Torsional Effects in the Pushover-Based Seismic Analysis of Buildings

Abstract

The general trends of the inelastic behaviour of plan-asymmetric structures have been studied. Systems with structural elements in both orthogonal directions and bi-axial eccentricity were subjected to bi-directional excitation. Test examples include idealised singlestorey and multi-storey models, and a three-storey building, for which test results are available. The response in terms of displacements was determined by nonlinear dynamic analyses. The main findings, limited to fairly regular and simple investigated buildings, are: (a) The amplification' of displacements determined by elastic dynamic analysis can be used as a rough, and in the majority of cases conservative estimate in the inelastic range. (b) Any favourable torsional effect on the stiff side, which may arise from elastic analysis, may disappear in the inelastic range. These findings can be utilised in the approximate pushover-based seismic analysis of asymmetric buildings, e.g. in the N2 method. It is proposed that the results obtained by pushover analysis of a 3D structural model be combined with the results of a linear dynamic (spectral) analysis. The former results control the target displacements and the distribution of deforrn&ions along the height of the building, whereas the latter results define the torsional amplifkations. The proposed approach is partly illustrated and evaluated by test examples.

Analysis in seismic provisions for buildings: past, present and future

“…Baros and Anagnostopoulos (2008) wrote: ''The nonlinear static pushover analyses were introduced as simple methods … Refining them to a degree that may not be justified by their underlying assumptions and making them more complicated to apply than even the nonlinear response-history analysis … is certainly not justified and defeats the purpose of using such procedures. '' In this section, the extended N2 method (Kreslin and Fajfar 2012), which combines two earlier approaches, taking into account torsion (Fajfar et al 2005) and higher mode effects in elevation (Kreslin and Fajfar 2011), into a single procedure enabling the analysis of plan-asymmetric medium-and high-rise buildings, will be discussed. The method is based on the assumption that the structure remains in the elastic range in higher modes.…”

“…Nevertheless, based on the preliminary results, the clause ''Procedure for the estimation of torsional effects'' was introduced, in which it was stated that ''pushover analysis may significantly underestimate deformations at the stiff/strong side of a torsionally flexible structure''. It was also stated that ''for such structures, displacements at the stiff/strong side shall be increased, compared to those in the corresponding torsionally balanced structure'' and that ''this requirement is deemed to be satisfied if the amplification factor to be applied Comparison of normalized roof displacement in plan obtained by NRHA analysis (mean values) for different intensities, elastic response spectrum analysis, pushover analysis, and the extended N2 method for the SPEAR building (adapted from Fajfar et al 2005) to the displacements of the stiff/strong side is based on the results of an elastic modal analysis of the spatial model''. ASCE 41-13 basically uses the same idea of enveloping the results of the two analysis procedures in order to take into account the higher modes in elevation.…”

Accuracy of Advanced Methods for Nonlinear Static Analysis of Steel Moment-Resisting Frames

“…In order to include the effects of higher modes, some advanced modal pushover procedures based on the elastic modal decomposition concepts have been developed in literature. Many of these procedures consider higher modes in lateral load pattern in order to take into account higher mode effects both in plan and in elevation [18][19][20][21][22][23]. In particular, in the well-known modal pushover analysis (MPA) proposed by Chopra and Goel [18] higher mode effects are considered by analysing each mode as an equivalent single-degree-of-freedom system including nonlinear properties related to that mode.…”

Effect of Material Variability and Mechanical Eccentricity on the Seismic Vulnerability Assessment of Reinforced Concrete Buildings

“…Also, non-uniform distribution of the columns material in r.c. buildings can produce torsional effects [24][25][26][27]: hence the definition of "material eccentricity" e m . Conventional analyses on existing buildings generally consider uniform materials by assuming an average value obtained from an experimental test.…”