| This study addresses the engineering, analysis, evaluation, and interpretation of the seismic records, for both the input ground excitation and the output structural response, of the Vincent-Thomas Suspension Bridge, Los Angeles Harbor. Concepts for seismic protection systems are also presented. This well instrumented and well studied bridge was shaken by two strong earthquakes: the October 1, 1987 Whittier, California, of magnitude Mw = 5.9, and the January 17, 1994 Northridge, California, of magnitude Mw = 6.7. Both the Whittier and Northridge earthquakes provided several valuable lessons regarding seismic performance, seismic energy absorbing devices, and damping augmentation. These lessons included not only the seismic protection of this suspension bridge, but also all the existing or newly constructed suspension bridges. The two earthquakes generated the most comprehensive set of data records, on the structural response and the multiple-support excitation of such horizontally extended bridges, ever recorded from damaging earthquakes anywhere in the world.; Temporal and spectral analyses of the recorded motions of the Vincent-Thomas bridge during the 1994 Northridge earthquake are presented. The manipulation of the records in the time and frequency domain, through the addition and subtraction, enhanced the identification process of the directional lateral, vertical, and torsional natural modes of vibration. Moreover, the study provides evidence of spatial variation of the ground motion along the bridge substructure, the strong structural and dynamic coupling among vibrational modes, and the overall dynamic behavior of the bridge during two different magnitudes of earthquakes. The damping characteristic of the bridge are thoroughly investigated by conventional methods and sophisticate system identification to determine optimal values to be used in the dynamic analysis of these types of bridges. Moreover, the resulting values of damping were utilized in the ongoing seismic retrofit and strengthening design.; In addition, modal properties were extracted from a 3-D finite element model of the Vincent-Thomas bridge. A comparative study is presented of the mode shapes and their associated frequencies from these analytical investigations are used to calibrate and validate the theory with observation. A comparative study of the ambient vibration surveys and the identified properties from earthquake records is also presented. These comparative studies were used to investigate the validity of the existing 3-D finite element model of the bridge.; Straining actions such as displacements, forces and moments of some selected structural components were evaluated and compared with their existing capacities. The Northridge earthquake results showed some vulnerabilities of specific members, joints, and supports of the bridge. Conceptual energy absorbing devices and damping augmentation techniques are presented and need to be addressed before large magnitude earthquakes hit the vicinity of the bridge.; The lessons learned from the Northridge earthquake are highlighted, contrasted, and compared with previous analytical studies and measurements. These results confirm the lessons learned from the 1987 Whittier earthquake. Both events provide practical and valuable guidelines for the seismic retrofit design, including strengthening and supplementary damping concepts and devices, and elaborate structural monitoring systems to address the integrity of such vital transportation links in the metropolitan regions. |
