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Influence of Vertical Ground Motion on Bridges Isolated with Friction Pendulum Bearings
AdvisorRyan, Keri L.
Civil and Environmental Engineering
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AbstractInfluence of Vertical Ground Motion on Bridges Isolated with FrictionPendulum BearingsThe motivation for this project comes from test of a full scale building isolated withtriple friction pendulum bearings on E-defense shake table at Japan. The test demonstratedexperimentally that the vertical motion can amplify both the base shear and the storyacceleration in the isolated building. Vertical shaking introduced high frequency variationin the axial force of the bearings, and consequently a high frequency component in thebearing lateral force, which excited higher structural modes in the building. Since bridgesare flexible in the vertical direction because of long spans, similar effects may be observedin bridges.The objectives of this study are to develop physical understanding of responsesamplification in bridges isolated with spherical sliding bearings, and to develop asimplified method to predict base shear amplification of bridges isolated with multispherical bearings subjected to a 3-D shaking. A series of 2-D and 3-D motions with wideintensity range of horizontal and vertical components were applied such that the directeffect of vertical shaking could be evaluated. The selected ground motion suite was dividedinto three groups based on vertical peak ground acceleration (PGAV). Multi-span concretebox girder bridges were selected for this study, as they are a prominent bridge type inCalifornia and are suitable for seismic isolation. Models were developed for a 3-span, 45ft wide, multicolumn Base Model bridge, and various superstructure and isolation systemparameter variations were implemented to evaluate effect of these variations on theiiamplification of base shear. Response histories were compared for a representative motionfrom each ground motion group under 2-D and 3-D shaking. Modal and spectral analyseswere conducted to understand dynamic properties and behavior of the bridge undervertical motion. Based on simplified theory, two methods to estimate the amplified baseshear were developed: Non-amplified method (NA method) and spectrally amplifiedmethod (SA method). The accuracy of the simplified methods were assessed by twodifferent error estimates, amplification error or AE, and base shear normalized error orBSNE.Response history analysis showed significant amplification of base shear under 3-D motion implying that exclusion of vertical component could lead to under estimation ofdemand shear forces on bridge piers. Deck acceleration spectral response at differentlocations revealed that a higher vertical bridge mode 7th mode was excited under 3D motionin base model bridge. Consequently, a closely spaced 6th structural mode was also excited.In other bridge models vertical motion has excited dominant vertical mode and modalcoupling is expected in some bridge models. NA method was seen to estimate underconservative values under one low vertical intensity ground motion. The SA method wasshown to be reliable compared to NA method and is recommended as a sufficientlyconservative approach to estimate base shear amplification for moderate to low intensityvertical motion. Variations in the bridge modeling parameters have little effect on baseshear since the deck acts as single mass sliding on isolators, and therefore simplified theorycan be applied to a range of bridge models. Amplification of base shear coefficientdecreased as the isolation period increased, thus estimated base shear was increasinglyconservative for long period isolation systems.