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

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7 2 DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES 2.1 Introduction Engineers have recognized the need to take into account the plastic design in the design of framed structures subjected to seismic actions. Regarding the seismic design, the interest is focused on dissipation of input seismic energy. The basic design parameter in this approach is ductility, considered as the ability of the structure to undergo serious plastic deformations without losing strength. In design practice it is generally accepted that steel is an excellent material for this purpose objectives due to performance in terms of ductility. But in the last decades specialists have recognized that the so-called good ductility of steel structures may be, in some particular conditions, only a dogma, which disagrees with reality. In fact, the recent earthquakes of Mexico City (1985), Loma Prieta (1989), Northridge (1994), and Kobe (1995) have seriously compromised this idyllic image of steel as a perfect material for seismic areas. In some cases the performance of steel joints and members was very bad and large damage was produced, showing that the present design concepts are not sufficient in special conditions. Aiming at making up for this lack, modern codes for building in seismic areas allow to design high-ductility structures and to provide designers with some constructional rules considering whose fulfilment is suppose to assure a good ductility. But the above mentioned bad behaviour of steel structures has shown that this conception is not proper and the verification of structure ductility must be quantified at the same level as the strength and stiffness. In fact, in the conventional practice for non-seismic loads, structures are designed only according to demands of strength and rigidity, which correspond to a good structural performance. The strength checking, including stability, is related to the ultimate limit state, assuring that the force level developed in the structure remain
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES in the elastic range, or some limited plastic deformations can occur in agreement with the design assumptions. The rigidity check is generally related to the serviceability limit state, for which the structure displacements must remain within certain limits, which assure that no damage occurs in non-structural elements. Although it is recognized that damage is also due to deformations, strength checking plays the leading role in designs for conventional loads. Conversely, in case of earthquake design, a new demand must be added to the two previous ones, that is the ductility demand. The performance of a structure under strong seismic actions relies on its capacity to deform beyond the elastic range, and to dissipate seismic energy through plastic deformations. So, the ductility check controls whether the structure is able to dissipate the given quantity of seismic energy considered in structural analysis or not (Gioncu and Mazzolani, 2002). 2.2 Ductility in seismic design 2.2.1 First developments of ductility concept The preliminary design concepts were conceived after the severe earthquakes at the beginning of the 20 th century. The great builder Gustave Eiffel had the intuition to model the earthquake forces by means of an equivalent wind load. The city of San Francisco was rebuilt after the 1906 great earthquake using a 1.4 kPa equivalent wind load. It was not until after the Santa Barbara earthquake in 1925 and the Long Beach earthquake in 1933, that the concept of lateral forces proportional to the mass was introduced into practice. The buildings have been designed to withstand lateral forces of about 7.5 percent for rigid soil and 10 percent for soft soil of their dead load. This rule was a consequence of the observation that the great majority of well designed and constructed buildings survived strong ground motions, even if they were designed only for a fraction of the forces that would develop if the structure behaved entirely linearly elastic (Fajfar, 1995). In 1943, the Los Angeles city code recognized the influence of flexibility of structures, and considered the number of structure levels in design forces. The San Francisco recommendations were the first ones where the influence of the fundamental period of the structure was introduced with a relation stating that the seismic forces are inversely proportional to this period (Bertero, 1992, Popov 1994). 8
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5. SEISMIC BEHAVIOUR OF RC COLUMNS
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6. SUMMARY, CONCLUSIONS AND FUTURE
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