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

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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 23
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES 24 were damaged by fracture of connections during the Northridge and Kobe earthquakes without global collapse, gives rise to the idea that the local fracture of these connections can transform the original rigid structure into a structure with semi-rigid joints. The positive result of the weakening is the reduction of the seismic actions at a level, which can be supported by the damaged structure, taking into account that the duration of an earthquake is very short. This is not the case of far-field earthquakes, for which the effect of long duration can induce the collapse of the structure. Required ductility on the other hand is associated with the global behaviour of the structure, which is a function of members of plastic hinges as well as of the amount of plastic rotation they undergo. For the plastic analysis of a moment resisting frame, the methods available to the designer are either monotonic static non-linear analyses, i.e. pushover type, or dynamic time-history analyses. Of course the last ones are more effective, but they require special computer programs, which are not available to all design offices. At the same time, time-history methods are large computation time consumer and they are very expensive. The pushover methods, if the conditions of load and local behaviour are properly designed, may provide sufficient information on the expected behaviour for design purposes. 2.4.1 Non-linear static pushover (NSP) analysis The purpose of the pushover analysis is to evaluate the expected performance of a structural system by estimating its strength and deformation demands in design earthquakes by means of a static inelastic analysis, and comparing these demands to available capacities at the performance levels of interest (Krawinkler and Seneviratna, 1998). The evaluation is based on an assessment of important performance parameters, including global drift, interstorey drift, inelastic element deformations (either absolute or normalized with respect to the yield value), deformations between elements, and element and connection forces (for elements and connections that cannot sustain inelastic deformations). The inelastic pushover analysis can be viewed as a method for predicting seismic force and deformation demands, which takes into account in an approximate manner the redistribution of internal forces occurring when the structure is subjected to inertia forces that can no longer be resisted within the elastic range of the structural behaviour. The pushover analysis is expected to provide information on many response characteristics that cannot be obtained from an elastic static or dynamic analysis. The following are examples of such response characteristics: • the realistic force demands on potentially brittle elements, such as axial force demands on column, force demands on brace connections, moment
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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