Report - PEER - University of California, Berkeley

introduce the elastic torsional influences (displacement shapes in the plan) at the topof the building, while preserving the absolute values of top displacements in masscentre and the vertical distributions (ratios) of seismic demand determined by twopushover analyses. Only amplification due to torsion is taken into account. Beneficialtorsional effects, i.e., de-amplification of displacements compared to thedisplacements in the mass centre, is neglected.10.5 ma)YX9.7 mCMb)2.5 m 5.0 m 5.0 mFigure 3. Test structures: (a) SPEAR building; (b) infilled frame.In order to illustrate the proposed procedure, a three-storey RC frame structure(Fig.3a), tested pseudo-dynamically in full-scale in ELSA laboratory in Ispra withinthe SPEAR project, has been analysed. The mathematical model corresponds to pretestanalyses. Non-linear time-history analyses were performed with 7 two-componentsemi-artificial accelerograms, fitted to Eurocode 8 elastic spectrum. In Fig.4envelopes of normalized displacements at the top of the building, obtained by timehistoryanalyses for different intensities of ground motions, are compared with theresults of elastic spectral analysis. Fig.4 indicates that, on the flexible sides (i.e., theupper and right hand side in Fig.3a), the torsional amplification generally decreaseswith increasing intensity of ground motion. On the stiff sides, the de-amplification issmaller than suggested by elastic spectral analysis. It may completely disappear oreven an amplification may occur. The coefficient of variation of normalizeddisplacements amounts to about 0.13 in X-direction and about 0.1 in Y-direction (atPGA = 0.3 g). According to the proposed extension of the N2 method, thedisplacement demand in both directions follows the elastic spectral line on theflexible sides and the horizontal line 1.0 on the stiff sides. The correction factors indifferent locations are determined as the ratios between the lines defined above andthe lines obtained by the pushover (static) analysis. The target displacement, which isby definition the displacement demand at the top at mass centre, is determined by theusual procedure, applied in the N2 method. Comparison of the capacity diagram anddemand spectra normalized to PGA = 0.3g is presented in Fig.5. There is a noticeabledifference in pushover curves due to different sense of loading in Y-direction, while2.7 m 2.7 m 2.7 m 2.7 m362