Extensive experimental investigations were conducted on the Vincent-Thomas Suspension Bridge at Los Angeles Harbor to determine natural frequencies and mode shapes of vertical, torsional and lateral vibrations of the structure. These ambient vibration tests involved the simultaneous measurements of both vertical and lateral vibrational motions caused by traffic. Measurements were made at selected points on different cross sections of the stiffening structure. Comparison with previously computed mode shapes and frequencies shows good agreement with the experimental results, thus confirming both the accuracy of the experimental determination and the reliability of the method of computation.
[Show abstract][Hide abstract] ABSTRACT: The present study is focused on field measurements and numerical analyses of dynamic characteristic of cylindrical liquid container. A liquid Storage tank which its diameter and height is similar of 12.190m, is considered. Ambient vibration tests were carried out on the tank and the classical peak picking spectral method was used to determine the natural frequency of the tank. The ambient forces which excite the tank are the result of wind currents and microseismic waves. Numerical values of natural frequencies and corresponds mode shapes of tank are derived form free vibration analysis of the finite element model which governs the fluid-shell interaction. Comparison between numerical and experimental frequencies and mode shapes shows good agreement.
[Show abstract][Hide abstract] ABSTRACT: The system identification and damage detection of long-span structures has been an area of considerable interest due to the critical role such structures often play in civil infrastructure systems. Because of their inherent length, such structures must be viewed as having multiple inputs at the base during strong-ground motions. The Vincent Thomas Bridge in the Los Angeles metropolitan area, is a critical artery for commercial traffic flow in and out of the Los Angeles Harbour, and is at risk in the seismically active Southern California region, particularly because it straddles the Palos Verdes fault zone. A combination of linear and nonlinear system identification techniques is employed to obtain a complete reduced-order, multi-input-multi-output (MIMO) dynamic model of the Vincent Thomas Bridge based on the dynamic response of the structure to the 1987 Whittier and 1994 Northridge earthquakes. Starting with the available acceleration measurements (which consists of 15 accelerometers on the bridge structure and 10 accelerometers at various locations on its base), a multistage, time-domain identification procedure is applied to the data set to develop an equivalent nonlinear, multi-degree-of-freedom model. This self-starting identification method uses least-squares parameter estimation methods, combined with nonparametric identification techniques, to generate a reduced-order nonlinear mathematical model suitable for use in subsequent studies to predict, with good fidelity, the response of the bridge under arbitrary dynamic environments. Results of this study yield measurements of the equivalent linear modal properties (frequencies, mode shapes and non-proportional damping) as well as quantitative measures of the extent and nature of nonlinear interaction forces arising from strong ground shaking. It is shown that, for the particular subset of observations used in the identification procedure, the apparent nonlinearities in the system restoring forces are quite significant, and they contribute substantially to the improved fidelity of the model. Also shown is the potential of the identification technique under discussion to detect slight changes in the structure's influence coefficients, which may be precursors to damage and degradation in the structure being monitored.
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