Spectral analysis of cable stay vibration (Louisiana)
Description
Cable stay vibration data was obtained from the cable-stayed suspension bridge at Luling, Louisiana. Cable stay motion is harmonic in nature and is modeled as a linear system consisting of a set of second-order resonances. While traditional processing of this data using harmonic analysis provides information about the harmonic frequencies and the average excitation at these frequencies, in this investigation additional features of this data such as the instantaneous modal excitations and estimates of modal damping coefficients were obtained using the methods of complex demodulation and the ZOOM-FFT At each mode of vibration, the axis of the cable is assumed to move in an elliptical trajectory. The method of complex demodulation was expanded and refined to allow the calculation of the time-varying parameters describing the elliptical trajectory at each significant mode of vibration. Using the computed parameters, excitations in two adjacent cables were compared and found to be highly correlated. Wind-cable interactions were investigated by comparing the simultaneously recorded wind conditions with the computed parameters. For certain modes of vibration, correlations between the horizontal wind direction angle and the modal excitations were observed Modal resonance characteristics of the cable stay vibration were examined using the ZOOM-FFT technique. The input to the system is assumed to be band limited white noise having a constant power spectrum over a very narrow band of frequencies. The output of the system is either the output from a single accelerometer or the semi-major axis of the elliptical trajectory computed from the output of two accelerometers. Estimates of modal damping coefficients were obtained for both representations of the output