Issue 47

V. Alecci et alii, Frattura ed Integrità Strutturale, 47 (2019) 161-167; DOI: 10.3221/IGF-ESIS.47.13 166 Figure 9 : Guggenheim Museum, Bilbao Figure 10 : Nationale-Nederlanden Building, Prague Nowadays, structural architects are increasingly becoming architects/artists who reject the written rules and the structural grids, giving space to creativity, dreams, free inspiration. This cultural trend inevitably influences the new master architects, the young generation of architects, the architectural Schools and Associations which enhance the expressiveness of architectural and structural designers but, at the same time, represents a sort of challenge that architectural designers can overcome if equipped with a solid structural and seismic background. They must know irregularity conditions, their relations with seismic loads and how to mitigate their effects. They must have a three- dimensional view of the structural and seismic issues (and not just plane) evaluating the opportunity to apply the simplifications that several calculation methods assume (rigid diaphragm, uniform seismic action, etc.), also performing an additional evaluation in the design stage of some effects which usually are secondary in ordinary buildings, such as torsional moments on individual structural elements, P-Delta effects, etc. A NALYSIS METHODS OF IRREGULAR STRUCTURES eismic behavior of irregular structures is usually investigated by single-storey models. Although single-storey models represent the most extreme idealization of plan irregular buildings, they still attract many researchers, as they remain adequate to obtain general information on torsional behavior of asymmetric buildings, particularly from a qualitative point of view. They have been widely used in the past also due to their capability of clarifying the influence of the governing parameters and derive effective design criteria. However, in recent years multi-storey building models have been used to study more realistically inelastic earthquake response of asymmetric buildings due, firstly, to the shortcomings of one-storey models in predicting torsional behavior of real structures and, secondly, for the development of powerful computational tools able to make extensive and refined numerical analyses of 3D multi-storey building structures. An accurate prediction of seismic behavior of irregular structures cannot be done by applying a standard Non-linear Static Procedure (NSP) because, in virtue of an uneven plan displacement demand, one target displacement is not sufficient and an evaluation of the amplification of displacement demand due to torsional effect is necessary. Furthermore, no allowances are considered for dynamic effects of lateral-torsional coupling and large uncertainties remain when results obtained from independent pushover analysis in the two horizontal directions are combined. Nowadays, the main approach available in literature to adapt standard NSPs for plan-irregular structures is represented by different displacement amplification methods. One such method, proposed by Fajfar et al. [18], considers to properly including torsional effects combining the results obtained by pushover analysis of a 3D structural model, based on the N2 method, with the results from a linear dynamic (spectral) analysis. The N2 method controls the target displacements and distribution of deformations along the height of the building, whereas the linear dynamic analysis is used to define the S