But these theoretical large plastic
rotations may not be achieved because some secondary effects occur. Flexural-
torsional instability, local buckling or brittle fracture of members usually imposes
the limitation to plastic rotation. A proper available ductility must be determined
taking into account that the members and joints belong to a structure with a
complex behaviour. But this is a very difficult task owing to the great number of
factors influencing the behaviour of actual members and joints. The problem of
evaluating the rotation capacity has recently been of primary interest, as witnessed
by the numerous published papers, presenting different methods which can be
classified as theoretical methods, based on the use of FEM, or integrating the
moment-curvature relationship; approximate methods, based on the use of the
collapse plastic mechanism; and empirical methods, based on statistical analysis
of experimental tests.
Joint ductility depends on the importance of all component behaviours. For welded
joints ductility is given by the plastic shear deformation, by crushing of web of joint
panel or weld fracture, while for bolted joints ductility results from plastic
deformations up to fracture of the column flanges, connection elements, i.e. end
plates, or by fracture of bolts or welds as summarized in Figure 2.13.
Recent great seismic events have shown that the concentration of inelastic
phenomena into joints leads to a brittle fracture of welds. Therefore, in the last
period great efforts were devoted to the definition of adequate different detailing of
joints able to provide a more satisfactory behaviour. New types of joint have been
proposed, based on the idea of moving the plastic hinge away from the column-
beam interface, in the field where the welding or bolts do not govern the node
behaviour. This solution can be obtained by weakening the specific beam near to
the connection by trimming the beam flanges, i.e. the dog–bone solution proposed
by Plumier (1994) or by strengthening the specific beam near to the connection by
adding vertical ribs or cover plates. The weakening of the beam offers the
possibility of reducing the dimensions of columns, while strengthening requires an
increase of these dimensions, showing the superiority of the dog–bone solution.
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