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Report - PEER - University of California, Berkeley

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4 x 50 m = 200 mC1 C2 C3h u = 7 mh c = 2x, 1x, 3x h uV2130.4m3.2m0.4m0.4m 1.2m 0.4m2.0m4.0mFigure 1. Irregular, torsionally sensitive viaduct V213.2.1 Single Mode ProceduresElastic single mode method (SM) grossly underestimates the displacement at the stiffside <strong>of</strong> the analyzed viaduct when compared with the results <strong>of</strong> the elastic multi mode(MM) method (Fig. 2). This is why the SM excites only first — torsional(asymmetric) mode, while the second — translational mode predominantly influencesthe displacement on the stiff side.8.00dis pl. [cm ]V213PSM6.00MM - envelopeMM - 1st modeMM - 2nd mode4.002.000.00A 1 P2 P3 P4 A 5-2.00pier/abutmentnormalized displacement1.000.750.50IA - 0.7g N2 - 0.7g0.25IA - 0.35g N2-0.35gElasticbent/abutment0.00A1 B2 B3 B4 A5Figure 2. Influence <strong>of</strong> higher Figure 3. Displacements <strong>of</strong> viaduct V213.modes in viaduct V213.Similar trend is observed in the case <strong>of</strong> inelastic analysis, but the importance <strong>of</strong> highermodes has diminished by increased level <strong>of</strong> yielding in piers. Results <strong>of</strong> standard,single mode push-over based procedure N2 (Fajfar and Fischinger, 1988) andinelastic time-history analysis (IA) are compared in Fig. 3 for two different loadintensities (a g,max = 0.35g and 0.7g). Results are further compared with elastic MMmethod (see also Fig. 2). It is obvious that with the increased pier plastification thesingle mode associated with the (elastic) deck deformation prevails in the response.The conclusion, that in general single mode static methods can not be used fortorsionally flexible structures has been frequently mentioned in the case <strong>of</strong> buildings(i.e., Rutenberg and Tso, 2004).The research (Isaković et al. 2003) has indicated other important parameters thatenhance the influence <strong>of</strong> higher modes and consequently increase the irregularity <strong>of</strong>bridge response as well as preclude the use <strong>of</strong> single mode methods. First <strong>of</strong> all this isthe ratio <strong>of</strong> the stiffness <strong>of</strong> the deck and that <strong>of</strong> the piers (affected also by the level <strong>of</strong>plastification in piers). Others include eccentricity and type <strong>of</strong> constraints at theabutments.295

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