Probabilistic performance evaluation of Vincent Thomas Bridge under seismic, wind, and traffic loads.

摘要

The Vincent Thomas Bridge, connecting Terminal Island with San Pedro, serves both Los Angeles and Long Beach ports, two busiest ports in the west coast of USA. The bridge carries an overwhelming number of traffic with an Annual Average Daily Traffic (AADT) volume of 100,000, many of which are cargo trucks. Based on the recent finding that the main span of the Vincent Thomas Bridge crosses directly over the Palos Verdes fault, which has the capacity to produce a devastating earthquake, the bridge underwent a major retrofit in spring 2000, mainly using visco-elastic dampers.This study focuses on performance evaluation of the retrofitted bridge under seismic, wind, and traffic loads. A member-based detailed three-dimensional Finite Element (FE) as well as panel-based simplified models of the bridge are developed. In order to show the appropriateness of these models, eigenproperties of the bridge models are evaluated and compared with the system identification results obtained from measured ambient vibration and response under strong earthquake. Tornado diagram and first order second moment (FOSM) methods are applied for evaluating the sensitivity of different parameters on the eigenproperties of the FE models.Considering a set of strong ground motions in the Los Angeles area, nonlinear time history analyses are performed using the FE models developed and seismic fragility curves are derived comparing the ductility demand with the ductility capacity at critical tower sections. Effect of spatial variability of ground motions on seismic displacement and force demands is also investigated. To generate spatially correlated nonstationary acceleration time histories compatible with design spectrum at each location. A new algorithm is developed involving evolutionary power spectral density function (PSDF) and with the aid of spectral representation method. It has been found that, in some locations on the bridge deck, the response is higher when the spatially variable ground motion is considered as opposed to the uniform ground motion time histories having the highest ground displacement.To record actual wind velocity and direction, three anemometers are installed at three different locations of the bridge. Measured fluctuating component of the wind velocity are used for simulation of fluctuating component of wind velocity throughout the span and along the tower on the basis of three different simulation methods (i) newly developed non-Gaussian conditional method, (ii) Gaussian conditional method, and (iii) Gaussian unconditional method. Response of the bridge is computed under wind velocity using these three different methods. It is observed that the non-Gaussian conditional simulation technique yields higher response than both Gaussian conditional and Gaussian non-conditional techniques. Finally, analysis of the bridge under traffic load is also carried out and a critical evaluation of shear force in deck shear connectors is performed.