âComputational Civil Engineering - "Intersections" International Journal

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“Computational Civil Engineering 2005”, International SymposiumIaşi, România, May 27, 2005Multiple exciter techniques for structural identificationCristian-Claudiu Comisu1 Catedra Cai de Comunicatii si Fundatii, Facultatea de Constructii, Iasi, 70050, RomaniaSummaryThe objectives of this paper are to integrated analytical research with theexperimental techniques. These techniques can be further sub-divided into twocategories - single exciter techniques and multiple exciter techniques. The varioustechniques used for obtaining the modal parameters, will be viewed from threeaspects: the way in which the structure is excited an d the instrumentationrequired, the data obtained and its presentation and the analysis of the data.Furthermore, the advantages and disadvantages of the various exciter techniqueswill be discussed.This paper is focus to find an algorithm to quantify modal parameters obtained bythe multiple exciter techniques.For each frequency of excitation there are as many characteristic phase angle asthere are number of degree of freedom, corresponding to certain sets of forces.If the structure is non-proportionally damped, the structure can be excited in itsprincipal mode, only at the corresponding natural frequency by a set of monophaseforces. In this case, the response will be in quadrature with the forces at allpoints on the structure.If the structure has n degrees of freedom, n number of shakers is required ideally toisolate a mode.For each characteristic phase lag there is a corresponding mode shape whichvaries with frequency. At the undamped natural frequencies one of the modeshapes is identical to the corresponding undamped principal mode.The mode shapes depend on the shape of the damping matrix and not on theintensity of damping.In each mode the response at the coordinates are all in phase, but lag behind the0excitation force by an angle θ. At the undamped frequncy θ = 90 for one of themodes which is the principal mode.KEYWORDS: structural identification, exciter, analytical model, modalparameters, flexibility, inertia, damping
238 C. C. Comisu1. INTRODUCTIONSince a superstructure of bridge when excited vibrates in several modessimultaneously, and thus causes difficulties in the analysis of the results, theunwanted modes have to be somehow eliminated. This can be achieved for simplestructures by placing the exciters or pick-ups at nodal points of the unwantedmodes, or by making use of the symmetrical and anti-symmetrical properties of themode shapes. In complex structures such as superstructures of bridge, this is notalways possible, and systematic methods have to be used for exciting the structurewith multiple shakers and forcing them to vibrate in their principal modes.However, this requires rather sophisticated equipment both on the excitation side,as well as on the data acquisition side, because of the large number of pick-ups,necessary for determining the mode shapes. To facilitate understanding of themultiple exciter techniques, it is necessary to first illustrate in practical terms thetheoretical considerations and describe the instrumentation required, and thefeatures that have been incorporated, that are obligatory for functioning of the test[1], [2].2. EXPERIMENTAL PROCEDUREThat if a structure is proportionally damped, it can be excited at any frequency by aparticular set of forces which are in phase or anti-phase with each other, such thatthe measured responses at all points are all in phase, or anti-phase, and that thecommon phase lag between the force and the response is unique at this frequency.At this frequency there are as many characteristic phase lags with their associatedlinearly independent force distribution, as there are degrees of freedom in thestructure. The structure when excited in this manner for a particular ratio of forceswill vibrate in the principal mode, and thus as a single degree of freedom system.If the structure is non-proportionally damped, the structure can be excited in itsprincipal mode, only at the corresponding natural frequency by a set of monophaseforces. In this case, the response will be in quadrature with the forces at allpoints on the structure.If the structure has n degrees of freedom, n number of shakers is required ideally toisolate a mode. To exemplify this statement, consider the mode shapes of the firstthree degrees of freedom of a free-free beam shown in figure 1.In order to excite the third mode only, three shakers in phase would be required,one at each of points A, B and C. If only two shakers with equal force amplitudeswere used in phase at A and B, the second mode could be eliminated, as points Aand B move in anti-phase, however, the first mode would be excited together withthe third one. Again, if only two shakers were used in phase at A and C, the first
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COMPUTATIONAL CIVIL ENGINEERING2005
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“Computational Civil Engineering.
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