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

are evaluated and presented in Figures 3.12 to 3.33. It should be noted that some of themode shapes involve the coupled motion in two or three directions. Their correspondingfrequencies are expected to be notable in the Fourier spectra of acceleration responses inthe relevant direction. Theoretically, because the time interval of the measured data is0.005 sec., the maximum frequency that can possibly be identified may reach up to100Hz, as included in Figure 3.10 for the vertical acceleration response at midspan of themain cable-stayed span. It can be seen from Figure 3.10 that the amplitude of Fourierspectra beyond 15Hz is very small with little significance in structural response. For thisreason, only frequencies lower than 15Hz are concentrated on in this study, especially thefrequencies lower than 1.0Hz as shown in Figure 3.11 for the longitudinal acceleration ofthe main cable-stayed span. Also illustrated on Figure 3.11 are several natural frequenciesidentified from the measured data, which will be verified with their corresponding modeshapes. A complete set of the identified frequencies from acceleration measurements aregiven in Table 3.3. The Fourier spectra for other significant acceleration components atstrategic locations are shown in Figures 3.12 to 3.33 for both low and high frequencyranges.32.52Amplitude1.510.500 10 20 30 40 50 60 70 80 90 100Frequency(Hz)Figure 3.10 Fourier spectrum of acceleration at HNZ-C229