“Computational Civil Engineering - "Intersections" International Journal

“Computational Civil Engineering 2005”, International Symposium 239mode could be eliminated, as point A and C move in anti-phase, however, thesecond mode could not be eliminated.From this example, it can be appreciated how the symmetrical and antisymmetricalproperties of the mode shapes can be used to advantage, for judiciouspositioning of the shakers. In complex structures, however, this is not alwayspossible. Furthermore, as continuous structures have infinite number of degrees offreedom, infinite number of shakers would be required. In practice, obviously alimited number of shakers are used, such that only the mode of interest isdominantly excited in a particular frequency range.FREE - FREE BEAMFIRST MODEABSECOND MODEABTHIRD MODEACBFig. 1. Positioning of shakers to excite the third mode of a free-free beamThe testing of large structures, such superstructure of bridges, is carried out in twostages. In the first step the number of modes and their natural frequencies areroughly established using single shaker sweeps, by either plotting the peakamplitude or the quadrature component as a function of frequency, or preferablyusing the complex plot. The response should be monitored at several points toensure that none of the modes is missed out in the frequency range of interest.Once the existence of a mode has been established, the second step involves tuningof the mode using multiple shakers.The test is started using a single exciter, placed preferably at an anti-nodeof the mode to be excited, with an arbitrary force, and the response from anaccelerometer placed at the same shaker, or an anti-node, is fed back to thegenerator. The natural frequency of the mode to be examined is now found byadjusting the frequency of the generator, and observing the relevant Lissajous