Dynamic Assessment of Curved Cable-Stayed Bridge by Model Updating

Bridges are indispensable components of the infrastructure of modern society, and their assessment via techniques of structural dynamics is assuming greater importance. This assessment concerns performance of the as-built structure compared to the design and can also extend to the assessment of structural deterioration or damage. Simple validation of numerical results by dynamic testing has met some success; however, feedback from testing into analysis is usually crude, and only recently have systematic techniques been developed that can be supplied to such structures. This paper investigates the application of sensitivity-based model updating technology to the dynamic assessment of the Safti Link Bridge, a curved cable-stayed bridge in Singapore. Based on the measured modal data from prototype testing, the simulated dynamic properties obtained via finite- element analysis have been significantly improved by modification of uncertain structural parameters such as Young's modulus of concrete and structural geometry. Cable-stayed bridges with modern distinctive styles are in- creasing in number worldwide. These bridges are now built in more unusual styles for structural and aesthetic reasons (Menn 1996; Rito 1996). Examples include the Lerez Bridge (Troy- ano et al. 1998), a single inclined tower bridge; the Katsushika Harp Bridge (Takenouchi 1998), with a single pylon and S- shaped deck; the Marian Bridge (Kominek 1998), with a single L-shaped pylon; the Alamillo Bridge (Casa 1995), with a sin- gle inclined pylon; and the Safti Link Bridge (Tan 1996), which has a curved deck and single offset pylon. The unique structural styles of these bridges beautify the environment but also add to the difficulties in accurate structural analysis. The accurate assessment of these and other types of bridges using dynamics-based methods has become of increasing concern due to their infrastructural role. Dynamics-based assessment (Severn et al. 1989; Felber 1995; Law and Ko 1995; Felber and Cantieni 1996) of these unusual bridges is based on a comparison of the experimental modal analysis (EMA) data obtained during full-scale tests with the finite-element analysis (FEA) predictions. One purpose of the comparison is so that the finite-element (FE) models can be used to predict performance during unusual loads such as earth- quakes (Dumanoglu et al. 1991; Brownjohn et al. 1992). Even if the resulting response is large enough that material and ge- ometric nonlinearities become significant, the starting point for nonlinear analysis would be a realistic linear model. For ex- ample, in the case of a suspension bridge, the large cable os- cillations will vary the geometric stiffness, which can be accom- modated in nonlinear analyses if the operating tension is known. Material nonlinearities can also be incorporated based on the low level characteristics. Other motivations for dynamics-based assessment include the validation of design assumptions em- bodied in the FE model and health assessment (i.e., the iden- tification of structural deterioration or damage). Confidence in using FE models for performance predictions may be lacking due to relatively large differences between experimental and analytical modes. The differences come not 1