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

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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. 35
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES 36 Figure 2.13. Joint collapse type after Gioncu and Mazzolani (2002) Due to the relevant number of influencing parameters, a macroscopic view of the joint obtained by subdividing it into individual basic components has proved to be most appropriate. This approach, which allows to determine the local strength and rotation capacity, is known as the component method (Eurocode 3, 2001) The assumption considered in this method allows us to determine the overall rotation as the sum of the all components and the node ductility by the ductility of the weaker component (see Figure 2.14). Figure 2.14. The component methodology
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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