Abstract
In this study, a procedure is developed to evaluate and improve the seismic performance of long‐span suspension bridges based on the performance objectives under the fragility function framework. A common type of suspension bridge in China was utilized in the proposed procedure, considering its approach structures according to earthquake damage experience and fortification criteria. Component‐level fragility curves were derived by probabilistic seismic demand models (PSDMs) using a set of nonlinear time‐history analyses that incorporated the related uncertainties such as earthquake motions and structural properties. In addition, one step that was covered was to pinpoint the capacity limit states of critical components including bearings, pylons, and columns. The stepwise improved seismic designs were proposed in terms of the component fragility results of the as‐built design. Results of the comparison of improved designs showed that the retrofit measure of the suspension span should be selected based on two attributes, i.e., displacement and force, and the restraint system of the approach bridges was a key factor affecting the reasonable damage sequence. Necessarily, from the comparison of different system vulnerability models, the mean values of earthquake intensity of system‐level fragility function developed by the composite damage state indices were used to assess the overall seismic performance of the suspension bridge. The results showed that compared to the absolutely serial and serial‐parallel assumptions, the defined composite damage indices incorporating the thought of component classification and structural relative importance between the main bridge and approach structures were necessary and were able to derive a good indicator of seismic performance assessment, hence validating the point that the different damage states were dominated by the seismic demands of different structures for the retrofitted bridges.
Highlights
Long-span suspension bridges serve as the highly important transportation links during their service lives, and there are no expectations for these bridges to be retired in the foreseeable future
A procedure for investigating the reasonable seismic performance of long-span suspension bridges was developed under the fragility function framework. e seismic performance of a long-span suspension bridge was examined and improved using the proposed procedure, from which the results revealed distinguishable conclusions as follows: (1) e definition of damage states for bearings considered the interactions between the adjacent structures and self-damage to bearings, in accordance with the actual design and experimental conclusions. e damage states of pylons in terms of curvature ductility were defined by developing modal pushover analysis and idealized bilinear fitting methods in the equivalent pylon fiber models of the suspension bridge
(3) e stepwise improved seismic designs were proposed in terms of the component fragility results of the initial design
Summary
Long-span suspension bridges serve as the highly important transportation links during their service lives, and there are no expectations for these bridges to be retired in the foreseeable future. E backbone of the proposed procedure is fragility-based improvement of seismic performance for long-span suspension bridges, including the definition of engineering demand parameters (EDPs) and the capacity limit state thresholds associated with the case-dependent components. (4) e optimum structural seismic performance is selected in this study, taking the bridge system-level fragility as the objective function according to the definition of damage states and fragility results of various components obtained from steps 2 and 3. E composite damage state in equations (4) and (5) can be incorporated in the PSDM when the correlation between concerned components with the use of covariance matrix is known In this case, Monte Carlo simulation is used to compare the demands obtained from the joint probabilistic seismic demand models [8] and multidimensional component capacities in the M ( 105) random samples at a range of the IM values.
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