In many seismic countries in the world (e.g. Europe, Northern USA, Japan, Turkey, etc.), the assessment of existing structures is a priority, since the majority of the building heritage was designed according to out-of-date or even non-seismic codes. The uncertainties about the nonlinear behaviour of the structures are, therefore, important and the nonlinear response should be treated directly, with a correspondingly strong increase in complexity of the assessment procedure. The assessment of regular reinforced concrete frame buildings has been performed, according to the Italian Seismic Code, Eurocode 8 and the CNR DT-212 guideline. A lumped plasticity model has been used with the aim of quantifying the differences between a fixed and a continuously updated shear span and between the use of inelastic springs located at the member ends or continuously along the beam elements, and with the purpose of considering the influence of axial-bending-shear interaction on the global capacity of the buildings.
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The recent seismic events and the importance of seismic prevention, increasingly growing in the last few years, have highlighted the necessity of assessing the capability of the existing building heritage to sustain earthquakes, in order to improve the average safety level of the population. The adequate modelling of existing Reinforced Concrete (RC) frames is a crucial issue (Elnashai & Di Sarno, 2008; Plevris, Mitropoulou, & Lagaros, 2012), related as well to the maintenance and to the structural upgrading possibility. The evaluation of the seismic vulnerability of existing buildings has a key role in determining and reducing the impact of an earthquake (Elnashai & Di Sarno, 2008).
This high vulnerability of RC buildings is due to many aspects, mostly related to the age of the buildings, the low standards of construction execution and maintenance, and the legislation in force at the time, which did not effectively address the seismic problem, even allowing the design for gravity loads in some earthquake-prone areas (Asteris & Plevris, 2015), erroneously considered as non-seismic zones. For this reason, old existing RC framed buildings are today characterized by:
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Poor quality concrete (Fiore, Porco, Uva, & Mezzina, 2013; A. Masi & Chiauzzi, 2013),
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Inefficient construction details (Angelo Masi & Vona, 2012),
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A lack of the fundamental principle of the capacity design, and
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Low column ductility mainly due to the inadequate use of stirrups (Verderame, De Luca, Ricci, & Manfredi, 2011).
Moreover, high shear forces, usually determined by global torsional effects, often result in brittle collapse susceptibility (Francesco Clementi, Quagliarini, Maracchini, & Lenci, 2015).
In most of these structures, the uncertainties about the nonlinear behavior are significant (Mpampatsikos, Nascimbene, & Petrini, 2008): generally, the presence and location of potential inelastic zones, as well as their ductility capacity, are not known. It is, therefore, very hard to define a direct correlation between the nonlinear internal forces that develop in the system during the real seismic excitation and those experienced by an equivalent indefinitely elastic structure. For this reason, a force-based assessment obtained by using an elastic analysis and reducing the internal forces by the behavior factor “q”, does not achieve, in general, satisfactory results (Mpampatsikos et al., 2008). Hence, the nonlinear behavior of the structure should be faced directly, with a corresponding considerable increase in complexity of the assessment procedure.
In this work, the assessment of RC frame buildings has been performed according to:
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The Italian Seismic Code (NTC 2008, 2008),
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The European code (CEN (Comité Européen de Normalisation), 2005b), and
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The new Italian Guideline (CNR, Consiglio Nazionale delle Ricerche, 2013).
All the Codes consider the nonlinear methods of analysis as the normal way to evaluate the seismic demand. Concerning the assessment of the response, all Codes require a force- (strength-) based procedure for the brittle mechanisms (shear in beams, columns, walls, and joints) and a displacement-based approach for the ductile ones (flexure in beams, columns, and walls): the evaluation of both deformation and shear capacities of the structural members of a building subjected to a ground motion of a certain intensity requires, in general, lengthy and not simple calculations.