Assessment of the Bill Emerson Memorial Bridge - FTP Directory ...

Table 3.3Natural frequencies from measured data (Hz)No.Frequency1 0.3382 0.4383 0.5004 0.5885 0.6506 0.7137 0.7758 0.8259 1.07510 1.33811 1.72512 2.03813 2.33814 3.26315 7.62516 8.77517 10.363.6. Mode shapes extracted from measure dataAfter picking the natural frequencies corresponding to the peaks of a number of Fourierspectra, the magnitudes of the Fourier spectra at each natural frequency were divided bytheir respective magnitudes of the spectra at baseline stations to create a relativemagnitudeplot for the bridge, relating the magnitudes of different stations to those of areference station (where occurred the maximum response). The relative magnitudes foreach point along the bridge were then plotted at each of the picked-peak frequencies todetermine the mode shapes of the bridge. Note that the sign of the mode shapes isdetermined by observing the phase differences in time domain. The first three modeshapes are depicted in Figures 3.35-3.37.38