OBJECTIVE EVALUATION OF THE Q FACTOR OF IRREGULAR RC BUILDINGS DESIGNED ACCORDING TO EC8 - DESIGN AND ANALYSES PROCEDURES

Abstract eng:
The seismic design of reinforced concrete (RC) mass or stiffness irregular buildings, in accordance with the classifications of Eurocode 8, is adjusted through the quantification of irregularity, through the use of irregularity indices and corresponding irregularity criteria. As a consequence, the design behavior factor in this case is no longer constant over a group of structures of similar form and building material, but varies on a case by case basis, since it involves modification of both behavior factors defining the design q, namely q due to structural overstrength (qΩ) and q due to the local ductility supply of the members (qμ). The reliability of this design approach is evaluated herein through the investigation of the inelastic performance of a form of irregular building with different degrees of irregularity per EC8. Focusing primarily herein - for the sake of simplicity - on vertical irregularity, an automated and therefore objective design analysis procedure is presented, which has been developed on the Open System of Earthquake Engineering Simulation platform (OpenSees). Following the definition of the basic framing configuration, characteristic loads and material grades, the procedure includes the following automatic steps: i) successive analysis of the frame for all EC8 required combinations, ii) evaluation of the internal design forces and envelopes thereof, iii) evaluation of flexural and shear reinforcement in all member critical regions, iv) evaluation of the local ductility supply of the members, v) formulation of the inelastic threedimensional model of the structure (base is assumed fixed), using spread damage fiber section finite elements, vi) three dimensional analysis of the model both statically (modal load profiles) and in the time domain (incremental base excitation); and vii) establishment of response details and damage indices, such as seismic shear and deformation profiles, the collapse pattern, local and global ductility and supplied q. Given the module versatility, different levels of vertical and/or torsional irregularities can be established by removing selected structural members in plan, by adjusting selectively the plan or height geometry or by locally increasing the acting vertical loads. The process is applied to a typical tower; the code prescribed design procedure and irregularity indices are imposed and their reliability is established through local damage comparisons with the assumed design values.

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