To investigate the seismic behavior of the wall pier of a covered bridge, the multi-layer shell element in OpenSEES was used to carry out the numerical analysis under a horizontal low cyclic loading in the weak axis direction. The effects of the mesh divisions of the wall pier models were evaluated and the hysteretic curves, skeleton curves, bearing and energy dissipation capacity, stiffness degradation and displacement ductility of the wall pier with different lateral loading modes were mainly researched. The results demonstrate that the discretization of layers along the thickness direction of the multi-layer shell element model has a very limited effect on the hysteretic and skeleton curves. The mesh division of the horizontal direction depends on that of the vertical, and the vertical mesh spacing shall be longer than the plastic hinge with a length of 0.5744 m. The arrangement of the loading points is critical for the seismic behavior of the wall pier. The pier suffering the force from the five points presents a relatively strong energy dissipation and larger ductility, but this layout may cause a more concentrated force at the local position. When the loading points are evenly distributed, the capacity and displacement changes sharply and the ductility diminishes. Successively, the seismic performance indexes at the local position of the wall pier tend to be more consistent with the increasing loading points. The deformation and energy dissipation capacity of the nearby position with the denser side loading points becomes larger, but this has a minor impact on the seismic performance of the position far from the points. The wall pier without the bent cap and with three bearings set is supposed to be more reasonable for the covered bridge through the overall analysis of seismic performance.

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.

Unbonded LRBs (laminated rubber bearings) are commonly applied in small-to-medium-span bridges in China. The frictional sliding characteristics of LRBs have a vital influence on the seismic response of the bridge. Nine square LRBs were subjected to the quasi-static displacement loading test in this paper, and the differences in sliding characteristics of LRBs at the interface of steel and concrete test pad were investigated. The variation of the friction coefficient during sliding was then analyzed. Based on the experimental data, a three-fold mechanical constitutive model of LRBs that considers the breakaway-sliding friction characteristics is established. Further, the bridge seismic demands in longitudinal directions with different friction interfaces are compared by nonlinear dynamic analysis on a typical LRB-supported concrete bridge. The results show the causalities of the displacements and decreases of the friction coefficient of the LRB. The breakaway coefficient of friction of the concrete surface was generally greater than that of the steel in the pre-sliding stage, while the sliding coefficient of friction of the steel interface in the post-sliding stage was greater than that of the concrete. Moreover, the proposed three-fold constitutive model is able to simulate the frictional sliding behavior of LRBs accurately. Lastly, the seismic design of small-to-medium-span bridges should take into account the breakaway-sliding friction effect of the LRBs and the preference for steel as friction pads for LRBs is recommended.