| Small-to-medium-span simply supported bridges are the major types of highway bridges in China because of their simplicity in structure,convenience in construction and economy in application.The damage investigation after the Wenchuan earthquake showed that such bridges suffered excessive movement of superstructure,accompanied by sliding of laminated rubber bearings and shearing off of shear keys.Most of the substructure(including the columns and foundations),however,remained in good conditions.This kind of damage modes,which was dominated by the transverse movement of superstructure and sliding of rubber bearings,was inconsistent with the ductility design philosophy recommended by current design code in China(in which the hysteretic behavior of the plastic hinges of pier columns is used to dissipate the seismic energy),leading to failure of seismic design.Furthermore,with the increase of service time,bridges continue to deteriorate due to the effects of corrosive media in the service environment,resulting in changes in the mechanical properties of bridge components such as stiffnesses and strengths.Thus,the seismic response and damage modes of the bridges are changed.However,the influences of structural deterioration of bridges have not yet been considered in the current seismic design codes all over the world.For the above reasons,typical multi-span simply supported bridges were used as the research examples in this study;and the influences of structural deterioration of the bridges under the general atmospheric environment were investigated and formulated;finally the seismic damage modes of the bridges in the transverse direction were discussed in the context of bridge component and system levels.This study should contribute to the further development of seismic design of small-to-medium-span simply supported bridges.The main research contents and conclusions are as follows:(1)Typical simply supported bridges in the Wenchuan earthquake area were taken as examples to establish the finite element models using the Open SEES software.Based on the previous research,the time-dependent degradation characteristics of reinforced concrete material properties were studied under the general atmospheric environment.Finite elements models were constructed respectively for different service lives,different shear key capacities and different bridge configurations.The dynamic characteristics of the bridges were compared and discussed under the above different cases.The results show that the corrosion rate of reinforcing steel increases with the increase of bridge service life,leading to continuous decrease of the diameter and strength of the reinforcing steel.Furthermore,the corrosion of stirrup causes decrease of the ultimate strength and strain of core confined concrete.The natural vibration periods of the bridges and the corresponding mass participation coefficients of basic vibration modes increase with the increase of bridge service life.(2)Using the procedures of capacity/demand method(C/D)and incremental dynamic analysis(IDA),a method of seismic analysis and performance evaluation was proposed based on the seismic demands of bridge components.The damage states and corresponding indexes were proposed for the bridge components.And then according to the damage states of the bridge components,the damage states and modes were defined for the bridge systems.Nonlinear time history analyses were conducted to investigate the effects of different parameters on the seismic damage modes of bridges in both the component and system levels.The results show that regardless of bridge configurations the structural deterioration has little effect on the damage states of bearings and shear keys,but notably aggravate the damage degree of pier columns,and even change the damage modes of bridges.Increasing the capacity of shear keys may also change the damage modes of bridges,and thus adding the difficulties of post-earthquake repair.(3)Based on the principle of nonlinear static analysis(Pushover analysis),pushover analysis procedures of bridges along the transverse direction were proposed using modal force distribution.According to the pushover curves of bridges,the damage states and damage indexes were defined in the structural system level.Then,a method of seismic damage analysis method was proposed based on the seismic capacities of bridge systems.Additionally,the analysis method for the bridge systems were further extended to be used in the bridge component level according to the pushover curves of individual components.The results show that the bridge service lives,the shear key capacities,and the bridge configurations are all critical parameters which may change the damage modes of bridges.When the shear keys are designed with very large capacities,the structural deterioration significantly increases the inelastic responses of pier columns,thus changing the damage modes of the bridge systems.(4)In general,with the increase of service life,the seismic damage of small-tomedium-span simply supported highway bridges in the transverse direction gradually transfers from superstructure to substructure.In some cases,the structural deterioration causes significant change of the seismic damage modes of bridges from excessive movement of superstructure to flexural failure of pier columns.With larger capacities,the shear keys are more efficient in restraining the seismic movement of superstructure.However,if the capacities of shear keys are too largely designed,the pier columns will probably suffer flexural failure because of the additional forces transferred by the shear keys,causing changes of damage modes of bridges.Especially when the structural deterioration is considered,the capacity of shear keys plays a more significant role in the seismic damage modes of bridges.Therefore,to research and improve the transverse seismic design strategy of small-to-medium-span simply supported highway bridges should be based on their life-cycle performance,and rigorously define and determine the role of shear keys in their seismic damage modes. |
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