“Computational Civil Engineering - "Intersections" International Journal

228 C. C. ComisuTo determine the mode shapes, therefore, one can either excite the structure at theresonance frequencies or measure the structural deformation in each vibrationmode, or they can be deduced from the frequency responses measured at variouspoints on the superstructure of bridge.The simplest and most commonly used technique is the peak amplitude method, inwhich the structure is excited by a sinusoidal force from a single exciter and theresponse curves of total amplitude, obtained at several points on the superstructureof bridge, are recorded as a function of frequency. The required information is thenextracted from these curves. This method has inherent deficiencies, in that notenough is measured, and what is measured is displayed unsatisfactorily. Since thevibratory response of a structure is due to the response in all the principal modessimultaneously, the measured mode shapes are often distorted. This problem isfurther exacerbated in the case of structures with close natural frequencies whereseparation of modes becomes mandatory.In contrast to the traditional sinusoidal excitation, measurement of response to wideband excitation signals has been made possible. In this technique, the frequencyresponse function of the superstructure of bridge can be measured at a single point,due to impulse excitation at various points on the structure, or the superstructurecan be excited at a single point using various forms of wide band random signals,and the frequency response function measured at several points. The modalparameters are then extracted by analytic curve-fitting the measured data in boththe time and the frequency domains [2], [3].The testing a superstructure of bridge is carried out in two steps.In the first step the number of modes and their resonant frequencies are roughlyestablished using single shaker sweeps. These presences of modes are indicated byresonant phenomena and phase shifts in the response, and can be difficult to detectwhen modes have similar shapes and natural frequencies.Once the existence of a mode has been established, the second step involvesisolation (tuning) of the mode. This is achieved by distributing the availablenumber of shakers around the superstructure of bridge, guided by experience andheuristic reasoning, and adjusting (appropriating) the amplitudes of the monophaseforces on the shakers, such that only the mode of interest is dominantlyexcited in that particular frequency range.The superstructure thus responds predominantly in the principal mode as a singledegree of freedom system, and the modal parameters can then be easily calculated.The number of shakers used and their judicious distribution around thesuperstructure govern the accuracy of the results.