Predicting torsion‐induced lateral displacements for pushover analysis: Influence of torsional system characteristics

Abstract

The increasing popularity of simplified nonlinear methods in seismic design has recently led to many proposals for procedures aimed at extending pushover analysis to plan asymmetric structures. In terms of practical applications, one particularly promising approach is based on combining pushover analysis of a 3D structural model with the results of linear (modal) dynamic analysis. The effectiveness of such procedure, however, is contingent on one fundamental requirement: the elastic prediction of the envelope of lateral displacements must be conservative with respect to the actual inelastic one.This paper aims at verifying the above assumption through an extensive parametric analysis conducted with simplified single-storey models. The main structural parameters influencing torsional response in the elastic and inelastic range of behaviour are varied, while devoting special attention to the system stiffness eccentricity and radius. The analysis clarifies the main features of inelastic torsional response of different types of building structures; in this manner, it is found that the above-mentioned method is generally suitable for structures characterized by moderate to large torsional stiffness, whereas it cannot be recommended for extremely torsionally stiff structures, as their inelastic torsional response almost always exceeds the elastic one. TORSION-INDUCED LATERAL DISPLACEMENTS FOR PUSHOVER ANALYSIS1371 displacements and the distribution of lateral forces for each resisting element; (2) a planar pushover analysis for each resisting element of interest (typically resisting elements located on the building perimeter), using the lateral force distribution and target displacement defined in step 1). However, although the procedure is convenient, in that it requires conducting only a small number of planar pushover analyses, it may be ineffective in the presence of significantly uneven distributions of stiffness and strength in buildings, since in such cases load distributions obtained simply through elastic analysis may differ considerably from the actual inelastic one [16].More recently, an important step towards incorporating torsional effects into pushover analysis has been made by Fajfar et al. [18,19]. They proposed combining the results obtained by pushover analysis of a 3D structural model based on the N2 method (i.e. the simple 2D nonlinear method developed in [2, 3]) with the results from a linear dynamic (modal) analysis. In particular, the N2 method controls the target displacement at the centre of mass and the distribution of deformations along the building height, whereas the linear dynamic analysis is used to define the plan-wise torsional amplifications of lateral displacements. The use of linear dynamic analysis is justified by the assumption that the elastic envelope of lateral displacements at the flexible edge is conservative with respect to the inelastic one [20,21]. According to their procedure, only amplifications due to torsion with respect to the target displacement at the mass centre are...

The extended N2 method considering higher mode effects in both plan and elevation

“…In general, higher mode effects decrease both in elevation and in plan, with increasing ground motion intensity. Some exceptions, representing extremely torsionally stiff structures, are discussed in De Stefano and Pintucchi (2010); Erduran and Ryan (2010). So, it is difficult to develop a relatively simple approach which would be able to simulate the structural response over the whole range of ground motion intensities that a structure may experience.…”

Pushover Analysis for Plan Irregular Building Structures

“…as defined by De Stefano and Pintucchi (2010), i.e. the polar moment of inertia of system strength computed with respect to the axes parallel to the z direction and passing through C R ; -Strength radius of gyration d r ¼ 1 ρL ffiffiffiffiffi I p, f F q non-dimensionalized with respect to the mass radius of gyration ρ.…”

“…Nevertheless, the basic assumption of the conservativeness of the elastic envelope of lateral displacements with respect to the inelastic one surely needs to be further investigated. De Stefano and Pintucchi (2010) performed a wide parametric analysis on one-storey models and found that the method lose its conservativeness for very torsionally stiff structures, such as shear-walled buildings, for which the inelastic response almost always exceeds the elastic one. This is mainly due to the development of a strength eccentricity related to the failure of one or more components of the structural system, leading to a significant increase of the inelastic torsional effects.…”

Assessing the seismic response of existing RC buildings using the extended N2 method

“…This kind of buildings constitutes the common case in real life so it is urgent to implement updating formulations in these procedures turning the structural response as realistic as possible. Some efforts have been made to take this generation of NSPs to a higher performance level (Chopra and Goel 2004;Fajfar et al 2005;Bento et al 2010;Stefano and Pintucchi 2010;Erduran and Ryan 2010). The Eurocode8-1 (4.3.3.4.2.7 Procedure for the estimation of the torsional effects) includes some guidelines for the determination of torsional effects based on elastic analysis (e.g.…”