Seismic Design and Retrofit of Bridges

To study the effects of ground motion spatial variations on curved bridges, this paper presents a shake-table test for a 1/10-scale typical curved bridge under spatial ground motions, including wave passage effect, local site effect, and ground motion multidimensionality. The experimental results show that both wave passage and local site effects have significant influence on seismic responses of curved bridges. Wave passage effect on curved bridges is attributed primarily to the higher asymmetric modes and pseudostatic displacements induced by asynchronous excitations, whereas local site effect is more obvious for structures with fundamental frequencies close to the predominant frequencies of the sites. Compared with straight bridges, the curvature radius causes curved bridges to be more sensitive to ground motion spatial variations, especially the local site effect. Therefore, curved bridges may be damaged more seriously than straight bridges during the same earthquake. Furthermore, seismic responses of irregular curved bridges are more sensitive to ground motion multidimensionality than are those of straight bridges. In this experiment, the effects of bridge abutments and nonuniform sites along the bridge were not considered.

Shake-Table Test for a Typical Curved Bridge: Wave Passage and Local Site Effects

Based on observed damage patterns from previous earthquakes and a rich history of analytical studies, asynchronous input motion has been identified as a major source of unfavorable response for long-span structures, such as bridges. This study is aimed at quantifying the effect of geometric incoherence and wave arrival delay on complex straight and curved bridges using state-of-the-art methodologies and tools. Using fully parametrized computer codes combining expert geotechnical and earthquake structural engineering knowledge, suites of asynchronous accelerograms are produced for use in inelastic dynamic analysis of the bridge model. Two multi- degree-of-freedom analytical models are analyzed using 2,000 unique synthetic accelerograms with results showing significant response amplification due to asynchronous input motion, demonstrating the importance of considering asynchronous seismic input in complex, irregular bridge design. The paper, Part 1 of a two-paper investigation, presents the development of the input motion sets and the modeling and analysis approach employed, concluding with sample results. Detailed results and implications on seismic assessment are presented in the companion paper: Effect of Asynchronous Motion on Complex Bridges. Part II: Results and Implications on Assessment.

Effect of Asynchronous Earthquake Motion on Complex Bridges. I: Methodology and Input Motion

Nonuniform seismic excitation has been shown through previous analytical studies to adversely affect the response of long-span bridge structures. To further understand this phenomenon, this study investigates the response of complex straight and curved long-span bridges under the effect of parametrically varying asynchronous motion. The generation process and modeling procedures are presented in a companion paper. A wide-ranging parametric study is performed aimed at isolating the effect of both bridge curvature and the two main sources of asynchronous strong motion: geometric incoherence and the wave-passage effect. Results from this study indicate that response for the 344 m study structure is amplified significantly by nonsynchronous excitation, with displacement amplification factors between 1.6 and 3.4 for all levels of incoherence. This amplification was not constant or easily predicable, demonstrating the importance of inelastic dynamic analysis using asynchronous motion for assessment and design of this class of structure. Additionally, deck stiffness is shown to significantly affect response amplification, through response comparison between the curved and an equivalent straight bridge. Study results are used to suggest an appropriate domain for consideration of asynchronous excitation, as well as an efficient methodology for analysis.

Effect of Asynchronous Earthquake Motion on Complex Bridges. II: Results and Implications on Assessment

Anti-symmetric mode excitation and seismic response of base-isolated bridges under asynchronous input motion

The assumption that earthquake response of extended structures, of which bridges is one example, may be studied ignoring the possibility of out-of- synch motion of various supports is examined in this paper. The purpose is to assess whether the reduction of dynamic response is sufficient to offset the increase in relative displacements, due to independent motion of different supports. The paper starts with a review of possible damage patterns due to asynchronous motion. Thereafter, a brief literature review is undertaken, followed by an assess- ment of the status of seismic design codes concerning this issue. Advanced inelas- tic analysis, using sophisticated material models and analysis techniques, is then employed to explore characteristics of input motion and structural configuration, which may lead to unconservative results from conventional (synchronous) analy- sis. Natural and artificial earthquake records are applied, representing travelling wave as well as geometric incoherence effects. Two model structures of medium span RC bridges are studied, subjected to different boundary conditions that influence the mode contributions. This is undertaken under transverse, longitudinal and verti- cal earthquake motion. The large volume of results, represented as displacement and force time-histories as well as Fourier amplitude spectra of the acceleration response, are distilled and used to assess the balance between dynamic de-tuning and static relative displacements. It is concluded that the conventional synchronous case provides conservative results for vertical vibrations. However, unconservative results, of up to 30%, are obtained for transverse and longitudinal response of short periods of structural vibrations, as well as cases where the higher modes of response are likely to be excited.

Inelastic Dynamic Response of RC Bridges Subjected to Spatial Non-Synchronous Earthquake Motion:

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Inelastic Dynamic Response of RC Bridges Subjected to Spatial Non-Synchronous Earthquake Motion: